A COMPARISON OF THE EFFECTS OF PREDATORS AND NEIGHBOURING

PLANTS ON LrrHRUM SALICARIA L. AND VERBENA HASTATA L.

A Thesis

Presented to

The Faculty of Graduate Studies

of

The University of Guelph

by

JENNIFER RACHICH

In partial fulfilment of requirements

for the degree of

Master of Science

July. 1997

O 3. Rachich, 1997

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A Cornparison of the Effects of Predators and Neighbouring Plants on Ljdhrurn salicarfa L and Verbena hastata L.

Jennifer Rachich University of Guelph, 1997

Advisor: Dr. Richard Reader

A field expriment was conducted to test the hypothesis that the introduced wetland plant

purple loosestrife (Lythrum salicaria L.) was less affected by predators and competition

from neighbours than a morphologically similar (i.e. similar seed size, plant height.

germination tirne) native wetland species, blue vervain (Verbena hastata L.). Predator

exclusion and neighbour removal treatments were used to assess the effects of predators

and competition from Phalaris neighbours on the shoot mass of the two target species.

Lythrum and Verbena. In most cases, Lythrum and Verbena were equally affected by

potential effects of predation and competition as well as the combined effect of competition

and predation, except at one site. where potential competition effects and the combined

effect of cornpetition and predation reduced Lythrum's shoot mass more than Verbena's.

None of the results of this study indicated that introduced Lythrum was less affected by

predators or more cornpetitive than native Verbena.

ACKNOWLEDGEMENTS

I would like to start off by thanking my advisor Richard Reader. who has not only been a

great teacher but a good friend. I appreciate al1 of his input and the long hours he spent

revising nurnerous venions of this thesis. Without his guidance. encouragement and

wisdom. this project would not have been possible. I thank my cornmittee. Ooug Lanon

and Man Husband, for their time, patience and comments. I would also like to thank to

Larrry Peterson for taking the time to revise this thesis and for his comments at my

defense.

I owe a great deal of thanks to Sheila Mc Nair for her assistance in the field. The endless

hou= we spent in the field swatting mosquitoes. clipping grass and spraying Orthene will

not be forgotten. Her friendship and encouragement (especially when it would not stop

raining) was greatly appreciated. Thanks also goes to Teresa for her help in the lab.

entering data and her help in the field. harvesting the test species. I would also Iike to

thank my fiancé. Greg Thomas who has been supportive from day one. I could not have

done this without his patience, encouragement and understanding.

A number of othen offered words of encouragement and understanding including; ApA.

Eden. Marcy. Tracy, Paul. Carole Ann and Angela. Thanks also goes to my fiancé's

parents and family who have offered a great deal of support. Lastly, I would like to thank

my parents and my sister Meagan. This work refiects the love, support and encouragement

that they have always given me.

CONTENTS

INTRODUCTION .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Objectives ........................................................ 7

METHODS .......................................................... 8

Choiceoftestspecies ............................................... 8

Choice of test habitat . . ............................................. 9

Study sites vegetation and biomass .................................... 9

ExperimentalDesign .................................................. 13

ExperimentalProcedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Caging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Neighbourremoval ................................................ 17

Seed collection and treatment . . ...................................... 17

Transplanting .................................................... 18

Shoot growth of L . salicana and V . hasfata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Comparison of treatment affects on L . salicaria and V . hastata . . . . . . . . . . . . . . . 19

Effect of experimental treatments on abiotic resources . . . . . . . . . . . . . . . . . . . . . 21

Soil variables - nutrient availability for test species . . . . . . . . . . . . . . . . . . . . . . . . 21

Photosynthetically active radiation (PAR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Relationship between PAR and mean shoot rnass . . . . . . . . . . . . . . . . . . . . . . . . 23

RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Vegetationbiomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

. . . . . . . . . . Potential effects of predation and cornpetition on the shoot mass of 24

L. salicaria and V . hastata

l i

................................... Predation with neighbours removed 24

................................... Cornpetition with predators excluded 30

........................... Combined effect of cornpetition and predation 32

..................... Effect of experimental treatments on abiotic resources 39

SoiIvanables ..................................................... 39

............................................................ PAR 41

Relationship btween PAR and mean shoot mass ........................ 41

DISCUSSION ....................................................... 44

Predation with neighbours removed ................................... 44

Effect of predation on introduced versus native species .................... 46

L . salicana versus V . hastata . Why are they equally . . . . . . . . . . . . . . . . . . . . . . 50

affected by potential predators?

Corn petition with predators excluded ................................... 51

Effect of cornpetition on introduced versus native species . . . . . . . . . . . . . . . . . . 53

L . salicana and V . hastata - Why were they equally . . . . . . . . . . . . . . . . . . . . . . . 57

affected by the potential effect of competition?

Combined effect of cornpetition and predation ........................... 58

Choosingspeciespairs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Study strengths and limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

iii

APPENDIX 1 (Half cage treatments) ...................................... 70

APPENDIX 2 (Before and after herbicide application) ......................... 73

APPENDIX 3 (Initial transplant height) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

APPENDIX 4 (Insect List) .............................................. 79

LIST OF TABLES

TABLE PAGE

1. Mean plant mass of Phalaris anrndinacea at three study sites near . . . . . . . . . -25 Guelph, Ontario, Canada.

2a. Analysis of variance on the potential effect of predation with . . . . . . . . . . . . . . . . 26 neighbours removed-.

2b. Analysis of variance for predation index values . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3a. Analysis of variance on the potential effect of cornpetition with target species . . .31 in a full cage.

3b. Analysis of variance for cornpetition index values . . . . . . . . . . . . . . . . . . . . . . . . -31

4a. Analysis of variance on the cornbined effect of predation and cornpetition . . . . . . 36

4b. Analysis of variance for predation plus cornpetition index values . . . . . . . . . . . . 36

5. Mean values of six substrate factors with vegetation present and . . . . . . . . . . . .40 vegetation absent at three study sites near Guelph, Ontario.

6. Mean percentage of incoming PAR measured at the top of each target plant . . .42 in experirnental treatments at three study sites near Guelph, Ontario.

7. Analysis of variance on the potential effect of shading for Lythrum . . . . . . . . . . .70 target species.

8. Analysis of variance on the potential effect of shading for Verbena . . . . . . . . . .71 target species.

9. Mean shoot mmass of Lythrum and Verbena between treatments . . . . . . . . . . . . -72 at three sites near Guelph, Ontario.

10. Mean values of six soi1 variables in plots with vegetation or without . . . . . . . . . .74 vegetation before and after herbicide application at site 1.

11. Analysis of variance for initial plant height for Lythnrm . . . . . . . . . . . . . . . . . . . . 76

12. Analysis of variance for initial plant height for Verbena . . . . . . . . . . . . . . . . . . . 77

13. Mean initial plant height for Lythrum and Verbena in experimentaf . . . . . . . . . 78 treatments at three sites near Guelph, Ontario.

14 . List of insects found at site 1 ....................................... -79

1 5 . List of insects found at site 2 ....................................... -80

16 . List of insects found at site 3 ....................................... -81

Figure LIST OF FIGURES

Page

1. Site map showing the location of the three study sites relative . . . . . . . . . . . . IO to Guelph, Ontario.

2(a,c,e) Mean shoot mass for Lythmm and Verbena in cleared plots with a . . . . . . . 28 full cage or with no cage at three sites near Guelph, Ontario-

2(b,d,f) Mean predation index values for Lythnrm and Verbena at three sites . . . . . + 28 near Guelph, Ontario.

3(a,c,e) Mean shoot mass for Lythrum and Verbena in fully caged plots . . . . . . . . . . 33 with neighbours removed or with neighbours present at three sites near Guelph, Ontario.

3(b,d,f) Mean competition index values for Lythrum and Verbena at three sites . . . . 33 near Guelph, Ontario.

4(a,c,e) Mean shoot mass for Lythnrm and Verbena in control plots . . . . . . . . . . . . ..37 with neighbours present and no cage versus plots with neighbours removed and plots fully caged at ttiree sites near Guelph, Ontario.

4(b,d,f) Mean competition plus predation index values for Lyfhrum and . . . . . . . . . . .37 Verbena at three sites, near Guelph, Ontario.

vii

INTRODUCTION

One of the major goals of plant ecology is to understand how environmental factors control

the distribution and abundance of plant species. Recently, many ecologists (Mooney et al.

1986; Crawley 1987; Rejmanek 1989; Hobbs and Heunneke 1992; Blossey and Notzhold

1995) have focused on environmental controls of the distribution and abundance of

introduced plant species largely because it has been suggested that they rnay displace

native plant species (Femald 1940; Stuckey 1980; Vitousek 1986; Schofield 1989; Mal et

al. 1997). An introduœd plant species is one that has originated outside of North America.

One of the most common questions asked is, what allows an introduced plant species to

become abundant in a new habitat? To answer this question, a number of studies have

focussed on habitat conditions and life history traits of introduced species that could explain

their success in a new habitat (Baker 1974; Bazzaz 1979; Mack 1981; Fox and Fox 1986;

Crawley 1987; di Castri 1990). According to Mack (1981) four main factors that facilitate

the rapid spread of introduced plant species are:

1) habitat conditions to which the introduced species is preadapted;

2) habitat modification at the time of entry;

3) leaving behind predators which may have once controlled its distribution and

4) native plant competitors which are inferior to the introduced species.

A num ber of researchers (e-g. Darwin 1859; Batra et al. 1 986; Thompson et al. 1 987; flig ht

1990; Hight and Drea 1991 ; Malecki et al. 1991 ; Blossey and Notzhold 1995; Trowbridge

1995; Edwards et al. 1995) consider factors three and four to be especially important. They

have hypothesized that introduced plant species are Iess affected by predators and more

cornpetitive than many native species. They have proposed that introduced species can

I

become abundant in a new habitat because they have presumably left their natural

predaton behind. Some ecologists have noted the larger plant size of some introduced

species in a new habitat cornpared to that of their native habitat and suggested that this

larger size may reflect a lack of natural phytophagous enemies (Crawley 1987; Blossey and

Notzhold 1995; Edwards et al. 1995). They reasoned that introduced plant species could

invest more energy in growth than native plant species which presumably invest some

energy for both defence mechanisms and growth (Blossey and Notzhold 1995). lntroduced

species rnay also be better cornpetitors for lirnited resources than native species with

similar Ife history traits (Le. sirnilar seed size, plant height, germination time. etc.; hereafter

referred to as morphologically sirnilar) since introduced species could allocate more of their

energy to resource acquisition than native species which are morphologically sirnilar. This

combination of low predator pressure and high competitive ability may explain why

introduced plant species become abundant in a new habitat.

This hypothesis, that introduced species are less affected by predaton and more

competitive than native species, rnay explain the successful invasion of the introduced

wetland plant purple loosestrife (Lythrum salicana L., henceforth called L) into North

America from Europe (Batra et al. 1986; Thompson et al. 1 987; Hight 1 990; Hight and Drea

1991 ; Malecki et al. 1991 ; Blossey and Notzhold 1995; Edwards et al. 1995). L was

intrcduced to North America from Europe over a century ago and since that time has

invaded numerous wetland systems (Stuckey 1980; Thompson et al. 1987; Edwards et al.

