Appendix B. The relationship between Bythotrephes biomass and absolute prey abundance in the hypolimnion.
In the extensive field survey, Bythotrephes biomass had a negative influence on the percentage of prey in the epilimion and a positive influence on the percentage of prey in the hypolimnion (Figs. 1 and 2). We interpreted these patterns as reflecting an induced vertical migration that was an adaptive behavioral response to predation risk. Here we evaluate an alternative possibility, namely that the observed changes in prey distribution are due to predation by Bythotrephes; i.e., predation by Bythotrephes, which was found primarily in the epilimnion, would also cause a reduction (increase) in the percentage of prey in the epilimnion (hypolimnion).
Consider the abundance of prey species in the hypolimnion. If Bythotrephes predation, not induced prey migration, were responsible for changes in prey distribution, then Bythotrephes would not affect the abundance of prey in the hypolimnion. Therefore, there would not be a correlation between Bythotrephes biomass and prey density. In contrast, if changes in prey distribution resulted from induced vertical migration, then we would expect prey abundance in the hypolimnion to be positively correlated with Bythotrephes biomass. Therefore a positive correlation would be consistent with our interpretation of induced migration but not predation. Note that these predictions require that there is no correlation between total prey abundance (in the water column) and Bythotrephes biomass. Because we expect either consumption or induced vertical migration to negatively affect prey abundance, the inference that a positive correlation between Bythotrephes biomass and abundance in the hypolimnion resulted from induced vertical migration is conservative. We used linear regression analysis to examine the relationship between prey abundance in the hypolimnion and Bythotrephes biomass. Prior to analysis, absolute prey abundance and Bythotrephes biomass were log-transformed to normalize distributions of dependent and independent variables (Zar 1999). There was a positive relationship between Bythotrephes biomass and the abundance of Daphnia mendotae and Bosmina longirostris in the hypolimnion (D. mendotae: t12 = 2.02, P = 0.03; B. longirostris: t13 = 1.69, P = 0.05) in Lake Michigan, and of B. longirostris and D. retrocurva abundance in the hypolimnion (B. longirostris: t52 = 2.37, P = 0.01; D. retrocurva: t40= 2.42, P = 0.01) in Lake Erie. Therefore, analysis of the absolute abundance of Bythotrephes prey are consistent with our interpretation that the prey relative abundance at different levels was affected by Bythotrephes-induced changes in vertical migration rather than predation.
LITERATURE CITED
Zar, J. H. 1999. Biostatistical analysis, Fourth edition. Prentice-Hall, New Jersey, USA.