Transcript Laboratory 1: Nearest Neighbor

Predation Experiments
Methods

Take 6 mud crabs and 6 hermit crabs
Tether 3 of each to artificial seagrass unit (ASU’s)
 Tether 3 of each to hooks placed in bare sediment
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Data: CW/sex mud crabs & hermit shell length
Methods
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Tether crabs with monofilament line and superglue
Methods
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For three 24 hour trials:
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Every 24 hours, check tethered crabs
Record identity and size of consumed animals
 Replace lost animals each day so that original density
of 3 animals/tether maintained over next 2 trials
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Tidal information available at:
http://www.mobilegeographics.com:81/locatio
ns/1501.html
Tidal Predictions
Deploy tethers
Data Example
Date
Habitat CW Shore Crab Consumed? Size Hermit Crab Consumed?
7/29/2009 G
7
N
4
N
7/29/2009 G
8
Y
5
N
7/29/2009 G
9
Y
6
Y
7/29/2009
S
6
Y
5
Y
7/29/2009
S
7
N
5
N
7/29/2009
S
8
N
4
Y
7/30/2009
7/30/2009
7/30/2009
7/30/2009
7/30/2009
7/30/2009
G
G
G
S
S
S
7
6
4
5
7
8
4
5
7
8
5
3
Data Analysis

Compare % of mud crabs, hermits consumed on each
of the two habitats using Chi-Square
Also evaluate whether crab size or type of a hermit
crab’s shell was correlated with percentage of prey
taken by predators
% Eaten per 24 hours
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100
90
80
70
60
50
40
30
20
10
0
1
2
3
4
5
Crab Size (CW or Shell Length)
6
7
Chi-Square
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Discrete data (counts) as opposed to continuous
Compares theorized predictions vs. obs’d data
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Example coin toss proportions
Chi-Square (χ2) =
[ Sum (#observed – #expected)2 ] / [ #expected ]
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Calculate χ2 value and compare to Table 2 at
p=0.05 (on page 55)
Questions
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1. Are there sig. diff’s in predation rate among
habitats? Why do these exist?
2. Does prey size influence the results?
3. Does the kind of shell carried by hermit crabs
influence vulnerability to predation?
4. Is predation likely to be an important factor in
the ecology of the species studied?