Non-consumptive predator effects in a pelagic food-web: Species- and developmental stage-level variation in behavioral responses to an invasive predator within a zooplankton assemblage

Background/Question/Methods

Predators can influence the structure and function of ecological communities, both through direct consumption of prey, as well as by inducing changes in prey traits. These trait changes can limit prey growth and survival (a non-consumptive effect of the predator, NCEs) and modify the way prey interact with other community members (trait-mediated indirect interactions, TMIIs). In nature, predators often have multiple prey species; thus, understanding the importance of NCEs and TMIIs in a system requires understanding differential investment in antipredator responses and the associated fitness tradeoffs among different prey species. Here, we combine field and laboratory studies to examine whether different species and developmental stages in the zooplankton community in a large pelagic ecosystem (Lake Michigan) exhibit variation in their behavioral responses to the invasive planktivore Bythotrephes longimanus and the potential for NCEs and TMIIs in this system.

Results/Conclusions

Field surveys in Lake Michigan conducted over several years indicated that higher Bythotrephes abundance was correlated with some species and developmental stages of zooplankton, but not others, occupying lower depth strata in the lake and increasing diel vertical migration. Such behavioral responses presumably diminish predation from Bythotrephes due to reduced spatial overlap, but also potentially reduce birth rate due to exposure to cooler water temperatures. This latter effect can result in a strong NCE on some species. Laboratory mesocosm experiments corroborated field patterns, showing that species that occupied lower depths and vertically migrated in the field also responded to water-borne cues from Bythotrephes by migrating to lower depths in experimental water columns. Variability in behavioral responses of zooplankton may reflect species- and developmental stage-level differences in ability to vertically migrate and vulnerability to Bythotrephes. We discuss how species-specific behavioral variation in zooplankton may alter their interactions with one other, their resources, and other predators (including fish), suggesting the complex effects predators can have on food webs through non-consumptive pathways. Our results further demonstrate the potential importance of NCEs in deep pelagic systems such as the world’s oceans.