Rapid evolution or adaptive phenotypic plasticity as a mechanism of the indirect effects of invasive lake trout on zooplankton in Yellowstone Lake

There is very little research into the indirect effects of invasive or introduced species, yet it is critical to our full understanding of ecology and the nega tive impacts associated with human interference. One ecosystem demonstrating these indirect effects is Yellowstone Lake. Lake trout, Salvelinus namaycush , were illegally introduced into Yellowstone Lake in the mid - 1980s. Lake trout prey upon the native cut throat trout, Oncorhynchus clarkii bouvieri , and have drastically reduced their population size. The cutthroat trout feed on zooplankton in the water column and since the reduction in their population size, natural selection on the zooplankton has been wea kened. Due to this, zooplankton life history traits should be different now than they were before the introduction of lake trout. I analyzed life history traits of the zooplankton species Daphnia pulicaria in samples from Yellowstone Lake from before and a fter the introduction of lake trout. The results indicated that after the invasion of lake trout, clutch sizes were smaller and fewer individuals were brooding. These results are consistent with the predictions of both the mechanisms of rapid evolution and adaptive phenotypic plasticity. More research into the cause of the life history shifts is needed to distinguish between these two mechanisms