Dispersal dynamics and the interactive effects of the invertebrate predator, Notonecta undulata and enrichment on pond zooplankton community diversity
What determines species diversity of local communities remains an ongoing question in community ecology with important implications for understanding and predicting future diversity loss due to human impacts. Recent studies suggest the necessity of understanding the interactive effects of predation and productivity on species coexistence and prey diversity. Models of keystone predation predict that coexistence of prey species with different competitive abilities can be achieved if inferior resource competitors are also less susceptible to predation and if productivity and/or predator mortality are at intermediate levels. Hence, predator effects on prey diversity are predicted to be highly context dependent: enhancing diversity from low to intermediate levels of productivity (or low to intermediate predation pressures) and reducing diversity of prey at high levels of productivity (or high levels of predation pressure). While several studies have examined the interactive effects of herbivory and productivity on primary producer diversity, experimental studies of such effects in predator-prey systems are much rarer. We tested these predictions, using an aquatic field mesocosm experiment in which initial density of the zooplankton predator Notonecta undulata and productivity were manipulated to test their interactive effects on zooplankton species diversity. We used open systems to allow for natural dispersal and behaviorally-mediated numerical responses by the flight-capable predator.
We observed significant positive, density-dependent emigration by Notonecta from the experimental mesocosms. However, these effects were moderated by productivity, with high productivity treatments maintaining higher realized Notonecta densities by the end of the experiment. The effects of predators on zooplankton composition and diversity depended on productivity level. At low productivity, predators reduced zooplankton diversity regardless of realized predator density. However, at high productivity, low densities of predators increased zooplankton diversity when compared to treatments without predators or with high predator densities. Thus, our findings are consistent with keystone predator model predictions in which predators facilitate prey coexistence and diversity at intermediate levels of productivity and/or predation intensity.