Predator presence dramatically reduces copepod abundance through condition-mediated non-consumptive effects

Predators can induce changes in prey phenotype that reduce predation risk but also incur costs (so-called non-consumptive effects' or NCEs). Few studies, however, have examined NCEs over time scales sufficient to link individual- and demographic-level processes to changes in prey abundance. Using a laboratory experiment, we investigated how NCEs of the invasive planktivorous cladoceran Bythotrephes longimanus on the condition of native zooplankton from Lake Michigan affect population abundance. Presence of chemical cues (i.e. kairomones) from Bythotrephes led to a greater than 13-fold reduction in abundance of diaptomid copepods in the absence of any direct predation. This dramatic reduction was a result of reduced fecundity and likely reduced survival. The costly anti-predator response underlying the demographic effects was reduced food intake and/or assimilation as evidenced by reduced gut fullness, which led to poorer condition, as evidenced by lower lipid accumulation. These predator effects may remain undetected in studies of animal populations that fail to include sufficient efforts to search for them, and illustrate the far-reaching potential of predator non-consumptive effects (fear effects') to influence prey populations.