Substrate and Time of Day Effects on Benthic Organisms

1. Substrate and Time of Day Effects on Benthic Organisms

2. Trawling Data Friday data in Heck-share folder Thursday night meet at Verill dock for night trawl in sand/mud habitats

3. Bottom Trawling in Action • Effective sampling method • Commercial trawling is responsible for major depletion of continental shelves, enormous bycatch, and habitat destruction • Likened to clear cutting of forests (Watling and Norse 1999) • http://www.youtube.com/watch?v=zikSzUhUGtA

4. Similarity Calculations • Compare similarity in fish/invert species composition during day between: • 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)

5. Similarity Calculations If Sand has 22 spp. Mud has 20 spp. 10 spp. overlap J = 10 / (22+20-10) J = 0.3125

6. 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

7. 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

8. Euclidean Dist. Calculations 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 ]

9. 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) Describe similarities and differences in body shape, shell thickness, and general morphology of the taxa inhabiting various structures

10. Predation Experiments

11. Methods • 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 • Tidal information available at: http://www.mobilegeographics.com:81/locations/1501.html

12. Tidal Predictions

13. 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

14. 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 ] Calculate x2 value and compare to Table 2 at p=0.05 (on page 55)

15. Questions 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?