Top predator boosts species richness – but not in alien ecosystems

Posted on December 13, 2016

The spiny water flea Bythotrephes longimanus is a large cladoceran, a specialised carnivore and a top predator. Invasions of North American lakes by Bythotrephes have been linked to dramatic changes in food web structure and reductions in species diversity, which seems to contradict the expectation that top predators increase the evenness and diversity of aquatic communities by controlling subdominant species. In a recent study in Norway, where spiny water fleas are native, researchers analysed deep-water plankton data collected from 1735 lakes in summer, and compared the structure of plankton communities in lakes with and without Bythotrephes. To allow for the effects of different factors that might influence the species richness of zooplankton, they included four explanatory variables (altitude, lake surface area, pH and calcium concentration) in their model, as well as the presence / absence of spiny water fleas. All these factors proved to be highly significant, the number of species increasing with lake size and calcium and decreasing with altitude and pH. When the effects of the co-variables were factored out, the presence of Bythotrephes was found to increase species richness by over 25%, pointing to a cascading effect on the whole zooplankton community. There was also an effect on abundance: many species were more common in the presence of Bythotrephes, which had the effect of making plankton communities more similar. Interestingly, in the same lakes there was a similar community response (a 45-68% increase in species) to the presence of another major invertebrate predator, the water flea Leptodora kindti. Although an understanding of the processes controlling these population responses will depend on further research, these findings show that interactions between spiny water fleas and other species are fundamentally different in native and exotic contexts, and support the conventional wisdom that in settings where species have co-evolved, controls by top predators can maintain food chain length, species coexistence and community stability.

Reference: Walseng, B. et al. 2015. Higher zooplankton species richness associated with an invertebrate top predator.
Freshwater Biology 60, 903–910.