1995; Mal et al. 1992). Researchers c lah that it foms dense monospecific stands that

reduce the biotic diversity by displacing native vegetation which eliminates food and shelter

for many wildlife species (Femald 1940; Rawinski and Malecki 1984; Thompson et al.

2

In Europe, L may grow up to 120cm in height with the occasional stem reaching 2m,

whereas in North America it has been reported that L can grow up to an average height of

25m, with some stems reaching 3m (Edwards et al. 1995). Similady, under laboratory

conditions, Blossey and Notzhold (1 995) examined the growth of L, from two locations, one

with natural herbivory (Lucelle, Switreriand) and one without natural herbivory (Ithaca. New

York, USA). They found that L plants from Ithaca, New York grew taller and had a greater

plant biomass than L plants from Lucelle, Switzerland. However, cornparisons of more

populations would be needed from both continents and other plant species in order to

determine whether they were just comparing populations with different biomass allocation

patterns or whether plants from areas of introduction consistently produce more biomass

when they colonize a new non-native area. Still, nurnerous researchers speculate that this

increased height reflects greater allocation of energy to growth in the absence of predation

and that this increased growth has given L a competitive advantage over native species

(Thompson et al. 1987; Edwards et al. 1995). As a result, rnany researchers have

hypothesized that this combination of low predator pressure and superior competitive ability

has led to the invasion of L into numerous wetland systems in North America (Stuckey

1980; Thompson et al. 1987; Hight 1990; Hight and Drea 1991 ; Malecki et al. 1991 ; Mal

et al. 1992).

One way to test this hypothesis, that L is less affected by predators and more competitive

than native species is to compare the effects of predators and competition from

neighbouring plants on L and a morphologically similar native species. Yet, for LI there are

very few studies that have tested this hypothesis (Rawinski and Malecki 1984; Gaudet and

Keddy 1988,1995; Johansson and Keddy 1991 ; Mal et al. 1 997).

Those researchers who have atternpted to test this hypothesis have done so by examining

onty one aspect of the hypothesis, either predation (Rawinski and Malecki 1984) or

competition (Gaudet and Keddy 1988,1995; Johansson and Keddy 1991 ; Mal et al. 1997).

Rawinski and Malecki (1 984) atternpted to examine the arnount of predation damage done

by muskrats (Ondatra zibethicus) on introduced L and native Typha spp. (cattails, hereafler

referred to as Typha) in enclosures and open plots, both containing a mixture of each

species. On one out of nine sampling dates, they found Typha's density was Iower outside

cages than inside cages, and on two out of nine sampling dates they found mat L's density

increased in the open plots while Typha's density decreased. While their results seem to

suggest that predators reduced the density of the native species (Typha) more than the

introduced species (L), their experirnental design was confounded uy ~otential effects of

interspecific competition since target species were surrounded by neighbours. To measure

the effect of predation alone, they should have eliminated potentiat competition from

neighbours by removing above ground and below ground plant biomass in caged and

uncaged plots. Since it is still not c!ear whether L was less affected by predators, I decided

to examine the potential effect of predators on introduced L and another morphologically

similar native species.

As previously stated, rnany researchers have assumed that L outcornpetes native species

(Stuckey 1980; Thompson et al. 1987; Wilcox 1989). To test this possibility researchers

have compared the competitive ability of L and native species growing in pots (Gaudet and

Keddy 1988, 1995; Johansson and Keddy 1991) or in the field (Mal et al. 1997). In an

outdoor corn pound, Gaudet and Keddy (1 988, 1 995) exarnined the relative com petitive

ability of 44 herbaceous wetland plants (neighbours) to suppress the growth of a

phytometer (Lythmm salicaria L.). Plants were genninated simultaneously from seed and

grown as species pairs in 1-litre pots of sterile, organic, high-nutrient mix soil, when plants

were one month old, Under these optimal greenhouse conditions they found that L was the

top competitor. However, they did not use a native species to compare L's competitive

ability. Instead they surrounded L with 4 individuals of one native species which acted as

neighbours instead of test species. Without using another native species, they cannot Say

that L is more or less competitive than a native species when surrounded by neighbours.

Since it is still not clear whether L was less affected by competition from neighbours, I

decided to examine the potential effect of neighbours on introduced L and another

morphologicalIy similar native species.

In another greenhouse study, Johansson and Keddy (1991) examined the competitive

abilrty of L and an morphologically similar native species, blue vervain (Verbena hastata L.).

The growth of both target species was compared when grown with and without neighbours.

Neighbours included each other as well as Mimulus ringens, Cyperus rivularis, Eleochans

obtusa and Juncus bufonius. They found that L was more competiiive (Le. less suppressed

by neighbours) than hastata, which is consistent with the hypothesis that L is more

cornpetitive than a morphologically similar native species. However, it is important to note

that this experiment examined competition interactions between young plants (Le., one

5

month old) of the same ske, which would only occur naturally on newly-exposed mudflats

or following an artificial reduction of water level (Le., a drawdown). As a result, I decided

to examine whether L was less effeded by adult neighbours than a morphologically sirnilar

native species.

In a field study of competitive ability under drawdown conditions, Mal et al. (1997)

compared the relative competitive ability of introduced L and a dissimilar native species

Typha angustifdia L.. using a modified replacement series experiment. After four yean.

L appeared to be the better competitor which is consistent with the hypothesis that L is

more competitive than a native species. Unfortunately, this field study was potentially

confounded by the effect of predation since they did not protect target species from

potential predators. Since it is still not clear that L is more competitive than a native

species. I decided to examine whether L was less affected by potential competition from

adult neighbours than a morphologically similar native species.

In surnmary, past studies provide soma support for the hypothesis that introduced L was

less affected by predatow and competition from neighbours than a native species when

plants are young on bare ground. However, no study to my knowledge has compared the

ability of L and other morphologically similar native species to compete with adult

neighbours in intact stands of vegetation. Thompson et al. (1 987) documented that L can

invade intact stands of Typha and reed canary grass (Phalaris anrndinacea L.). Therefore,

I decided to compare the effect of competition from adult neighboun on L and a

morphologically similar native species in intact stands of adult vegetation rather than on

bare ground.

6

Objectives

The objectives of this study are to 1) compare the potential effect of predators on

introduced L and a morphologically similar native species, 2) compare the potential effect

of cornpetition from adult neighbouring plants on introduced L and a morphoIogicalIy similar

native species, and 3) compare the combined effect of predators and cornpetition on

introduced L and a rnorphologically similar native species.

Since other studies provided only weak evidence that L was less affected by predators and

more cornpetitive than native species, a field experiment was conducted to test the

hypothesis that the introduced wetland plant L is less effected by predators and more

competitive than a morphologically similar native wetland species.

METHODS

Choice of test species

Two test species, L and blue vervain (Verbena hastata, henceforth called V), were used

for this study. I chose V because it is a native species (Gleason and Cronquist 1994) that

is morphologically similar to the introduced species L (Shipley and Parent 1991; Boutin and

Keddy 1993). Boutin and Keddy (1993) classified 43 species of wetland plants based on

27 traits such as relative growth rate (RGR), height of juveniles/adults, rates of shoot

extension, above-and belowground biomass allocation, photosynthetic area (cm2), total

number of tillers or shoots, and % of flowering during first year. They found that three traits

(Le., % of flowering during first year, life span and photosynthetic area) separated the 43

species into two main groups, perennials and ruderals. Further analysis showed that the

ruderals consisted of two further groups: obligate annuals which flowered in their first year

of growth and died at the end of the growing season and facultative annuals which also

flowered during their first year of growth but did not die at the end of the growing season.

Boutin and Keddy (1 993) classified both L and Vas facultative annuals with tall erect. fast-

growing main stems, which may flower in their first year of growth. Othennrise, shoots may

emerge the following year from the base and again produced tall plants with a narrow, erect

growth form topped by an inflorescence. In another study, Shipley and Parent (1991)

examined 64 wetland species, including L and V, in relation to five traits including;

germination time, maximum number of seeds produced in 1 day, % of seeds genninated

in 30 days, seed weight, and seedling relative growth rate. In their study they also

classified L and V as facultative annuals. Past studies confinn that L and V are found in

similar wetland habitats (Collette 1983; Thompson et al. 1987; Mal et al. 1997).

Choice of test habitat

Wetiand habitats dominated by reed canary grass (Phalaris arundinacea L.) were chosen

as test habitats for tw reasons. First, Thompson et al. (1987) considered habitats

dominated by Phalaris to be highly susceptible to invasion by L. Second, species traits

such as predator effects and competitive ability are likely to be important determinants of

plant performance in wetlands dominated by P. amndinacea because it has relatively high

biomass (e-g. 579 g/m2, Bonser and Reader 1995) for a wetland. Results of a number of

past studies indicate that predation and plant competition are intense in habitats with this

amount of biomass (Grime 1979; Oksanen et al. 1981; Wilson and Keddy 1986 a & b;

Keddy 1990; Shipley et al. 1991 ; Wilson and Keddy 1991; Bonser 1994; Bonser and

Reader 1995).

Study sites vegetation and biomass

Three sites were chosen for the study (Fig. 1). Two sites (hereafter referred to as sites 1

and 3) were approximately 19 km north of Guelph, Ontario, Canada (43" 33 N, 80" 15 W)

and approximately 4km apart, while the third site (hereafter referred to as site 2) was

approximately 50 km south of Guelph, Ontario, Canada. Sites 1 and 3 were located

alongside Cox Creek, while site 2 was located alongside Fairchild Creek.

The three sites were chosen because vegetation consisted almost entirely of Phalaris

arundinacea which provided a relatively uniforrn habitat for the experiment, V was present

at each of the three sites while L was absent. Since V is restricted to certain wetland

habitats (i.e., fertile wetland sites) and L is found in a wider variety of wetland habitats (Le.,

Figure 1: Site map showing the location of the three study sites relative to Guelph. Ontario, Canada.

Infertile sites), I felt that it was more important to find sites where V was present.

To estimate the amount of above ground biomass of vegetation present at each site, above

ground plant material was collected from five randomly selected 1-m2 areas per site on

August 12 (at site 2) and August 14, 1996 (at sites 1 and 3). This plant material was dned

(80" C) and weighed. Mean biomass was calculated for each site and the site means were

compared using an analysis of variance CANOVA) followed by Tukey's honestly significant

difference (HSD) test.

Experimental Design

To measure the potential effects of predation and competition from Phalaris on the above

ground shoot mass of the two target species (L and V), a randomized-block experiment with

the foltowing six treatments was set up at each of the three sites:

1) target species fully caged and neighbours removed;

2) target species not caged and neighbours removed;

3) target species fully caged and neighbours of the target species left intact;

4) target species not caged and neighbours of the target species left intact;

5) target species placed in a half cage and neighbours removed; and

6) target species placed in a half cage and neighbours of the target species left intact.

With this experimental design the potential effect of predators on the growth of L and V

could be measured by comparing treatments 1 and 2 (Le., predaton excluded by a full cage

versus predators not excluded by a cage, without any neighbours present). When

measuring the potential effect of predators on target species, neighbours were removed in

order to eliminate potential confounding effects of competition. As a result, when

measuring the potential effect of predation, data for treatments 3 and 4 would not be used.

