Transcript Final Labs - DISL Sharepoint Site

Substrate and Time of
Day Effects on Benthic
Organisms
Trawling Data
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Friday data in Heck-share folder
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Thursday night meet at Verill dock for night
trawl in sand/mud habitats
Bottom Trawling in Action
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Effective sampling method
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Commercial trawling is responsible for major
depletion of continental shelves, enormous
bycatch, and habitat destruction
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Likened to clear cutting of forests (Watling and
Norse 1999)
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http://www.youtube.com/watch?v=zikSzUhUGtA
Similarity Calculations
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Compare similarity in fish/invert species
composition during day between:
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Mud-Sand; Mud-Shell; Sand-Shell (Day and Night)
Use Jaccard similarity index:
J = C / (A+B-C)
 A = No. of spp. on substrate 1
 B = No. of spp. On substrate 2
 C = No. of spp. shared by both substrates


Value varies from 0 (no common spp.) to 1 (all
common)
Similarity Calculations
Mud Sand Shell
Mud 1
Sand
1
Shell
1
If Sand has 22 spp.
Mud has 20 spp.
10 spp. overlap
J = 10 / (22+20-10)
J = 0.3125
Euclidean Distance

Considers the distribution of individuals with
species in each collection:

DeltaJK = Sqrt [ Sum (Xif – Xik)2 ]
DeltaJK = Euclidean Distance
 Xif = Number indiv’s of species “i” in collection “j”
 Xij= Number indiv’s of species “i” in collection “k”
 n = Total number of species

Euclidean Distance

Compensate for fact that Euclidean Distance
increases with number of species in a sample by
calculating average distance:

djk = [ Sqrt ( Delta2jk) ] / (n)
djk = average Euclidean distance b/n sample j and k
 Deltajk = Euclidean Distance
 n = number of species in the samples being
compared

Euclidean Dist. Calculations
Site 1 Site 2 Site 3
Sp. 1 2000 1000 500
Sp. 2 20 10
5
Sp. 3 0
5
0
ED1,2 = sqrt [ (2000-1000)2 + (20-10)2 + (0-5)2 ]
ED1,3 = sqrt [ (2000-500)2 + (20-5)2 + (0-0)2 ]
ED2,3 = sqrt [ (1000-500)2 + (10-5)2 + (5-0)2 ]
Questions

Explain why you found the similarity values you
did, using material from lecture, notes, text,
observations

Discuss why Jaccard and Euclidean distances
showed different patterns (if they did)
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Describe similarities and differences in body
shape, shell thickness, and general morphology
of the taxa inhabiting various structures
Predation Experiments
Methods
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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
Data Analysis

Compare % of shore 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


Use discrete data (counts) as opposed to
continuous data (e.g. 1.234 m)
Compares theorized predictions vs. observed
data – (e.g. coin toss proportions)

Chi-Square (x2) = [ Sum (#observed – #expected)2 ] /
[ #expected ]
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Calculate x2 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?