Higher zooplankton species richness associated with an invertebrate top predator

A dataset of 1735 Norwegian lakes, spanning a wide range of geographical, physical, chemical and biological properties, was analysed to assess the role of the top invertebrate predator, Bythotrephes longimanus, on the crustacean species richness and community structure. Bythotrephes was associated with a 25.5% increase in mean richness of other zooplankton species, and this effect could not be related to confounding factors. Based on presence-absence data, community composition did not differ significantly between lakes with and without this top predator. Neither fish predation nor lake area, altitude, and geographical or physicochemical parameters offer any obvious impact on the observed diversity patterns. We suggest that Bythotrephes facilitates species richness by reducing the abundance of species controlling subdominant species. This pattern of increased diversity offers an interesting contrast to North American lakes recently invaded by Bythotrephes, where species richness typically has decreased, suggesting different short-term and long-term effects of predatory invaders.