Temporally variable niche overlap and competitive potential of an introduced and a native mysid shrimp

Temporal patterns of interspecific interactions and their correspondence with life history events are of key significance for community dynamics, but often overlooked in characterising the mechanisms of species invasions and predicting competitive outcomes. Here, we examine the role of seasonality in moderating interactions between a globally important invasive mysid shrimp (Hemimysis anomala) and an ecologically similar native species (Mysis salemaai) by combining distribution and diet surveys with experimental quantification of comparative resource exploitation rates and mutual aggressive interactions. We found partial overlap in winter, but not summer horizontal distributions of the two species. Diets were highly similar in the seasonally overlapping range, potentially resulting in competition for the high-quality, but seasonally scarce, zooplankton prey in invaded lakes. Despite its smaller size, H. anomala had similar feeding rates to M. salemaai on three key zooplankton prey species. H. anomala was more aggressive toward M. salemaai than M. salemaai was toward H. anomala, although there was no evidence for intraguild predation. Our findings indicate that spatio-temporal niche partitioning permitted invasion by H. anomala and coexistence with M. salemaai, in spite of their high trophic similarity.