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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
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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
Marc Hasenbank Ecology Conference 2006 - Wellington
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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
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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)