To compare potential predator effects on two species at three sites, a three-way ANOVA

for a randomized block experiment was used to test for differences in shoot mass between

treztments in cleared plots. Values were square root transformed + 0.1 to meet

assumptions of ANOVA. Main effects of site, species and cage (Le., caged venus uncaged

target species) were examined, al1 of which were fixed effects. Interactions between site

' cage, cage species, site * species, and site cage species were also examined. In

order to compare caging treatments Tukey's HSD test was used.

13

The potential effect of competition from Phalaris could be measured by comparing the

shoot mass of L and Vin treatments 1 and 3 (Le., neighbours removed versus neighbours

left intact, with predators excluded by a full cage). When measuring the potential effect of

wmpetition from Phalaris on target species, target species were caged in order to eliminate

the potential confounding effect of predation. As a result, when measuring the potential

effect of competition from Phalaris data for treatments 2 and 4 would not be used. To

examine potential competition effects from Phalaris on the two species at the three sites,

a three-way ANOVA for a randomized block experiment was used to test for differences in

growth between treatments for caged target species. Values were log, + 0.1 transformed

to meet assumptions of ANOVA. Main effects of site, species and neighbours (Le.,

neighbours present versus neighbours removed) were examined, al1 of which were fixed

effects. Interactions between site neighbours, neighbours species, site ' species, and

site neighbours species were also examined. In order to compare neighbour treatments

Tukey's HSD test was used.

The combined effect of competition and predation on L and V could be measured by

comparing growth of target species in treatrnents 1 and 4 (Le., predators excluded by a full

cage and neighbours removed versus predators not excluded by a cage and neighbours

left intact). To examine the combined effect of predation and competition on the two

species at the three sites, a three-way ANOVA for a randomized block experiment was

used to test for differences in growth between treatments. Values were log, + 0.1

transformed to meet assurnptions of ANOVA. Main effects of site, species and treatment

(Le., caged target species and neighbours removed versus uncaged target species with

neighbours intact) were examined, al1 of which were fixed effects. Interactions between site

14

* species, site treatment, treatment species, and site treatment species were atso

examined. In order to compare individual treatments (Le., caged target species and

neighbours removed versus uncaged target species with neighbours intact) Tukey's HSD

test was used.

The two hatf cage (HC) treatments (5 and 6) were included in the experimental design in

order to test for a potentially confounding effect of shading by the full cage. The half cage

also shaded the target plant but it did not excluded predators (as described in more detail

in the experimentaf procedure to foltow). To assess the effect of shading, the growth of L

(and V) was compared in treatments 2 and 5 (i.e., no cage versus haif cage, with

neighbours removed) and in treatrnents 4 and 6 (Le., no cage versus half cage. with

neighbours intact). To examine the potential effects of shading on the two species at the

three sites, a three-way ANOVA for a randomized block experiment was used to test for

differences in growth between treatments for each species separately. Main effects of site,

cage (no cage versus half cage) and neighbours (Le., neighbours removed versus

neighbours intact) were examined, where al1 effects were fixed. Interactions between

neighbours ' cage, site cage, site * neighbours, and site neighbours cage were also

examined. Results for no cage versus half cage versus treatments did not differ

significantly for any of the six combinations of 3 sites x 2 neighbour treatments for L or V

(Appendix 1). As a result, shoot mass values for L and Vin the no cage treatment were

used instead of half cage values at atl sites.

Experimental Procedure

For the three sites, each of the six treatments was assigned randomly to one of the twelve

1 .5-m2 plots set up along a transect (or block) running parallel to the stream (Le., 6 plots

per species). Within each block, plots were approximately 5 m apart. Five blocks were set

up at Ieast 5 m apart at both sites 1 and 2, while four blocks were set up 5 m apart at site

3 due to its smaller size.

Caging

For the full cage and half cage treatments a target species was either fully enclosed or

partially enclosed, respectively. A full cage consisted of a wire mesh (6rnm) cylinder (0.5

m diameter x 1 m tall) that was supported by two bamboo poles. The cylinder was open

at the top and bottom. The bottom of the cylinder was placed 10 cm into the ground to

exclude small mammals. When target species were almost 1 m tall, additional cylinder

cages were attached as needed to the original cage in order to exclude tall vertebrate

herbivores such as deer. lnvertebrates were discouraged by spraying the target species

weekly with a systemic insecticide (Le., an 8.5 % solution of acephate [O, Sdimethyl

acetylphosphoramidothioate], tradename Orthene, as recommended by the distributor).

Initial trials in which L and V were sprayed with Orthene or water (five replicates each)

indicated that Orthene did not visually affect plant growth. A half cage treatment consisted

of the same wire mesh and bamboo poles, but was placed only on the south side of the

target species to give about the same amount of shading expenenced by a plant in a full

cage. Also, half cage treatments were sprayed weekly with water in order to compensate

for the addition of water when plants were sprayed with Orthene.

Neighbour removai

At sites 1 and 2, al1 aboveground vegetation and litter was removed by applying a herbicide

(Le., a 1% solution of glyphosate (N-phosphenomethylglycine], tradename Roundup). The

herbicide was applied as soon as there was no standing water present and plots were

raked the following week to remove al1 dead vegetation: at site 1, plots were sprayed on

June 3,1996 and raked on June 10, 1996; due to fi ooding at site 2, plots were not sprayed

until July 8, 1996 and rakeâ on July 15 1996. Soil samples were taken on May 28, 1996

(before herbicide application) and on June 20, 1996 (after herbicide application) in five

cleared plots and five uncleared plots at site 1, in order to determine if killing vegetation with

a herbicide effected the amount of nutrients in the soil. Roundup had no detectable affect

on soi1 nutrient content (Appendix 2)- To maintain cleared plots, herbicide application was

repeated once more during the growing season on August 19 and 20, 1996 at sites 1 and

2, respectively. To avoid spraying the target species, an aluminum cylinder (1 .Sm x 1 rn x

l m tall) was placed around the target species while the plot was sprayed with Roundup.

For neighbour removal treatments at site 3, al1 aboveground vegetation and litter was

rernoved by clipping on June 3 and 4, 1996 since the landowner did not allow me to rernove

vegetation with a herbicide. Potential root interference was controlled along the perimeter

of each plot by severing roots and rhaomes of Phalaris arundinacea with a spade. Clipping

was repeated bi-weekly to maintain cleared plots.

Seed collection and treafmenf

Target species were grown from seeds collected locally in 1995. L seeds were collected

frorn the banks of the Speed River in Guelph, Ontario while V seeds were collected from

Cootes Paradise near Hamilton, Ontario. To simulate winter conditions, seeds of both

species were stored at 4°C during the winter months in 1995/1996. Seeds of V were

placed in moist sand to fulfill any requirernent for breaking seed dorrnancy. Seeds of L

were stored in a dry state since this species did not require cold stratification to promote

germination (Shamsi and Whitehead 1977). On ApriI 25, 1996 (for sites 1 and 3) and on

June 26, 1996 (for site 2) seeds of both species were germinated in Petri dishes and

seedlings were transferred to small pots (4 x 3 cm x 5 cm deep) that contained a

commercially prepared potting medium (Promix). Seedlings were allowed to become

established in the greenhouse, then transplanted into field plots.

Transplanting

Young plants (8 wks. old at sites 1 and 3; 5 wks. old at site 2) of each species were

transplanted into experimental plots on the following dates: June 17 and 21, 1996 at site

1; June 17, 1996 at site 3; and July 30 and 31, 1996 at site 2. Ideally, al1 target species

would have been the same age at the three sites, but due to an extremely wet spring,

planting had ta be delayed at site 2 until conditions were suitable. Since target plants at site

2 were planted much later in the season, they were watered with approximately 0.2 L of

Stream water dunng their first week in order to minimize transplant shock. One target plant

suffered from transplant shock at site 1 and was replaced on July 2, 1996. Target species

were transplanted at each site so that plant size and age could be standardized among

treatments. The average initial height of target species did not differ significantly among

treatments for either L or Vat any of the three sites (Appendix 3). One individual of either

L or V was planted at the centre of each 1.5m plot. A flagged bamboo pole was placed

18

next to each target plant to be able to relowte the transplant.

Shoot growfh of L and V

Target species were hanrested (aboveground shoot mass only) once leaves began to

senesce, which was site dependent: site 3 was harvested on Septernber 26, 1996, 15

weeks after plants were transplanted; site 1 was harvested on October 9, 1996, 17 weeks

after plants were transplanted; and site 2 was harvested on October 17, 1996, 12 weeks

after plants were transplanted. Plants were drkd at 80°C for 48 hours and weighed. Mean

values of shoot mass were calculated and the statistical significance of differences in mean

shoot mass among the three treatments (Le.. predator effeds, neighbour effects and

combined effect of predators and neighbours) were tested using a three-factor, complete

block ANOVA (for statistical analysis details refer to pages 13 - 15).

Cornparison of tresünent effects on L and V

Since L and V have the potential to attain different final shoot mass values, and abiotic

conditions may Vary among sites, indices were used to standardize shoot mass values in

order to compare the effects of predation, competition and the combined effect of predation

and competition among species and sites. To compare the effeds of predation (PI),

competition (CI) and competiiion plus predation (CPI) on the two target species, values of

the following indices were calculated for each species, based on recommendations of

Grace (1 995):

PI = (FC - NC)/FC,

19

where FC and NC are the mean shoot masses of target species within a full cage or no

cage respectively, in plots where neighbours were removed (i-e., effect of competition

excluded);

CI = (NR - NI)INR,

where NR and NI are the mean shoot masses of target species in treatments with

neighbours removed and left intact, respectively, in fully caged plots (Le., effect of predation

excluded);

CPI = (FCNR - NCNI)/FCNR, (3)

where FCNR is the mean shoot mass of target species within a full cage and with

neighbours removed and NCNl is the mean shoot mass of target species not in cages and

with neighbours left intact.

For each index, the reduction in potential target species mass due to predation and/or

competition (Le., the numerator) was expressed as a fraction of the potentiat target species

mass at that site (Le., the denominator). This accounted for variation in potential target

species mass among species and sites due to differences in abiotic conditions. An index

value of O would indicate that predation (or competition) did not reduce a target species

mass from its potential value at a site. An index value of 1 would indicate that predation (or

competition) reduced a target species potential mass to zero. Mean index values (either

PI, CI or CPI) for each species were calculated and the statisücal significanœ of diierenœs

in index values between species and sites was tested using a two-way ANOVA. Data were

arcsine square root transforrned to meet assumptions of ANOVA. Main effects of species

and site (which were fixed), as well as the interaction between species and site were

examined. Tukey's HSD test was used to compared diKerences among species.

