Distribution of a plant parasite on its host: gall distribution on the flowering dogwood, Cornus florida

BIOL3060

Overview

(A) Parasite: Gall Midge Fly (Resseliella clavula)

(B) Host: Flowering dogwood (Cornus floria)

• Process– Female fly lays eggs near terminal dogwood bud– Eggs hatch & larvae enter dogwood shoots– Abnormal swelling (galls) form on twig ends– Larvae get protection and food source– Plants expend more energy regenerating tissue

Purpose• To determine the distribution of midge galls

on dogwood trees and to investigate some environmental factors that might influence distribution.

Questions1) How are the galls distributed spatially?

• Random vs. clumped vs. uniform

2) Could gall number or distribution be related to:• Size/age of the plant? Nearest neighbor

distance? Sunny versus shady microhabitat?

P1: Random Distribution

• Parasites are randomly distributed within the environment. If midges flew until they “hit” a dogwood haphazardly …

4020

20

30

30

20

2020

10

2030

4030

20

20

30

30

30 20

20

40

40

P2: Even Distribution

• “Regular”: Organisms are evenly spaced in the environment. In our case, every tree would have the same number of galls. This would suggest that the parasites are competing for the trees. 10

10

10

10

1010

1010

10

10

10

10

10

10

10

P3: Clumped Distribution

• Most trees have few parasites, and some a lot.

• Typically associated with:– disease outbreaks– tree densities (“contagious”)– age-related effects

030

30

3030

0

0

00

0

0

0

0

0

00

0

0 030

Question 2) Number of galls per tree related to the environment?

• Tree size• Older and infirm trees might be more likely

to be attacked by the midge parasite.

• Distance to nearest neighbor• If trees are close together, they might be

more likely to “catch” galls.

• Collect data in two habitats• Some microhabitats might be better for

galls than others.

Environmental factors we are measuring

Hypotheses

1) HO: Dogwood galls are distributed randomly. HA: Dogwood galls are not distributed randomly.

prediction: Galls are either like a disease to dogwoods and they will be clumped, or there will be competition between them and their distribution will be even..

2) HO: The number of dogwood galls per twig on trees is the same between sunny areas and shady areas

HA: The number of dogwood galls per twig on trees is not the same between sunny areas and shady areas prediction: There is an environmental factor (possibly sunlight) that differs between the two habitats that causes the number of galls to differ between habitats.

3) HO: The number of dogwood galls per twig on trees is not correlated with tree size. HA: The number of dogwood galls per twig on trees is correlated with tree size.

prediction: Part of the biology of the dogwood midge involves choosing or avoiding larger trees.

4) HO: The number of dogwood galls per twig on a tree is not correlated with distance to nearest neighbor.

HA: The number of dogwood galls per twig on a tree is correlated with distance to nearest neighbor.prediction: Dogwood midges are more likely to lay eggs on trees closer to the ones they developed in.

Task 1. Student’s T-test1) # galls

2) dbh3) dnn

Also use the excel function TTEST

1 2

2 21 2

1 2

x xt

s sn n

Degrees of Freedom:Df = (n1-1) + (n2-1)

P=0.05

Probability Levels for Student’s t-Distribution (Two-sided Probability Level)

Degreesof Freedom

Sample Size

Value of p

0.5 0.2 0.1 0.05 0.01

3 5 0.77 1.64 2.35 3.18 5.84

5 7 0.73 1.48 2.02 2.57 3.36

10 12 0.70 1.37 1.81 2.23 3.17

15 17 0.69 1.34 1.75 2.13 2.95

30 32 0.68 1.31 1.70 2.04 2.75

38 40 0.68 1.30 1.69 2.03 2.71

40 42 0.68 1.30 1.68 2.02 2.70

50 52 0.68 1.30 1.68 2.01 2.68

100 102 0.68 1.29 1.66 1.98 2.63

1000 1002 0.67 1.28 1.65 1.96 2.53

Task 2. Histogram

Use countif statements

# galls/ tree

# trees in edge habitat

# trees in canopy habitat

0-2    3-5    6-8    9-11    12-14    15-17    18-20    21-23    24-26    27-29    30-32    33-35    36-38    39-41    

Task 3. What is the distribution of galls in the forest?

• Index of Dispersion (Id)

2 1d

s nl

x

Critical Values of the Chi-Square Distribution For p = 0.05

Degrees of Freedom Lower Upper

10 3.94 18.3115 7.26 25.0018 9.39 28.9019 10.10 30.1420 10.85 31.4125 14.61 37.6530 18.49 43.7735 22.47 49.8040 26.51 55.7645 30.61 61.66

50 even 34.76 rand 67.51 clump55 38.96 73.3160 43.19 79.0865 47.45 84.8270 51.74 90.5375 56.05 96.22

Task 4. Correlations1) dbh2) dnn

• Degrees of Freedom:• Df= (n-2)

• P=0.05

Task 5. Scatter plots (2)

0

2

4

6

8

10

12

14

16

0 5 10 15 20 25 30

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30

Before you leave today:

• Fill out chart on Pg 39

• 3 graphs (histogram, 2 correlation plots)