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 (late spring).

– Eggs hatch & larvae enter dogwood shoots.

– Abnormal swelling ( galls ) form on twig ends (summer).

– Larvae exit galls and overwinter in soil. Pupate in early spring. Become adults in late spring.

– 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.

Questions

1) How are the galls distributed spatially?

• Random vs. clumped vs. uniform 2) Could gall distribution be related to: • Size/age of the plant? Nearest neighbor distance? Sunny versus shady microhabitat?

Question 2. Could gall distribution be related to: • • Tree size Older and infirm trees might be more likely to be attacked by the midge parasite. How could this affect distribution?

• • Distance to nearest neighbor If trees are close together, they might be more likely to “catch” galls. How could this affect distribution?

• • Collect data in two habitats Warmer environments might be better for midges. Environments with more vigorous trees might affect midge choices. How could this affect distribution?

Clumped

Distribution

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

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

0 0

2 0 6 4 16 14 12 10 8

0 30 30 30 0

0 10 20

# Parasites

30 40

0 30 0 0 0 0 0 0 0 30 0 0 0

Even

Distribution

6 4 2 0 16 14 12 10 8 • “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 10 10 10 10 10 10 10 10 10 10

0 10 20

# Parasites

30 40

10

Random

Distribution

• Parasites are randomly distributed within the environment. If midges flew until they “hit” a dogwood haphazardly … 2 0 6 4 16 14 12 10 8 0 10 20

# Parasites

30 40

40 30 20 20 30 20 20 30 10 20 30 30 40 20 20 30 40 20 20 30 40 20

Hypotheses and predictions

H 1 Midge galls are like a disease for dogwoods, some are attacked while others are not.

prediction: the distribution of galls on dogwoods is clumped.

H 2 Gall midges compete for laying sites, and larvae cannot survive if they are too close to each other.

prediction: the distribution of galls on dogwoods is regular (even).

Task 1. Student’s T-test

1) # galls 2) dbh 3) dnn Also use the excel function TTEST

t



x

1 

x

2

s n

1 2 1 

s n

2 2 2 Degrees of Freedom: Df = (n 1 -1) + (n 2 -1) P=0.05

Probability Levels for Student ’ s

t

-Distribution (Two-sided Probability Level) Degrees of Freedom 3 5 10 15 30 38 40 50 100 1000 Sample Size 5 7 12 17 32 40 42 52 102 1002 0.5

0.77

0.73

0.70

0.69

0.68

0.68

0.68

0.68

0.68

0.67

0.2

Value of

p

0.1

1.64

2.35

1.48

1.37

2.02

1.81

1.34

1.31

1.30

1.30

1.30

1.29

1.28

1.75

1.70

1.69

1.68

1.68

1.66

1.65

0.05

3.18

2.57

2.23

2.13

2.04

2.03

2.02

2.01

1.98

1.96

0.01

5.84

3.36

3.17

2.95

2.75

2.71

2.70

2.68

2.63

2.53

Task 2. Histogram

# 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 Use countif statements 2 1 0 7 6 5 4 3 0-2 3-5 6-8 9-11 12-14 15-17 18-20 21-23

Number of galls

Sunny Shady

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

• Index of Dispersion (I d )

l d

 

s

2 

n

 1  

x

Critical Values of the Chi-Square Distribution For

p

= 0.05

Degrees of Freedom 10 15 18 19 20 25 30 35 65 70 75 Lower 3.94

7.26

9.39

10.10

10.85

14.61

18.49

22.47

47.45

51.74

56.05

Upper 18.31

25.00

28.90

30.14

31.41

37.65

43.77

49.80

40 45 26.51

30.61

50

even

34.76

rand

55.76

61.66

67.51

55 60 38.96

43.19

73.31

79.08

84.82

90.53

96.22

clump

Task 4. Correlations

1) dbh vs. # galls 2) dnn vs. # galls • • •

Degrees of Freedom: Df= (n-2) P=0.05

Task 5. Scatter plots (2)

16 14 12 10 8 6 4 2 0 0 5 10 15 20 25 30 35 30 25 20 15 10 5 0 0 5 10 15 20 25 30

2 0 6 4 16 14 12 10 8 16 14 12 10 8 6 4 2 0 0 10 20

# Parasites

30 40 0 10 20

# Parasites

30 40 16 14 12 10 8 6 4 2 0 0 10 20

# Parasites

30 40

Before you leave today:

• Fill out chart on Pg 51 • 3 graphs (histogram, 2 correlation plots)