Effect of experimental treaîments on abiotic resources

Soil variables - nutrient availabilily for test species

Soil was collected on July 22. 1996 at site 1 and on July 23, 1996 at sites 2 and 3 to

examine nutrient availability (i-e., pH. Mg, Kt NHrN, NOzN, and P) in vegetated and

cleared plots at each of the three sites. For each plot. six cores, 20 cm deep and 2 cm in

diameter, were taken with a soi1 auger and placed in a plastic bag, which was labelled by

transect and plot number. Wthin three hou= of collection al1 samples were placed in a

freezer. Once frozen, the samples were transported for analysis to the Analytical Services

Laboratory, University of Guelph. Ammonium and nitrate were extracted with 2M KCI,

filtered through no. 42 Whatman paper and compared colourimetrically to known standards

with a Braun and Lubbe Traacs 800 analyzer. Ammonium and nitrate concentrations were

expressed in mgkg of dry soil. For sodium bicarbonate-extractable phosphorus, available

magnesium, potassium and soi1 pH, soi1 was dried at 35" C for 24 hours and sieved through

a 2 mm mesh pnor to determination of concentrations. Phosphorus was extracted with

0.5M NaHCO,. filtered through No. 5 Whatman paper and measured with a Technicon Auto

Analyzer. Extractable phosphorous concentrations were expressed in mgA soil.

Magnesium and potassium were extracted with 1 M neutrat ammonium acetate, filtered with

No. 5 Whatman paper and measured by atomic absorption spectrophotometry. Available

21

magnesium and potassium concentrations were expressed in mg/i soil. Methods for soi1

analyses are described by Page et al. (1982). To determine whether values differed

significantly between plots (i.e., vegetated and unvegetated) and sites a two-way ANOVA

for a randomized block expriment was used. Main effects of site and plot (which were

fixed effects) and the interaction belween the two were examined. In order to determine

differences between sites and plots, Tukey's HSD test was used. The analysis was

conducted for each of the six variables separately. Only data for NH,-N needed to be

transformed log, + 1 prior ta analysis to meet assumptions of ANOVA.

Photosynfheticaliy active radiation (PAR)

PAR measurements were made to compare the effect of the neighbour removal and caging

treatments on the amount of PAR received by a target species at the three sites. PAR

measurements were taken on August 14, 1996 for site 1 and on August 9, 1996 for sites

2 and 3, using a quantum sensor (Li-Cor, USA) held at the top of each target plant. PAR

readings were taken between eleven in the morning and three in the afternoon when

readings in a cleared plot were no less than 1500 pmol per m2 per sec. The recorded

values were expressed as a percentage of available PAR (Le., PAR above the vegetation

canopy) measured for each plot. To examine whether PAR diiered among treatments (Le.,

neighbours and cages) and among sites, a three-way ANOVA for a randomized block

experiment was used to test for differences in percentages among treatments. Percentage

value were arcsine square root transformed prior to analysis. This analysis was carried out

for each of the two target species separately. Main effects of site, neighbours (neighbours

intact versus neighbours removed) and cage (Le., caged target species versus uncaged

target species) were examined, al1 of which were fixed effects. Interactions between site

22

* neighbours, neighbours cage, and site neighbours ' cage were also examined. In

order to compare individual treatments Tukey's HSD test was used.

Relationship between PAR and mean shoot mass

To determine whether L and V responded similariy to changes in PAR, I compared the

relationship between PAR and shoot mass for the two species at each of the three sites

separately. The statistical significance of the relationship between plant biomass and PAR

was deterrnined using linear regression analysis by site for each species. Shoot mass

values were transformed log, + 1 to linearize the relationship. Analysis of covariance was

used to determine if there was a significant difference between L and V in the slope of the

relationship between shoot mass and PAR.

RESULTS

Vegefation biomas

Phalaris biomass was not significantly different between the three sites, ranging from a

mean of 937 g/m2 to 994 g/m2 (Table 1).

Potential effects of predation and competition on the shoot mass of L and V

For each of the three analyses (the potential effect of predation, the potential effect

of competition and the combined effect of competition plus predation) the results of the

ANOVA table will be reviewed, followed by the results for individual treatments. Since

results were site-specific, each site will be discussed separately regarding the effects of

predation, competition and the combined effect of competition and predation. Once each

site has been discussed I witl then surnmarize across sites.

Predation with neighbours removed

Results of ANOVA for the potential effect of predation show that the main effects of site and

caging treatments (Le., caged versus uncaged) were significant, but the main effect of

species was not significant (Table 2a). The interactions between site cage and cage '

species atso were significant, indicating that the effect of cage differs among sites and

among species. The interactions between site species and site * cage species were not

significant. Results of the ANOVA for predation index values show that there were

significant differences among sites but not between species (Table 2b). The interaction

between species site also was not significant. Since several of the interactions were

significant for predation effects, the results are presented in figure 2 (a, b, cl dl e and f)

Table 1 : Mean (*1 SE) shoot mass of Phalaris anrndinacea at three study sites near Guelph, Ontario, Canada- Values with the same letter do not differ significantly (Pr0.05).

Study Site

Site 1

Site 2

Site 3

Shoot mass (glm2)

994 s 55A

987 i 1 O ï A

937 I 106*

Table 2a: Results of the analysis of variance on the potential effect of predation with neighbours removed, The analysis tested for the following differences: between sites, species and caging treatments (caged versus uncaged). It also tested for interactions between site caging, caging ' species, site species and site* caging speaes.

Source of variation 1 df 1 MS 1 F 1 P

site 1 2 1 44.87 1 32.56 1 0.001

species 1 1 ( 0.65 1 0.47 1 0.50

- 1 1 1 1

'Note no block effect was detected. therefore it was pooled wilh error term

site ' cage

cage * species

site ' species

site ' cage * species

Table 2b: Results of the analysis of variance for predator index values. The analysis tested for the following diMerences: between species and site. It also tested for interactions between species ' site.

2

2

1

2

Source of variation df

species 1

site 2

species * site 2

12.61

69.25

0.99

0.62

MS

226.44 --- 2101.27

200.22

9.15

50.25

0.72

0.45

0.005

0.001

0 -40

0.64

F

1.456

13.44

1.28

P

0.245

0.005

0.306

after evaluation with Tukey's HSD in order to compare individual treatments.

At site 1 the mean shoot mass of both species (L, V) was significantly greater for fully

caged plants than for uncaged plants (Fig 2a). Target species in caged treatments had a

mean shoot mass of 76.19 (V) and 84-59 (L), whereas target species in uncaged

treatments had a rnean shoot mass of 4.7 g (L) and 6.3 g (V) (Fig. 2a). Values of the

predation index (PI) did not differ significantly between the iwo species (Fig. 2b).

At site 2 the mean shoot mass of both target species (L, V) was not significantly greater for

fully caged plants than for uncaged plants (Fig 2c). Target species in caged treatments had

a mean shoot mass of 14-19 (L) and 22-79 (V) and mean shoot mass of uncaged plants

was 10.49 (L) and 20.09 (V) (Fig. 2c). Consequently, values of the predation index (PI) did

not differ significantly between the two species (Fig. 2d).

At site 3 the mean shoot mass of both target species was not significantly greater for fully

caged plants than for uncaged plants (Fig 2e). The mean shoot mass of caged target

species was 2.09 (V) and 11 -69 (L) and mean shoot mass of uncaged plants 0.79 (V) and

10.79 (L) (Fig. 2e). Again, values of the predation index (PI) did not differ significantly

between the two species (Fig. 2f).

In summary, predation only reduced shoot mass significantly at one of the three sites (Le.

at site 1) where the two target species were equally affected.

Figure 2 (a,c,e): Mean (I 1 SE) shoot mass for Lythmm and Verbena in cleared plots with a full cage (+ C) or with no cage (- C) at three sites near Guelph, Ontario. Values (potential predator effects ) with the same letter do not differ significantly (Pr 0.05). Values were square root transformed + 0.1 prior ta analysis to meet assurnptions of ANOVA.

Figure 2 (b,d,f): Mean (I 1 SE) predation index values for Lythrum and Verbena at three sites near Guelph, Ontario. Values with the same letter do not differ significantly (Pz 0.05). Values were arcsine square root transformed prior to analysis to meet assumptions of ANOVA.

Compeüüon with predatorr excluded

Results of the ANOVA for potential cornpetition effects show that the main effects of site

and neighbour treatments (Le., neighbours present versus neighbours rernoved) were

significant, but the effect of species was not significant (Table 3a). The interactions

between site ' neighbours and neighbours species were also significant, indicating that

the effect of neighbours differed arnong sites and between species. The interaction

between site ' species was also significant indicating that there was a difference between

species across sites. The interaction between site * neighbours species was not

significant. Results of the ANOVA for cornpetition index values show that there were

significant d-fierences among sites and between species (Table 3b). The interaction

between species site also was significant. Since several of the interactions were

significant, the results are presented in figure 3 (a,b, c, d, e and f) after evaluation with

Tukey's HSD in order to compare individual treatments.

At site 1 the rnean shoot mass of target species was significantly greater with neighbours

rernoved than with neighbours left intact (Fig 3a). Target species with neighbours removed

had a mean shoot mass of 76.19 (V) and 84.5g (L), whereas target species with

neighbours intact had a mean shoot mass of 0.2 g (L) and 0.5g (V) (Fig. 3a). Values of the

cornpetition index (CI) did not differ significantly between the two species (Fig. 3b).

At site 2 the mean shoot mass of target species was significantly greater with neighbours

rernoved than with neighbours left intact (Fig. 3c). Target species with neighbours removed

had a mean shoot mass of 14.1 g (L) and 22-79 (V), whereas target species with neighbours

intact had a rnean shoot mass of 0.6 (L) and 1.6 (V) (Fig. 3c). Values of the cornpetition

3 O

Table 3a: Results of the analysis of variance on the potential effect of cornpetition with target species in a full cage. The analysis tested for the following diirences: between sites, species and neighbour treatments (neighbours intact versus neighbours removed). It also tested for interactions between site ' neighbours, neighbours species, site species site* neighbours species.

I source of variation

l site s pecies

site neighbours

neig h bours species

1 site species

1 site neig hbours * species 'Note no block eifect was detected, then

1 ::3: 1 8.49

11 5 7

0.59 2.12 ore it was pooled with emr terni

Table 3b: Results of the analysis of variance for cornpetition index values. The analysis tested for the following differences: between species and site. It also tested for interactions between species site.

Source of variation F

species I I

species * site 1 2 1 97.54 4.88

index (CI) did not difFer significantly between the two species (Fig. 3d).

At site 3 the mean shoot mass of L was significantly greater with neighbours removed than

with neighbours left intact (Fig 3e). L species with neighbours removed had a mean shoot

mass of 11 -69, whereas with neighbours intact had a mean shoot mass of 0.89 (Fig. 3e).

In contrast, the mean shoot mass of Vdid not differ significantly with neighbours removed

(2.09) or when neighbours were left intact (0.69) (Fig. 3e). Values of the cornpetition index

(CI) did differ significantly between species (Fig. 3f), with L having a significantly higher CI

value than VI indicating that L was affected more than V by adult neighbours.

In summary, competition from Phalaris reduced shoot mass significantly at sites 1 and 2,

where the two target species were equally affected. At site 3, competition from Phalaris

reduced L's mean shoot mass significantly whereas V's mean shoot mass was not

significantly different when Phalaris neighbours were removed or left intact. As a result, L

had a higher CI than VI indicating that L was affected more than V by adult neighbours.

