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Egg laying on patchy resources and the importance of spatial scale.

By

Marc Hasenbank, Stephen Hartley

School of Biological Sciences, VUW

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Overview

•Background

•Aim

•Hypothesis

•Experimental setup

•Results Summer’06

•Distribution of cabbage white eggs measured at different spatial scales

Marc Hasenbank Ecology Conference 2006 - Wellington

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Background – the basic Question

Marc Hasenbank Ecology Conference 2006 - Wellington

= Cabbage plant

Cabbage white

(Pieris rapae)

How are the eggs distributed?

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“Resource concentration hypothesis“: specialized

herbivorous insects are more likely to find and stay in more

dense and less diverse patches of their host plants (Root,

1973, Ecol. Monog.).

Background – Key Hypothesis

Marc Hasenbank Ecology Conference 2006 - Wellington

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“Resource dilution hypothesis”: insects are likely to

locate diffused resource patches more often than expected

(e.g. Yamamura, 2002, Pop. Ecol.).

Background – Key Hypothesis

Marc Hasenbank Ecology Conference 2006 - Wellington

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Examples for resource concentration:

- cinnabar moth, Tyria jacobaea, utilising ragwort (Harrison,

1995; Kunin, 1999)

- chrysomelid beetle, Trirhabda virgata (Long, 2003)

Examples for resource dillution:

- seed-head fly, Botanophila seneciella, utilising ragwort

(Crawley, 1988)

- Pieris rapae butterflies ovipositing on Brassicaceae (Root,

1984)

Background – Species dependent response

Marc Hasenbank Ecology Conference 2006 - Wellington

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Background

Marc Hasenbank Ecology Conference 2006 - Wellington

= Cabbage plant

Cabbage white

(Pieris rapae)

How are the eggs distributed?

But it is already documented, that cabbage white females tend to spread their eggs among plants in low density stands.

But how is low and high density defined?

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By changing the scale of observation, different areas of

low or high resource density may be identified.

Background

Small scale

Marc Hasenbank Ecology Conference 2006 - Wellington

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By changing the scale of observation, different areas of

low or high resource density may be identified.

Background

Medium scale

Marc Hasenbank Ecology Conference 2006 - Wellington

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By changing the scale of observation, different areas of

low or high resource density may be identified.

Background

Coarse scale

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What at small scale seemed to be dense cluster might

become an area of low resource density at coarse scale.

Background

HighLowMedium

Marc Hasenbank Ecology Conference 2006 - Wellington

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We want to test whether it is possible to simultaneously

observe ‘resource concentration’ or ‘resource

dilution’ responses to patch density, simply by changing

the scale of analysis.

In particular…

Do female cabbage white butterflies (CW), Pieris rapae,

show scale-dependent responses to host plant density?

Aim

To answer these questions we measured the distribution

of CW eggs among cabbage plants.

Marc Hasenbank Ecology Conference 2006 - Wellington

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Hypothesis – Plant densities

H0 Eggs are evenly distributed among different plant

densities

HA’ More eggs are found on plants in low density

stands

Marc Hasenbank Ecology Conference 2006 - Wellington

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Hypothesis – Measurement at different spatial scales

H0 There is no difference between fine, medium and

coarse scale measurements

HA’ The observed response varies between different

scales of measurement

Marc Hasenbank Ecology Conference 2006 - Wellington

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Experimental setup

100m

150m

36m

36m P

E

F

G

H Number of Plants in Patches:

P: 180

E-H: 61

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Experimental setup

100m

150m

36m

36m

36m

18m

18m

1 4

40 16

4 4

44 4

40

404040

36m

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Experimental setup

36m

18m

1

36m

18m

4

40 16

4 4

44 4

40

404040

4 1 14

1 16 1 1

1 1 11

1 1 1 4

6m

6m

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Experimental setup

Plant densities at different scales of measurement:

1x1m: 1 4 16

6x6m: 1 4 16 40

36x36m: 61 180

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Results – Number of eggs/plant [mean+/-sem] vs plant densities at different scales for experiment Levin2

Marc Hasenbank Ecology Conference 2006 - Wellington

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Results – Number of eggs/plant [mean+/-sem] vs plant densities at different scales for experiment Levin2

Marc Hasenbank Ecology Conference 2006 - Wellington

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Conclusion

Marc Hasenbank Ecology Conference 2006 - Wellington

H0 Eggs are evenly distributed among different plant

densities

HA’ More eggs are found on plants in low density

stands

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Conclusion

Marc Hasenbank Ecology Conference 2006 - Wellington

H0 There is no difference between fine, medium and

coarse scale measurements

HA’ The observed response varies between different

scales of measurement

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Thanks to…

Marc Hasenbank Ecology Conference 2006 - Wellington

Stephen Hartley

Jim Barrit

Shirley Pledger

‘Bug Group’

Special thanks to John Clark and the staff of Woodhaven Farm (Levin)