Predator-spreaders: predation can enhance parasite success in a planktonic host-parasite system

The "healthy herds" hypothesis suggests that selective predators, by acting as parasite sinks, may inhibit the start of epidemics and reduce prevalence of infection. Here, we describe a counter-example using field patterns, experiments, and a model. The predator Chaoborus releases infective spores of a fungal parasite and, in doing so, may facilitate epidemics in Daphnia populations. In the field, epidemics occur in lakes with higher Chaoborus densities. Experiments revealed that nonselective Chaoborus release many of the spores contained in their prey. Since these released spores remain infective, this predator can catalyze epidemics when a lake's physical environment might otherwise impede them. Without Chaoborus, Daphnia dying of infection may sink to the lake bottom before releasing spores. A model tracking hosts and spores in the water column (where hosts contact spores) and in bottom sediments (where they cannot) illustrates this mechanism. Thus, by dispersing spores while feeding, this predator spreads disease. Many invertebrates are parasitized by obligately killing parasites, offering a variety of systems for additional tests of this "predator-spreader" hypothesis. In the meantime, this planktonic disease system prompts a very important, general warning: before we use predators to keep the herds healthy, we need to carefully think about the interface between predator feeding biology and the underlying epidemiology of wildlife disease.