Combined effect of competition and predation

Results of the ANOVA for the combined effect of competition and predation show that the

main effects of site and treatrnent (Le., caged target species with neighbours removed

versus uncaged target species with neighbours present) were significant, but species was

not significant (Table 4a). The interactions between site treatment and treatment *

species were significant, indicating that the effect of treatment differed among sites and

among species. Interactions between site species and site treatment species were

Figure 3 (a,c,e): Mean (î 1 SE) shoot mass for Lythnrm and Verbena in fully caged plots with neighbours removed (- N) or with neighbours present (+N) at three sites near Guelph, Ontario. Values (for potential cornpetition effects) with the same letter do not differ significantly (P 2 0.05). To meet assumptions of ANOVA, values were log, + 0.1 transformed prior to analysis.

Figure 3 (b,d,f): Mean (I 1 SE) campetition index values for Lyfhrum and Verbena at three sites near Guelph, Ontario. Cornpetition index values with the same letter do not differ significantly (P r 0.05). To meet assumptions of ANOVA, values were arcsine square root transformed.

also significant, indicating that there was a difference in species across the three sites for

each treatment. Results of the ANOVA for predation plus cornpetition index values show

that there were significant difkrences between species and among sitesgable 4b). The

interaction between species ' site also was signifiant. Since several of the interactions

were signifiant, the results are presented in figure 4 (a,b, c. d, e and 9 after evaluation with

Tukey's HSD in order to compare individual treatments.

At site 1, the mean shoot mass of target species was significantly greater with neighbours

removed and plants fully caged than for plants with neighbours Ieft intact and not caged

(Fig. 4a). Target species wlh neighbours removed and plants fully caged had a mean shoot

mass of 76-19 (V) and 84-59 (L), whereas target species with neighbours intact and plants

not caged had a mean shoot mass of 0.09 (L) and O.@ (V) (Fig- 4a). Values of the

competition plus predation index (CPI) did not differ significantly between the two species

(Fig. 4b).

At site 2, the rnean shoot mass of target species was significantly greater with neighbours

removed and plants fully caged (Fig. 4c). Target species with neighbours removed and

plant fully caged had mean shoot mass of 14.19 (L) and 22-39 (Ir), whereas target species

with neighbours intact and plants not caged had a mean shoot mass of 0.49 (L) and 1.59

(V) (Fig. 4c). Values of the competition plus predation index (CPI) did not differ significantly

between the two species (Figdd).

At site 3, the mean shoot mass of target species was significantly greater with neighbours

removed and plant fully caged (Fig. 4e). Target species with neighbours removed and

3 5

Table 4a: Results of the analysis of variance on the combined effect of predation and cornpetition. The analysis tested for the following differences: between sites, species and treatments (caged target species with neighbours removed versus uncaged target species with neighbours intact). It also tested for interactions between site species, site treatments, treatrnents species, and site * treatments species.

Source of variation 1 df 1 MS 1 F 1 P

site 1 2 1 2.98 1 16.96 1 0.001

species 1

site * treatment

treatment

site * species

0.03

Table 4b: Results of the analysis of variance for predation plus cornpetition index values. The analysis tested for the following differences: between species and site. It also tested for interactions between species ' site.

1

2

treatment * species

site treatment species

0.15 0.7

215.1

2.88

'Note no block effect was detected, therefore it was pooled with error terni

1

2

Source of variation

species

site

species site

1224.62

16.41

2.96

1 -1 8

df

1

2

2

0.001

0.001

16.88

6.73

MS

358.98

849.66

142.73

0.002

0.008

F

17.73

41.96

7.05

P

0.004

0.001

0.005

Figure 4 (a,c,e): Mean (I 1 SE) shoot mass of Lythrum and Verbena in control plots with neighbours present (+ N) and no cage (-C) versus in plots with neighbours removed (- N) and plots fully caged (+C) at three sites near Guelph, Ontario. Values (for competition plus predation) with the same letter do not differ significantly (P a 0.05). Prior to analysis, values were log, +0.1 transformed to meet assumptions of ANOVA.

Figure 4 (b,d,f): Mean (I 1 SE) index value for the effect of competition plus predation of the shoot mass on Lythrum and Verbena at three sites near Guelph. Ontario. Values with the same letter do not differ significantly (P z 0.05). Pnor to analysis, values were arcsine square mot transfomed to meet assumptions of ANOVA.

plants fully caged (mg. 4e). Target species with neighbours removed and plants fully caged

had a mean shoot mass of 2-09 (V) and 11.6g (L), whereas target species with

neighbours intact and plants not caged had a mean shoot mass of 0.49 (L) and 0.69 (V)

(Fig. 4e). Values of the competition plus predation index (CPI) were significantly higher for

L than for V, indicating that L was affected more than V by the combined effect of

competition and predation (Fig.49.

In summary, combined effed of competition and predation reduced shoot mass significantly

at al1 three sites. Values of the CPI did not dHer significantly between species at both sites

1 and 2, but did differ signifmntly at site 3, where L had a significantly greater CPI value

than V, indicating that L was affected more than V by the combined effect of competition

and predation.

Effect of experimental treatments on abiotic resources

Soi1 variables

Substrate conditions did not differ significantly among plots with vegetation present versus

vegetation removed at any of the three sites (Table 5). However, substrate conditions did

differ significantly across the three sites for al1 but one of the six substrate factors listed in

Table 5. Vegetated plots, at site 1 (9mgkg) had a significantly higher amount of ammonia

(NH,-N) than at site 2 (4 mglkg). Cleared plots, at site 1 (19 mglkg) had a significantly

higher amount of ammonia than sites 2 (5 mglkg) and 3 (6 mglkg). In the vegetated

treatment, site 1 (22 mgkg) had a significantly higher amount of nitrates (NO,-N) than site

2 (12 mglkg). Phosphorous (P) ranged from 11 to 16 mglkg at the three sites and inter-site

difference was not statistically significant. Magnesium (Mg) was significantly higher, at

39

Table 5: Cornparison of mean (I ISE) values of six substrate factors with vegetation present (+ veg) and vegetation absent (- veg) at three study sites near Guelph, Ontario. values with the same l&er do not differ significantly (P; 0.05).

Siibstrate Factor

NH,-N (mglkg)

NO,-N (mg/kg)

Site 1 + veg

Site 2 + veg

Site 2 - veg Site 3 ( * v g

Site 3 - veg

sites 1 and 2, 660 mglkg and 648 mgkg respectively, than it was at site 3, where it was

only 328 mg/@. Potassium (K) at site 2 (88mgjkg ) was significantly higher than at site 3

(56 mgkg), with the values at site 1 (74 mglkg) k i n g intemiediate. Lastly, across the three

sites pH ranged from a high of 7.4 at site 3 to a low of 6.2 at site 1 and diiered significantly

across the three sites.

PAR

At each of the three sites, both L and Vin plots where neighboun were removed received

a significantly higher percentage of incoming PAR than plants in plots where neighboun

were left intact (Table 6). Plants in plots where neighbours were removed received from

63 - 99% of incoming PAR, while plants in vegetated plots received only 249% of incoming

PAR. In vegetated plots, there was no significant difference in the percentage of incoming

PAR between species, with and wiaiout cages. Plants in caged plots with vegetation

removed, received 16 - 32% less PAR than plants without cages.

Relationship befween PAR and mean shoot mass

At site 1, mean shoot mass increased as PAR increased for both L and V. Variation in PAR

among treatments accounted for 93% of the variation in mean shoot mass for L (R2 =

0.9324) and 93% of the variation in mean shoot mass for V(f? =0.9308). The analyses of

covariance indicated that the dope of the relationship between mean shoot mass and PAR

did not differ significantly for L and V-

At site 2, mean shoot mass increased as PAR increased for both L and V. Variation in PAR

among treatments accounted for 90% of the variation in mean shoot mass for L (R2 =

0.9045) and 87% of the variation in mean shoot mass for V (R2 =0.8731). The analyses of

covariance indicated that the dope of the relationship between mean shoot mass and PAR

did differ significantly for L and V (P = 0.0005), with V attaining a higher

mean shoot mass at the same PAR levels.

At site 3, mean shoot mass increased as PAR increased for L but not V. As a result,

there is no significant difference between the relationship of PAR versus mean shoot mass

for the two speües. Variation in PAR among treatments accounted for 83% of the variation

in mean shoot mass for L (R2 = 0.8390) and 16% of the variation in mean plant biomass for

V (RZ =0.1642). The analyses of covariance indicated that the slope of the relationship

between rnean shoot mass and PAR did differ significantly for L and V (P = 0.0008). with

V attaining a higher mean shoot mass at the same PAR levels.

DISCUSSION

My results were not consistent with the hypothesis that the introduced species L was less

affected by potential predators and competition from adult neighbours than the

morphologically similar native species, V. At al1 three sites, I found no ditference between

L and V when examining the potential effects of predation, cornpetition and the combined

effect of predation plus competition except at one site where competition from neighbours

had a greater effect on the introduced species L than on the native species V. Since my

results depended on &O# the site and the effect examined (Le., predation and cornpetition),

I will discuss the effects of the three treatments (Le., predation, cornpetition, and predation

pius competition) for each site in tum. In each section, 1 first discuss rny results then I

cornpare my findings to those of previous studies involving L and other pairs of native and

introd uced s pecies.

Predation with neighbours removed

Potential predator effects on target species were measured by comparing the shoot mass

of target species in a full cage versus no cage in the absence of neighbours. Caged target

species had a significantly greater mean shoot mass than uncaged target species, at site

1, indicating that potential predation reduced the growth of uncaged target species

significantly. Weekly monitoring of target species revealed that uncaged target species

experienced predation from both mammals (i.e., stems were cut on an angle or clipped off

at ground level) and insects (Le., holes in leaves). Mammals such as deer and voles were

most likely the vertebrate predators and insects such as grasshoppers were likely the most

important invertebrate predators involved (for a list of insect species refer to Appendix 4).

Less potential predation on fully caged target species than on uncaged plants probably

accounted for the difference in their shoot mass. Predation indices showed that L and V

were both equally affected by predators. At site 2, caged target species did not have a

significantly greater mean shoot mass than uncaged target species, indicating that potential

predation did not have a significant effect on the growth of uncaged target species. Since

target species were exposed ta potential predators for only 12 weeks at site 2 compared

to 17 weeks at site 1, target species at site 2 may not have been exposed to predation long

enough to have detected a significant effect on mean transplant shoot mass. Both L and

V had low predation indices, indicating that predators did not reduce their shoot mass

significantly. At site 3, caged target species did not have a significantly greater mean shoot

mass than uncaged target species, indicating that potential predation did not have a

significant effect on the growth of uncaged target species at this site. Fewer insect

predators (particularly grasshoppers), were seen at site 3 than at site 1, which is consistent

with fewer plants k i n g damaged. Another possible reason why no difference was detected

between caged and uncaged target species could have been due to the slower growth of

target species at site 3 than at site 1. Even though target species at site 3 were planted at

the same tirne as site 1, target species at site 3 were much smaller (Le., caged target

species at site 3 were less than 209 compared to caged target species at site 1 which were

greater than 80g). The smaller size of caged target species at site 3 made it more difficult

to detect any potential difference between caging treatments. One reason for lower mean

shoot mass of caged target species at site 3 could be due to greater below-ground

cornpetition from intact Phalah roots. Below-ground portions of Phalaris were not removed

at site 3 (since plots were hand clipped) while at site 1, herbicide killed shoots as well as

roots. Both L and V had low predation indices, indicating that predators did not reduce their

45

shoot mass significantly.

