Ecological Archives E093-008-A7

Moisés A. Aguilera and Sergio A. Navarrete. 2012. Functional identity and functional structure change through succession in a rocky intertidal marine herbivore assemblage. Ecology 93:75–89.

Appendix G. Results of bootstrap t-test analyses.

Table G1. Results of bootstrap t-test analyses performed to compare the per capita (a–c) effect of the focal herbivore species between phase 1 (colonization) and phase 2 (establishments) of early succession in the field experiments. P values, the t statistic and 95% confidence intervals for each comparison are presented. Significant values using uncorrected probabilities (α < 0.05) in bold. A simple Dunn-Sidák correction for the four tests on each species, produce a corrected significance level of α < 0.0127 which doesn't alter conclusions.

Species P t CI 95%
(a) Ulvoids
(per capita effects: phase 1 vs. phase 2)
Chiton granosus 0.005 3.776 0.006
0.026
Fissurella crassa 0.722 -0.368  -0.002 
0.013
Scurria araucana 0.351 -0.978  -0.023 
0.009
Siphonaria  lessoni 0.666 0.444 -0.297 
0.445
(b) Porphyra
C. granosus 0.320 1.045 -0.003 
0.008
F. crassa 0.504 -0.669  -0.604 
0.298
S. araucana 0.829 0.221 -0.019 
0.015
S. lessoni 0.671 -0.437  -0.001 
0.001
(c) Blidingia
C. granosus   0.343 -0.995  -0.007 
0.003
F. crassa 0.846 0.198 -0.011 
0.014
S. araucana   0.248 -1.227  -0.021 
0.006
S. lessoni   0.447 -0.791  -0.003 
0.002

Table G2. Results of a bootstrap t-test analysis, performed to compare per capita effects of herbivores on the ephemeral algae Porphyra columbina recorded during field and laboratory experiments for phase 1 (a) and phase 2 (b). P values, t statistic and 95 % confidence interval are presented for each comparison. Significant values (α < 0.05) are presented in bold.

Species P t CI 95%
(a) Porphyra Columbina
(per capita effects: Field(phase 1) vs. Lab)
Chiton granosus 0.250 -1.154  -0.008 
0.001
Fissurella crassa 0.491 0.691 -0.001 
0.002
Scurria araucana 0.782 0.277 -0.001 
0.002
Siphonaria  lessoni 0.468 -0.726  -0.0004 
0.0002
(b) P. columbina
(per capita effects: Field(phase 2) vs. Lab)
C. granosus 0.111 1.594 -0.0007 
0.0006
F. crassa 0.430 -0.791  -0.002 
0.001
S. araucana 0.316   1.006 -0.0006 
0.0019
S. lessoni   0.461 -0.738  -0.0002 
0.0007

Table G3. Results of a bootstrap t-test analysis, performed to compare per capita effects of herbivores on the corticated alga Mazzaella laminarioides recorded during field and laboratory experiments. P values, t statistic and 95 % confidence interval are presented for each comparison. Significant values (α < 0.05) are presented in bold.

Species P t CI 95%
Mazzaella laminarioides
(per capita effects: Field vs. Lab)
Chiton granosus 0.134 -1.633  -0.0432 
0.0066
Fissurella crassa   0.011 -3.123  -0.232 
-0.039 
Scurria araucana 0.133 1.647 -0.021 
-0.132 
Siponaria  lessoni   0.071 -2.023  -0.031 
0.002

Statistical Method

In order to test for differences between phases (colonization vs. establishment (Table G1), and between results of field and laboratory experiments (Tables G2–G3), a bootstrap t test procedure was conducted (Manly 1997). The procedure consists in generating bootstrap samples from adjusted data by removing group means (no between group differences). Then 1000 bootstrap samples are obtained by re-sampling with replacement, separately from each original group. A bootstrap t statistic was computed independently for each herbivore species. In this case, the suitable t test is: x1 - x2 / v(S21 / n1 + S21 / n2) where S2 = Σ(xij - X)2 / (nj - 1), and X is the mean value for group x1 and x2.


Literature Cited

Manly, B. F. J. 1997. Randomization, bootstrap and Monte Carlo methods in biology, Second edition. Chapman & Hall, Boca Raton.


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