Allelopathy and herbivory

Additional readings:– Hawkes CV, Sullivan JJ . 2001. The

impact of herbivory on plants in different resource conditions: A meta-analysis. Ecology 82:2045-2058.

Seminars

Outline1. Amensalism2. What is allelopathy and how is it inferred?3. “plus-minus” interactions4. Herbivory and importance in range and

forest5. Trophic relationships: top down VS bottom

up6. Effects of herbivory

• Aspects of herbivory in range and forest• Herbivore functional responses• Sublethal effects and compensation (example)

Allelopathy• Could be considered “interference

competition” for plants...• Definition:

• Often mentioned, but hard to prove. Why?

Allelopathy• Inferred by:

1. Spatial pattern/arrangement of plant community

2. Presence of chemicals in leaves, roots and/or soil

3. Demonstration that chemicals cause decline in growth or survival of surrounding vegetation

Allelopathy• Difficult to prove because:

1. Low [chemicals] in the field2. Mediated by 3rd

party (e.g. microbes and litter)

3. Trophic interactions similar to apparent competition: shrub harbours seed predator – causes “allelopathic” spatial arrangement.

Allelopathy• Recent work on Centauria maculata

(spotted knapweed) provides good evidence for allelochemicals.

• Knapweed is an important rangeland weed; what are implications of allelopathy?

Amensalism• Negative to one species,

inconsequential to other.• Often is actually very asymmetric

competition/pathogenesis• Example: allelopathy

“Plus-Minus” relationships• Include disease, parasitism, predation

and herbivory.• Effects on population (or biomass) can be

modelled using Lotka-Volterra equations to predict population of prey and predators.

dV/dt=V[b-aP]=f1(V,P)

dP/dt=V[kaV-d]=f2(V,P)

Where: V=#prey, P=#predators, b=prey growth rate, a=prey consumption rate by predator, k=rate of increase of predator per unit prey,

and d=predator death rate

Functional response• Relationship of predator (herbivore)

consumption of prey (plants) to density of prey (plants)

• 3 types – unsaturating (I), saturating(II), and sigmoidal (III)

• Type of functional response has implications for community structure and stability

• Discuss further in lab…

Herbivory• What is the importance of herbivory?

– Small amount of biomass removed: 10%• Tundra/alpine 3%• Forest 4%• Grassland 10-15%• Rangeland/grazing systems 30-60%

– “world is green” hypothesis (who?): there is more plant biomass than herbivores can eat.

– Why? Trophic interactions.• Top down control (predators)• Bottom up control (plant forage quality)

Herbivory• But – trophic cascade models too

simplistic; herbivory has more dramatic effects than they predict...

– Defensive compounds (coevolution)– Community composition– Productivity– Seedling survival and demography– Seed predation

Herbivory• What are some of the effects of

herbivores on plants and plant communities?

Effects and issues• Mortality of seedlings• Insects VS vertebrates• Herbivore outbreaks (insects especially)• Sublethal effects and herbivory escape• Compensatory growth and

overcompensation• Productivity• Herbivore functional responses• Resistance/defence

Forests• Mortality of seedlings• Insects • Outbreaks (insects especially)• Sublethal effects and herbivory escape• Defensive compounds?

Rangelands• Productivity• Compensation/overcompensation• Herbivore functional responses• Toxicity and herbivore resistance• Selectivity/preference• Diversity and coexistence

Herbivore selectivity• Plant stress VS plant vigour hypotheses:

– Herbivores attack already stressed plants– Herbivores favour plants in high resource areas

and with larger “plant modules”– Evidence for both– Also influenced by herbivore defenses

• Can alter species composition and lead to coexistence IF favoured species is best competitor

• Differs among herbivore species, therefore management of different species can change community composition

Compensation• Response of plants to defoliation can vary

widely: positive, negative or neutral• “Compensation” means plant growth

increases after herbivory to compensate for lost tissue

• “Overcompensation” much discussed: this means plants are stimulated to grow MORE after grazing/browsing.

• Is overcompensation possible?• Is compensation over extended periods

possible?

Sublethal effects of herbivory

• Herbivores don’t often cause mortality of adult plants.

• Can affect plants in other ways:– Reduced seed set/fruit abortion– Reduced size/growth rate– Change in architecture– Delay or prevention of maturity– These may all affect plant fitness (contribution

to next generation)

• Example: population consequences of herbivory on three Australian native plants. Allcock and Hik 2004. Oecologia 138:231-241.

Grazing experiment• Three groups of grazing animals: domestic

stock (sheep and cattle), native macropods (kangaroos and wallabies), and introduced rabbits.

• Four treatments: control, stock fence, kangaroo fence, rabbit fence.

• Two habitats: woodland (intact Eucalyptus canopy) and grassland (cleared “native” pasture)

• Three target species: kangaroo grass (Themeda australis), cypress pine (Callitris glaucophylla) and white box (Eucalyptus albens)

Grazing experiment• Plants placed in experimental plots in April

1998• Monitored until April 2001.• Generalized linear modelling used to

analyze factors affecting survival (habitat, grazing animals, competition)

• Survival and growth data used to parameterize stage-based population models for trees.

“Life history” transition diagram

• Transitions between 5 size (height) classes for trees; final stage is “escape from herbivory”.

• Models created for each habitat and treatment combination.

S5

S3

S2

S1

P33

P22

P44 P

55

P11

P35

P15

P25

P24

P13

P14

P12

P23

P34 P

45S4

P52

P53

P54

P51

P43

P42

P41

P31

P18

P21

S1

[<25 cm]

S2

[25 - 49 cm]

S3

[50 - 74 cm]

S4

[75 - 100 cm]

S5

[ >100 cm]

S1

S2

S3

S4

S5

0 P21

0 0 0

0 P22

0 0 0

0 P23

0 0 0

0 P24

0 0 0

0 P25

0 0 0

Matrix One

P11

P21

P31

P41

P51

P12

P22

P32

P42

P52

P13

P23

P33

P43

P53

P14

P24

P34

P44

P54

P15

P25

P35

P45

P55

S1

[<25 cm]

S2

[25 - 49 cm]

S3

[50 - 74 cm]

S4

[75 - 100 cm]

S5

[ >100 cm]

S1

S2

S3

S4

S5Matrix Two

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Eucalyptus albens

years since planting

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 160.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Callitris glaucophylla

Pro

port

ion

reac

hing

esc

ape

heig

ht (

100

cm)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Grassland Woodland

+r +k +s+r +k -s+r -k -s

-r -k -s

Results• Different herbivore species affected

different plant species (rabbits/kangaroos – cypress; stock – Eucalyptus)

• Plants in productive environment better able to compensate (more rapid growth)

• High herbivory rate in unfenced low productivity habitats prevented “escape”.

• This could have population consequences even though mortality was fairly low.

• Interaction between competition and herbivory…tradeoff.