In summary, potential predation either significantly reduced L and Vs mean shoot mass

equally (site 1) or had no effect on L and Vs mean shoot mass (sites 2 and 3). At site 1,

predation indices show that potential predators reduced plant biomass of both L and V

equally. As a result, the hypothesis that L was less effected by potential predation than the

morphologically similar native species V was rejected. At site 2, predation indices showed

that L and V were effected similarly by potential predators and as a resuit the hypothesis

that L was less effected by potential predation than V was rejected. Even though at site 3

L's mean shoot mass in uncaged plots was significantly higher than Vs, predation indices

showed that potential predators effected both plants similarly. As a result the hypothesis

that L was less effected by potential predation than V was rejected for aIl three sites.

Effect of predation on introduced versus native species

To my knowledge, no previous studies have examined the effect of predators on the native

species V, whereas previous researchers have found that predators (both mammals and

insects) reduce L's shoot mass (Rawinski and Malecki 1984; Anderson 1995). Each of

these studies reported that L was consumed by mammalian predators. Rawinski and

Malecki (1 984) reported that L shoots grazed by white-tailed deer (Odocoileus virginianus)

were shorter and weighed less than shoots in a nearby enclosure. Anderson (1995) found

that 7% of the L population on the floodplain of the Ipswich River had been clipped by

mammals. He speculated that deer were the culprits based on abundant deer tracks and

droppings. Anderson (1995) also observed that rabbits (most likely Syhilagus fransaionalis)

had clipped L stems. One of these studies also reported insect predation on L. Anderson

(1995) observed 30 randomly selected L plants and found that they al1 had extensive fruit

damage by Popollia japonica.

While both of these studies provide evidence that the introduced wetland plant L is

consumed by predators, only one study attempted to compare predator effects on L and

a native species. ln a three year study, Rawinski and Malecki (1 984) examined the amount

of predation damage done by muskrats (Ondatra zibethicus) on introduced L and a

rnorphotogically dissimilar native Typha spp. (cattails, hereafter referred to as Typha) in

enclosures and open plots, both containing a mixture of each species at three sites. They

found that on one out of nine sampling dates Typha's average mid-summer density was

significantly lower in uncaged plots than in caged plots and on two out of nine sampling

dates they found that L's density increased in the open plots while Typha's density

decreased.

While these results suggest that predators reduced the density of the native species

(Typha) more than the introduced species (L), their experimental design did not examine

the potential effect of predation alone, since neighbouring plants were left intact.

Consequentiy, it is difficuIt to interpret the potential effect of predation alone on the target

species in their experiment.

Only a few other studies (Schierenbeck et al. 1994; Trowbridge 1995) have compared the

effect of predators on introduced species versus native species and both studies provide

evidence that introduced speaes and morphologically similar native species are equally

effected by predators. Schierenbeck ef al. (1994) compared the effect of predation on the

growth and biomass allocation patterns of the introduced Lonicera japonica and the native

congener Lonicera sempervirens. More specifically, they compared the percentage of

above-ground leaf loss between the two species and among three treatments (i-e., no

herbivory, insect herbivory and mammal and insect herbivory). Schierenbeck et a/. (1 994)

claimed that in the insect herbivory treatment the native species experienced more damage

that the introduced species at seven out of nine harvest dates which is consistent with a the

hypothesis proposed by DaMn (1 859)and others since then (Batra et al. 1986; Thompson

et al. 1987; Hight 1990; Hight and Drea 1991; Malecki et al. 1991; Blossey and Notzhold

1995; Trowbridge i 995; Edwards et al. 1 995) that introduced species are less affected by

predators and are more cornpetitive than native species. Yet, their results showed that the

introduced species was not less affected by predators than the native species because the

native had signifiant leaf loss due to insect herbivory on only one out of nine harvest dates,

not seven out of nine harvest dates. Similar to my results, they also found that an

introduced species and a morphologically similar native species were equally affected by

predation on most sampling dates. ln the mammal and insect herbivory treatment the

introduced species was less affected by predators beause the native species experienced

significantly more herbivory in four out of nine harvest dates, which is consistent with the

hypothesis that introduced species are less affected by predators than native species.

These results show that it is possible that an introduced species is less affected by

predation than a morphologically similar native species, but significant differences were not

ahvays evident. However, their predation estimates were probably underestimated because

damage from predation was measured as a percentage of leaf area eaten, but leaves

48

totally removed were not included in the estimate. As a result, if the top of the plant was

eaten. it would not have been included in their mammal and herbivory predation estimates.

A more appropriate way to measure potential predation effects would be to harvest the

above-ground shoot mass at the end of a growing season before leaves begin to senesce.

Therefore. their results for mammal and insect herbivory are difficult to interpret.

Trowbridge (1995) examined whether six common predators ate the introduced alga.

Codium fragiie ssp. tomentosoides as much as the native congener Codium convolutum.

In a pairwise laboratory feeding study, she found that out of the six species tested, one

species preferred the native species, another species showed no preference. a third

species ate neither alga and two predaton ate significantly more of the introduced species

than the native species. These resuits suggest that the effed of predaton on introduced

versus native algae, may depend on the species of predaton involved. In addition. none

of these predators were successful in controlling the spread of this introduced alga in New

Zealand, since it appeared that the alga had a high intertidal refuge from most of these

grarers in the field.

In summary. results from previous studies provide some evidence that introduced species

are less affected by predation than morphologically similar species, whereas rny results

show that introduœd L and native V are equally effected by potential predators. It appears

that native predators have the potential to affect introduced species as well as

morphologically similar native species. Research from previous studies shows that native

herbivores (both mammals and insects) not only eat introduced plants but in some cases

control their invasion (Strong et al. 1984; D'Antonio 1993; Creed and Sheldon 1995;

Sheldon and Creed 1995). Strong et al. (1984) found that native insects quickIy located

most species of introduced plants. They suggested that the rate of recruitment of insects

ont0 a new plant is positively correlated with the distribution of the plant and the taxonornic,

phenological, biochemical and morphological match between the introduœd plant and a

morphologically similar native species. D'Antonio (1 993) found that herbivory was a major

cause of mortality for an introduced species transplanted into three habitats. In two other

studies, Creed and Sheldon (1 995) and Sheldon and Creed (1 995) present evidence that

a native insect rnay have been responsible for producing the decline of an introduced

aquatic plant. These studies indicate that generalist predators have the potential to reduce

the shoot mass of introduced species. As a result, these introduced species would also

have to invest sorne energy for defence against predators, as do native plant species.

L versus V - Why are fhey egually affecfed by potenfial preâators?

Numerous researchers have claimed that L is less affected by predators than many native

species (Batra et al. 1986; Thompson et al. 1987; Hight 1990; Hight and Drea 1991;

Malecki et al. 1991 ; Blossey and Notzhold 1995; Trowbridge 1995; Edwards et ai. 1995),

yet I found that introduced L and a morphologically similar native species, V were equally

affected by potential predators. We know that there are some fundamental differences

between these two species since they belong to different famiiies and different genera. So

why did this introduced species, L and this morphologically similar native species, V

respond the same way to the potential effect of predators, especially since it has been

assumed that L is free of any native predators? In my opinion, the main reason that L and

V responded similarly to the potential effed of predation is a result of numerous generalist

predators including both vertebrates (Le., deer) and invertebrates (Le., grasshoppers) that

consumed both species. As previously stated, numerous researchers have found that

generalist herbivores not only eat introduced plant species, but also control their distribution

(Strong et al. 1984; D'Antonio 1993; Creed and Sheldon 1995; Sheldon and Creed 1995).

Another possible explanation is that young individuals of vertebrates and invertebrates may

be sampling a variety of different 'new' food sources which would result in both species

being eaten, since they were equally rare compared to the Phalaris.

Competition with predators excIuded

Potential competition effects on target species were measured by comparing the shoot

mass of target species with neighbours removed versus left intact in the absence of

predators. Target species with neighbours removed had a significantly greater mean shoot

mass than target species with neighbours present, at site 1, indicating that potential

competition reduced shoot rnass of both L and V. One explanation for increased plant

growth in removal plots is increased nutrient availability from dead or dying roots that were

not removed during herbicide application and raking. Since soi1 anaiysis before and after

herbicide application, showed that there was no significant increase in soi1 nutrients in the

removal plots, increased nutrient availability can not account for the increased plant growth.

A more likely explanation for increased growth in removal plots is an increase in PAR

availability. A strong relationship existed between PAR and mean shoot mass for both L

and V; as PAR decreased so did mean shoot mass. Since neighbour removal increases

the amount of PAR, an increase in PAR due to neighbour removal probably accounted for

the increase in mean shoot mass in the removal plots. Competition indices showed that

L and V were both equally affected by Phalaris neighbours. Target species with neighbours

removed had a significantly greater mean shoot mass than target plants with neighbours

present at site 2. Again, a linear relationship existed between PAR and mean shoot mass,

indicating that neighbours shaded both target species resulting in reduced plant growth.

V attained higher mean shoot mass values than L did at the same PAR levels, because

initially, V grows faster than L (greenhouse experiment relative growth rate). Even though

target species responded to the neighbour removal treatment, they did not attain the high

shoot mass values that target species did in removal plots at site 1 (Le., L and Vtarget

species in removal plots at site 2 were 149 and 229 cornpared to 84g and 769 at site 1).

Target species at site 2 were planted 5 weeks later than target species at site 1 which

would have attributed to their lower mean shoot mass. Campetition indices show that both

L and V were equally affected by the presence of Phalaris neighbours. Neighbour rernoval

had a significant effect on L but not on Vat site 3. L target species with neighbours

removed had a significantly greater mean shoot mass than L target species with

neighbours present. Again, a strong relationship between PAR and mean shoot mass

existed, indicating that when neighbours were present plants were shaded resulting in

reduced plant growth. Even though L plants responded to the treatment, they did not attain

the high biomass values that L plants did at site 1 (Le., target species in rernoval plots at

site 3 were 1 l g cornpared to 849 at site 1). Site 3 did not have Phalaris roots removed, and

as a result. below-ground cornpetition could have reduced L's above-ground shoot mass

in cleared plots. V target species seems to be more sensitive to below-ground cornpetition

than L target species because Vs mean shoot rnass did not increase when shoots of

neighbours were rernoved. Also, there was not a strong relationship between PAR and

mean shoot mass. In removal plots at site 3, Vs mean shoot mass (2.09) was significantly

less than it was at both sites 1 and 2 (76.lg and 22.79, respectively). Despite the fact thaf

V plants at site 3 were planted a month and a half earlier than at site 2. they were smaller

at site 3. Again, this difference could be attnbuted to greater below-ground competition

from intact Phalaris roots at site 3. Campetition indices show that 1's rnean shoot mass

was significantly reduced by Phalaris neighbours, whereas Vs mean shoot mass was

restricted by some other biotic andlor abiotic factor.

In summary, potential cornpetition significantly reduced L and Vs mean shoot mass equally

at sites 1 and 2, while at site 3 potential competition reduced the mean shoot mass of L

more than V. At sites 1 and 2, competition indices showed that the presence of Phalaris

neighbours significantly reduced both L and Vs mean shoot mass equally. therefore, the

hypothesis that L is more competitive than V was rejected. At site 3, L was more effected

by Phalaris neighbours than V and as a resuit, the hypothesis L was more competitive than

V was rejected for al1 three sites.

