A common belief among ecologists is that, in a way, predators help their prey by removing the weak and sick from the population and reducing the chance of disease spread. But that idea, known as the "healthy herds hypothesis," has been challenged by recent data, published in Functional Ecology, suggesting that some water fleas (Daphnia dentifera) are better at defending themselves against predators but are at great risk of parasitic infections. When Daphnia sense certain distinct chemicals exuded by their predators, they grow larger, making it more difficult for the predators to eat them. But larger Daphnia, it turns out, also consume greater quantities of a deadly yeast parasite, known as Metschnikowia. Furthermore, once inside a larger Daphnia, the parasite appears to release more spores, which go on to infect other water fleas.
“While some have argued for increasing predator densities to control disease, our results suggest that it is important to consider the indirect effects of predators, such as the one we found, in which trying to avoid one enemy increases the hosts vulnerability to another,” Meghan Duffy, assistant professor in Georgia Tech’s School of Biology, said in a press release. “They're sort of stuck between a rock and a hard place.”