Big versus small: Does Bythotrephes longimanus predation regulate spatial distribution of another invasive predatory cladoceran, Cercopagis pengoi?

Offshore-onshore spatial distribution and abundance of Cercopagis pengoi, a small non-indigenous predatory cercopagid, in Lake Michigan have been hypothesized to be regulated by the larger non-indigenous predatory cercopagid, Bythotrephes longimanus, through predation and/or competition. However, temperature and prey abundance are other factors that could be affecting Cercopagis. First, we examined all these factors on Cercopagis population abundance, life history traits and spatio-temporal distribution. In addition, we examined vertical spatial overlap between these species and determined predation rate of Bythotrephes on Cercopagis. Linear mixed effects analysis of spatial-temporal data showed that biomass of B. longimanus had the strongest effect, which was significantly negative on biomass, proportion of fecund females and mean clutch size of Cercopagis. Fecundity increased significantly with density of potential prey zooplankton, whereas Cercopagis total biomass increased significantly with the mean epilimnion temperature. Cercopagis and Bythotrephes overlapped vertically in the epi- and metalimnion, and neither of them showed any appreciable diel vertical migration. In predation experiments, Bythotrephes consumed Cercopagis at the same rate as Daphnia mendotae, a known preferred prey, when offered at equal concentrations. Overall, this observation, together with vertical overlap of Cercopagis with Bythotrephes implies that Bythotrephes predation has a strong influence on Cercopagis distribution; however, prey availability, temperature, and competition may be important secondary factors. These results imply that invasion success of Cercopagis may be limited by prior invasion by Bythotrephes.