Effect of cornpetilion on infroduced versus native species

In an outdoor compound, Gaudet and Keddy (1 988, 1995) examined the relative

competitive ability of 44 herbaceous wetland plants (neighboun) to suppress the growth

of a phytometer ( L y I h ~ m salicana). Plants were germinated simultaneously from seed and

grown as species pairs in 1-litre pots of sterile, organic. high-nutrient mix. when plants were

one month old. They found that Phalaris amndinacea suppressed L by 89% under these

laboratory conditions. In my field study Phalaris amndinacea suppressed the shoot mass

of L by 99.8%, 95.7% and 93% at sites 1, 2 and 3, respectively. Since Phalaris was an

adult neighbour in this study rather than a seedling neighbour, it probably created a more

corn petitive environment than seedling neighbours.

In another study, Johansson and Keddy (1 991) examined the competitive ability of both L

and Vwhen grown from seed with and without neighbours. Neighbours included L and V

as weIl as Mimulus nngens, Cypems rivuiaris, Eleocharis obtusa and Juncus bufonius.

They found that L was more competitive (Le. not suppressed by neighbours) than V. In

Johansson and Keddy's (1991) study target species and neighbours were the same size

initially (Le.. seedlings one month old), whereas in my study neighbours were adults which

would create a more competitive environment. My results show that adult neighbours

suppress young plants of both L and V equally. While, Johansson and Keddy (1 991) show

that L is a better competiior than V when neighbours are young, conditions more typical of

wetlands in a drawdown stage, rather than in intact stands of adult vegetation.

No other field studies have compared the competitive ability of L and V, but Mal et al.

(1997) compared the competitive ability of L and a morphologically dissimilar native

species, Typha angustifolia L. with a modified replacement series experiment having four

starting densities (Le., 64, 36, 16 and 1) and four relative proportions (Le., 25, 50, 75 and

100) of each species. In their four year field experiment, they found that durhg the first

year, T. augustifolia had a greater overall rate of ramet production (OPR). However, by the

fourth year, L appeared to be outcompeting T. augustifolia. This study was more realistic

than the other two cornpetition studies because it was done in the field. However, when

cornpetition experiments are done in the field, the plants should be protected from

herbivores. If they are not, as in the case of this experiment by Mal et al. (1997), how does

one know if they are measuring the effects of competition andior predation?

No other field studies have compared the competitive ability of L and V, but other field

studies have compared competition effects using other pairs of introduced and natnre

species (Harrington et al. 1989 a,b; Schierenbeck and Marshall 1993). Similar to my resuls

for L and VI these studies also found that introduced species were not more competitive

than native species. Schierenbeck and Marshall (1993) compared the competitive ability

of Lonicera japonica (introduced) and Lonicera sempervirens (native), using monthly diurnal

measurements of leaf photosynthesis, conductance, and transpiration for a year in a high

and low light environment. They found little diierence between the two species, with one

exception, that in the introduced species new leaves had signifkantly higher photosynthetic

rate than new leaves of the native species on two out of twelve sampling dates (August and

January) in the closed canopy and on one out of twelve sampling dates (January] in the

open canopy. Since the introduced species retained photosynthetically active leaves for

a longer period than the native species, they suggested that a greater annual carbon gain

for the introduced species may provide it with a competitive advantage. Yet, there was only

a significant difference in photosynthetic rates dumg the middle of the growing season

(January) which muid mean that at that time the introduced species had an advantage, but

by the end of the experiment there was no difference between native and introduced

species. These results provide weak support for the hypothesis that L is more cornpetitive

than V as they also found that there was no diierence between L japonica (an introduced

species) and L. sempervirens (a natives species), even though they claim that this

difference in photosynthetic rate, which occurred on one measuring date, gave the

introduced a cornpetitive advantage. It is difficuk to apply such small scale physiological

measurements to the larger picture of invasibility of introduced species. Even though they

claim that their results support the hypothesis that introduced species are more cornpetitive

than natives, I find these resuits to be more consistent with results from my study, as I also

found that by the end of the growing season both species were equally affected by potential

competition.

In a similar field study, Ha~ngton et al. (1989a) compared leaf photosynthetic rate of two

introduced shrubs (Rhamnus cathamca and Lonicera X bella) and two native shnibs

(Cornus racernosa and Prunus semtina) in understory and open habitats on 24 sampling

days between April and Novernber. When examining leaf properties and photosynthetic

characteristics they either found that there was no difference between native species and

introduced species or that there was as much difference between the two introduced

species as there was between native species and introduced species. For example, when

the species were ranked based on mean daily maximum photosynthesis, the two native

species were ranked at the high and low ends of the scale with the two introduced species

somewhere in the middle. In a cornpanion study, Hanington et al. (1989b) compared the

leaf growth rates between the same two introduced shrubs and the same two native shnibs

in open and understory habitats. Again, they found that there was either no differences

between native species and introduced species or there was as gluch difference between

the two introduced species as there was between native species and introduced species

when comparing wood production per unit Ieaf area, leaf area per unit wood biomass, leaf

mass per unit wood biomass, and annual net carbon gain. Therefore, both of these studies

are consistent with my study, as I also found that by the end of the grbwing season both

L and V were equally effected by potential competition.

In summary, the few field studies that have attempted to compare competition effGcts on

native and intrduced species have detected little difference either based on small scale

physiological measurements of growth (Harrington et al. 1989a. b; Schierenbeck and

Marshall 1993) or large sa le rneasurements of shoot growth (my study). The results of

these field studies provide weak support for the hypothesis that introduced species was

less affected by potential competition than morpholagically similar native species.

L and V - Why were they equally affected by the potential emct o f competition?

Numerous researchers have claimed that L is more cornpetitive than many native species,

including V (Stuckey 1980; Thompson et al. 1987; Gaudet and Keddy 1988, 1995; Hight

1990; Hight and Drea 1991; Johansson and Keddy 1991; Malecki et al. 1991; Mal et al.

1992; Mal et. al, 1997). whereas I found that L and V were either equally effected by

potential competition or that L was effected more than V. Aside from the fact that these two

species do have some fundamental differences, 1 feel that they responded similarly ta the

potential effect of competition frorn neighbours because they were competing with adult

neighbours, where competition is said to be more intense, rather than with seedling

neighbours, where competition is said to be less intense due to ô !%ver amount of plant

biomass (Grime 1979; Oksanen et al. 1981 ; Wilson and Keddy 1986 a & b; Keddy 1990;

Shipiey et al. 1991; Wilson and Keddy 1991; Bonser 1994; Bonser and Reader 1995).

These few studies have dernonstrated that under drawdown conditions where neighbours

are srnall, and competition is less intense (due to lower arnounts of plant biomass), L is the

top cornpetitor (Gaudet and Keddy 1980,1995; Johansson and Keddy 1991 ; Mal et. al.

1997) but where neighbours are large and cornpetition is intense (due to higher plant

biornass), L and V were equally effected by potentiad cornpetition.

Combined emct of competition and predation

The cornbined effect of cornpetiiion and predation on target species of L and V was

rneasured by cornparing the shoot mass of target species in a full cage with neighbours

removed versus target species without a cage and neighbours intact. At sites 1 and 2,

cornpetition and predation indices showed that L and V were effected equally by

cornpetition and predation. Therefore, the hypothesis that L was lass effected by predation

and more cornpetitive than V was rejected. At site 3, cornpetition and predation indices

showed that L was affected by cornpetiiion and predation more than V and as a result, the

hypothesis that L is less effected by predation and more cornpetitive than V was rejected

for al1 three sites.

No other published studies that have examined the combined effect of cornpetition and

predation on native and introduced plant species sa it is difficult to judge whether my results

for L and V are typical of al1 pairs of introduced and native species.

Choosing species pairs

For this study I chose an introduced species, L and a rnorphologically similar native

species, V. Since these species are not congeners or counterparts, were they a good

species pair? I felt that they were a good species pair based on a nurnber of wrnrnon

5 8

characteristics including:

similar life form (Le-, tall, erect, main stems);

the potentiat to flower in their first year of growth;

they are perennial in that shoots emerge the following year from the root stock;

both are dicots;

flower from August to September;

produce many small seeds;

visited by bees;

found in similar wetland habitats; and

have spike inflorescence.

Study strengths and limitations

I consider the four major strengths of my study to be as follows:

1. In this study I compared the potential effect of predators and competition from adult

neighbours on an introduced and a native wetland species. In doing so, I was able

to answer the question 'whether introduced L was less effected by potential

predators or more competitive than native V . Other studies that have attempted to

examine this hypothesis for L and other introduced species but they have not

compared an introduced species to a morphologically similar native species and as

a result were unabte to determine whether a native species or an introduced

species are less effected by predators andfor more competitive (Rawinski and

Malecki 1 984; Gaudet and Keddy 1986,1995; Anderson 1995).

Another strength of this study was that I not only exarnined both predator effects

and cornpetition effects but this was the first study to examine the combined effect

of competition and predation. Past studies have attempted to examine predation

(Rawinski and Malecki 1984) or campetition (Gaudet and Keddy 1986,1995;

Johansson and Keddy 1991), but not both. Since both predator and neighbour

effects were documented as reasons for the successful invasion of L, I felt that it

was important to examine both of these traits (Thompson et al. 1987; Hight 1990;

Hight and Drea 1991 ; Malecki et al. 1991 ; Blossey and Nohhold 1995; Edwards et

a!. 1995).

3. The third strength of this study was that its experimental design allowed the potential

effeds of predation and competition to be evaluated both singly and together using

robust statistical tests. Other studies that have compared the effects of predators

andfor competitive ability of L to other native species have not controlled for

potentially confounding effects. For example, Rawinski and Malecki (1984)

examined the effeds of predators on L and Typha, yet their experiment was either

confounded by neighbours or not set up properly to test the combined effect of

predators and neighbours. In another study, Mal et al. (1997) examined the

competitive ability of L and Typha, yet their experimental results were confounded

by herbivore effects since their plots did not exclude predators.

4. Lastly. this study examined potential competitive ability of L in the field. Most other

competition studies have been set up in the greenhouse studies under optimal

conditions (Gaudet and Keddy 1986,lggS; Johansson and Keddy 1991). Result

60

fmm these studies are difficult to extrapolate to field conditions because similar

conditions may not be found in the field.

All studies have limitations and rny study is no exception. Some limitations of this study are

as follows:

1. Since this study only examined one species pair, L and V, (one introduced species

and one native species) which provides only one replicate of each species,

generalizations about other native species in comparison with L or other pairs of

native and introduced species could not be made (Le., pseudo replication). As a

result, I feel that addiional work is needed in order to determine whether my

results reflect general differences between introduced and native species. For

example, from my own personal field observations, I know that introduced L and

native V were both less affected by predators than the native species boneset

(Eupatorium perfoliatum L). Therefore, I would recommend that a minimum of five

species pairs be used to test whether introduced L is less effected by predators and

more cornpetitive than native species.

