Kevin L. Pangle, Scott D. Peacor, and Ora. E. Johannsson. 2007. Large nonlethal effects of an invasive invertebrate predator on zooplankton population growth rate. Ecology 88:402–412.

Appendix A. Estimates of nonlethal effect produced using a prey somatic growth rate model.

We used a second method to estimate the nonlethal effect (Δb) of the predator on prey population growth rate. In this case, we used somatic growth rate as a surrogate for population growth rate. For cladoceran species like D. mendotae and B. longirostris, somatic growth rate correlates very strongly with population growth rate (r² = 0.99, Lampert and Trubetskova 1996). We estimated somatic growth rate (SGR, /d) for D. mendotae and B. longirostris using the equations:

respectively, where T is water temperature (ºC). These equations were derived by fitting nonlinear models to published data on D. mendotae (Hall 1964) and B. longirostris (Hanasato and Yasuno 1985). The mean somatic growth rate of each sampling profile (SGR_p) was calculated as:

(A.3)

where SGR_z was the estimated somatic growth rate at the depth z given its midpoint temperature, N_z was the prey density (number/m³) at depth z, and w_z (m) was the height of the water column represented by depth z. SGR_p's were averaged over the entire 24-h period using the same weights as Eq. 5 in the main text. The nonlethal effect of the predator was then calculated as the difference between the estimated somatic growth rate using the vertical distributions of prey when Bythotrephes was present and when Bythotrephes was absent. The former distributions came from the three intensive surveys, while the latter distributions were the deepest, the average, and the shallowest observed prey distributions in Bythotrephes absence from our extensive surveys. This method yielded qualitatively similar estimates of the nonlethal effect, relative to the lethal effect, of Bythotrephes on prey growth rate that of the more direct method based on life history of organisms collected in the field. For D. mendotae in Lake Michigan, the nonlethal effect on somatic growth rate was estimated as 0, 0.101, and 0.211/d, respectively, based on our three scenarios in which prey were at the deepest, the average, and the shallowest observed prey distributions in Bythotrephes absence from our extensive surveys. For B. longirostris in Lake Erie, estimates of nonlethal effects in the first survey were 0, 0.048, and 0.105/d, respectively, while estimates were 0, 0.016, and 0.050/d, respectively, in the second survey.

LITERATURE CITED

Hall, D. J. 1964. An experimental approach to the dynamics of a natural population of Daphnia galeata mendotae. Ecology 45:94–112.

Hanasato, Y., and M. Yasuno. 1985. Effect of temperature in the laboratory studies on growth egg development and first parturition of five species of Cladocera. Japanese Journal of Limnology 46:185–191.

Lampert, W., and I. Trubetskova. 1996. Juvenile growth rate as a measure of fitness in Daphnia. Functional Ecology 10:631–635.