Anti-predatory response of Daphnia to an invasive predatory cladoceran and ensuing non-consumptive effects: Influence of seasonally changing environmental variables and predator density

Background/Question/Methods

Nonconsumptive effects (NCEs, the focus of "the ecology of fear") can be as large as the consumptive effects of predators. The relative contribution of non-consumptive effects can vary greatly on the nature of the predator-prey interaction, the density of predator and prey, and environmental context. Estimating the magnitude of nonconsumptive effects in the field is therefore a challenge. Previous lab experiments indicate that Daphnia mendotae can perceive chemical cues from an invasive predatory cladoceran Bythotrephes and respond by migrating to deeper, colder waters. We examined Daphnia's vertical position response (i.e. mean depth) to Bythotrephes in Lake Michigan at different points in the season and over 6 years between 2004-06 and 2009-11. We further measured environmental variables (e.g. light, temperature) to examine if these factors influence the response of Daphnia to Bythotrephes, and to estimate costs of the response (i.e. NCEs). We use an optimization model, which balances predation risk and growth rate, to predict results. 

Results/Conclusions

Consistent with the optimization model, Daphnia displays a nearly uniform deep migration above an intermediate Bythotrephes density; other environmental variables therefore have little influence. At lower Bythotrephes density, in contrast, the vertical position (i.e. mean depth) of Daphnia is strongly correlated with environmental variables, including deeper distributions with increased Secchi depth (a measure of light levels) and shallower distributions with increased epiliminion (warmer upper water layer) depth. The relationship between Bythotrephes density and the magnitude of the phenotypic response of Daphnia has large effects on CEs and NCEs. The NCE of Bythotrephes on Daphnia increases as a saturating function of Bythotrephes density, whereas the CE decreases as a function of Bythotrephes density. The relative contribution of the NCE to the net effect of the predator therefore increases dramatically at higher Bythotrephes density. The relative contribution of the NCE changes seasonally, as the costs of the phenotypic response are dependent on changes in the vertical temperature gradient. Our results elucidate how Bythotrephes affects zooplankton in Lake Michigan’s food web, and, more generally, the dependence of predator density and environmental context on the relative contribution of NCEs to the net effect of the predator.