2. A second limitation of this study was that my results were site-dependent due to late

planting at site 2 and uncontrolled effects of below-ground cornpetition at site 3. Al1

sites should be treated equalIy to minimize sitedependence.

A third weakness of this study was that target species treated to eliminate al1

herbivory still experienced some herbivory, A few target species had their tops

chewed off by vertebrates (Le., likely deer) that were taller than the cage, while

other target species experienced herbivory (i-e., holes in leaves) from insect

herbivores (i-e., mainly grasshoppers). Lids on cages would have eliminated tall

vertebrates from reaching over the cages and topping off the plant, while a more

rigorous pesticide application may control grasshoppers. A fourth treatment which

consisted of fine mesh mveflng a cage was considered for this experiment, but was

not used because of a potential microclimate effect. Future studies may wish to use

a more complete caging technique.

4. A fourth weakness was that I was unable to use fecundity (Le., seed number) as a

measure of plant success in the three treatrnents, instead I weighed above-ground

shoot mass. I was unable to use seed number because, L has a heteromorphic

incompatibiltty system, which means that it produces three floral morphs that differ

in relative positioning of the stigma and anthers in the flowers. Full seed set is

achieved only if pollen is transferred between different morphs and from an anther

level that corresponds to the position of the receiving stigma. At my sites there was

a maximum of 30 L plants, none of which set seed. Agren (1996) found that

population size was positively correlated with number of fruits per shoot and with

total seed production per plant. As a result it would be difficult to use seed number

as a measure of plant success.

CONCLUSIONS

1. I reject the hypothesis that the introduced species Lythmm was less effected by

potential predators than a morphologically similar native species. Verbena because

predators either significantly reduced the shoot mass of Lythrum and Verbena

equally (site 1 ) or predators did not effect their shoot mass significantly (sites 2 and

3)-

2. My results show that L and V were equally effected by predators, which is

consistent with results of previous studies (Rawiniski and Malecki 1984;

Schierenbeck et al. 1994; Trowbridge 1995) that used other pairs of introduced and

native species. Therefore, 1 conclude that native species may not necessarily

experience greater predation han introduced species.

3. 1 reject the hypothesis that the introduced species Lythrum was less effected by

potential competition than the morphologically similar native species Verbena,

because competition frorn neighbours either significantly reduced the shoot mass

of Lythrum and Verbena (sites 1 and 2) or neighbours reduced Lythnrm's shoot

mass, but not Verbena's (site 3).

4. My results showed no evidence of greater competitive ability of the introduced

Lythrum compared to the native Verbena which was somewhat consistent with

results of previous field studies that compared the competitive ability of other pairs

of native and introduced species. Therefore, I conclude that cornpetition from intact

neighbouring plants may affect introduced species and native species equally.

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Appendix i

Table 7: Results of the analysis of variance on the potential effect of shading for L target species. The analysis tested for the following differences: between sites, cage (i-e., half cage versus no cage L plants) and neighbours (neighbours intact versus neighbours removed). It also tested for interactions between neighbours * cage, site cage, site neighbours, and site neighbours cage.

Source of Variation I df I YS I F I P - -

site

site cage 12 1 1.43 1 0.90 1 0.42

neighbours

neighbours cage

site ' neighbours 12 1 0.03 1 0.02 1 0.98

2

1

1

Main effects of site and neighbours were significant, while caging was not significant. None

of the interactions were significant. Since, there are no significant differences between no

cage and half cage treatments, values for the no cage treatrnents were used in the analysis

(Table 9)(Appendix 1 continued next page).

1 6.64

site neighbours cage

73.5

0.14

10.48

'Note no block effect was detected, therefore it was pooled with error terni 2

0.002

46.26

0.09

0.001

0.76

1 .1 0.69 0.51

Appendix 1 (continued)

Table 8: Results of the analysis of variance on the potential effect of shading for V target species. The analysis tested for the following differences: behnreen sites, cage (Le., half cage versus no cage V plants) and neighbours (neighbours intact venus neighbours removed). It also tested for interactions between neighbours cage, site cage, site neighbours, and site ' neighbours cage.

site 12 1 733.36 1 24.95 1 0.001

Source of Variation 1 df 1 MS

cage 11 1 45.47 1 1.55 1 0.22

F P

neighbours

site cage 12 1 15.86 1 0.54 1 0.59

neighbours ' cage

site neighbaun 12 1 625.68 1 21.28 1 0.001

1

1 1 59.97 1 2.04 1 0.16

site neighbours cage 12 1 22.45 1 0.76 1 0.47 Note no block effect was detected. therefore it was pooled with error tem

1 1 1 1

Main effects of site and neighbours were significant, while caging was not significant. The

1535.31

interactions between site neighbours was also significant, while neighbours ' cage, site

cage, and site * neighbours cage were not significant. Since, there are no significant

52.23

differences between no cage and half cage treatments, values for cage treatments were

0.001

used in the analysis (Table 9) (Appendix 1 continued next page).

Appendix 1 (continued)

There was no significant difference in mean shoot mass between no cage (NC) and half cage (HC) treatments for L or V transplants in

vegetated plots and cleared plots at all sites(Tabte 9).

Table 9: Mean ( I 1 SE) values of shoot mass (g/plant) of Lythmm and Verbena behiveen treatments with vegetation present (+veg) and absent (- veg) and with no cage (NC) and half cage (HC) at three sites near Guelph, Ontario.

Site

Site 1

Site 2

Site 3

L

+ Veg NC

O

0.4

0.4

+ Veg HC

O

0.7

0.3

Appendix 2

Methods

Soi1 samples were collected on May 28, 1996 (pnor to herbicide application) and on June

26, 1995 (after herbicide application) in five vegetated and five unvegetated plots to test

whether experimental removal of vegetation affected the availability of the following six soi1

variables: ammonium, nitrates, phosphorous, magnesium, potassium, and pH (NHhN, NO,-

N, P. Mg. K and pH respectively) (see page 16 for methods). To test whether percentage

values differed significantly (Pc 0.05) among soi1 variables and timing, a two-way ANOVA

for a randomized block expriment was used, followed by Tukey's HSD test. Only data for

NHhN needed to be transformed log, + 1 prior to analyses to meet assumptions of ANOVA.

Results

After herbicide application there was no significant difference between vegetated (+veg)

and unvegetated (-veg) plots for 5 of the 6 soi1 variables (Table 10). Removing vegetation

caused an increase in nitrates (N03-N) ftom 0.6 to 0.9 rngikg, which was only slightly

greater than the difference between vegetated and unvegetated plots before herbicide

application (Le., 1.0 vs. 0.8 mgkg) (Appendix 2 continued next page).

Appendix 2 (continued):

Table 10: Comparison of mean (I 1SE) values of six soi1 variables in plots with vegetation (+veg) or without vegetation (-veg) before and after herbicide applicatioi site 1. Values with the same letter do not differ significantly (P r 0.05).

Soil Variable

NH, - N (mglkg)

NO, - N (mglkg)

P (mgll)

Before Herbicide

May 28196

45 & 3

1.0 I 0.4O

13.6 f 1.8

+ Veg

After Herbicide

June 20196

- Veg + Veg

6 2 i 8 '

0.8 I 0.2

12.6 f 1.3-

- Veg

26i6%

0.6 k 0.1 A

8.6 f 1 . 3 ~ ~

25kSB

0.9 I 0.2'

6.6 f 1.5AB

Appendix 3

Meaiods

Data for initial transplant height was analyzed using a three-way randomized block

ANOVA. To meet assumptions of ANOVA, data for L was log, + 0.1 transformed. Main

effects were site, cage (Le., caged versus uncaged) and neighbours (Le., neighbours

intact versus removed), al1 of which were fixed effects. Interactions included neighbours

cage, site cage, site neighbours, and site neighbours ' cage.

Results

For L and V, main effect of site was significant, while caging and neighbours were not.

None of the interactions were significant (Table 11 and 12). In order to determine which

of the three sites differed significantly, Tukey's HSD test was used (Table 13). At each

of the three sites, initial transplant height for either Lythrum or Verbena did not differ

significantly between any of the four treatments (Appendix 3 continued next page).

Appendix 3 (continued)

Table 11 : Results of the analysis of variance for initial plant height for L. The analysis tested for the following differences: between sites. cages (Le., caged versus uncaged) and neighbours (neighbours intact versus neighbours removed). It also tested for interactions between neighbours cage. site cage, site neighbours. and site ' neighbours * cage.

1 Source of variation 1 df

1 neighbours 1 8.34 1 0.38 1 neighbours cage

1 site * neighbours * cage 1 2 1 10.0 1 0.45 1 0.64 'Note no block effect was detected, therefore it was pooled with error tenn

site * cage

site * neighbours

2

2

3.87

17.6

0.17

0.79

0.84

0-45

Appendix 3 (continued)

Table 12: Results of the analysis of variance for initial plant height for V. The analysis tested for the following differences: between sites, cages (Le,, caged versus uncaged) and neighbours (neighbours intact versus neighbours removed). It also tested for interactions between neighbours cage, site cage, site neighbours, and site neighbours cage.

Source of variation

neighbours 11 1 2.9

site

1 neighbours cage 1 1 1 0.6

d f

1 site cage 12 1 11-10

MS

2 1390.7

L - - 1

'Note no block effect was detected. therefore it was pooled

site neighbours

site * neighbours cage ith error tenn

2

2

3.29

0.95

Appendix 3 (continued)

Table 13: Comparison of mean ( î ISE) initial plant height (cm) for Lythrum and Verbena in experimental treatments with neighbours present (+veg) or absent (-veg) and no cage (NC) or full cage (FC) at three sites near Guelph, Ontario, Canada. initial height values with the same superscript capital letter do not differ significantly (P r 0.05).

Site

Site 1

Site 2

Site 3

+ Veg NC

Initial Plant Height of L I + Veg FC

-

- Veg NC - Veg FC I + Veg NC

Initial Plant Height of V

+ Veg FC

Appendix 4:

Table 14: List of insects found at site 1.

Order ORTHOPTERA

( grasshoppers, crickets, etc) ORTHOPTERA ORTHOPTERA

1 HOMOPTERA 1 Cercopidae 1 NIA 1 N/A 1 Sac) Suckers

HOMOPTERA HOMOPTERA

HEMIPTERA 1 Miridae 1 NIA 1 NIA 1 NIA

(Appendix 4 continued next page)

Type of Plant Damage Defoliation

Defoliation Defoliation

Family Tettigoniidae

Conocephalidae Acrididdae Aphididae

Cidacellidae

Species Metrioptera meselii

Orchelimum gladiator Melanollhis bivittatus

NIA NIA

NativelExotic Introduced

Native Native NIA N/A

Sap Suckers Sap Suckers

Table 16: List of insects found at site 3.

1 AOMOPTERA 1 Cereopidae 1 NIA NIA 1 Sap Suckers

Order

ORTHOPTERA ORTHOPTERA

Family Acrididae

Ra~hido~horidae

AOMOPTERA AOMOPTERA HEMIPTERA

Species

Melanoplus sp. Ceutholohilus meridionalus

Cicadeilidae Aleyrodidae

Miridae

NativelExotic Native Native

NIA NIA NIA

Type of Plant Defoliation Defoliator Defoliator

NIA NIA NIA

Sap Suckers Sap Suckers

NIA

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