When battling an epidemic¹ of a deadly parasite², less resistance can sometimes be better than more, a new study suggests. A freshwater zooplankton（浮游动物） species known as Daphnia dentifera endures periodic epidemics³ of a virulent⁴（剧毒的） yeast⁵ parasite that can infect more than 60 percent of the Daphnia population. During these epidemics, the Daphnia population evolves quickly, balancing infection resistance and reproduction.

A new study led by Georgia Institute of Technology researchers reveals that the number of vertebrate（脊椎动物的） predators⁶ in the water and the amount of food available for Daphnia to eat influence the size of the epidemics and how these "water fleas⁷" evolve during epidemics to survive.

The study shows that lakes with high nutrient⁸ concentrations and lower predation levels exhibit large epidemics and Daphnia that become more resistant⁹ to infection by the yeast Metschnikowia bicuspidata. However, in lakes with fewer resources and high predation, epidemics remain small and Daphnia evolve increased susceptibility to the parasite.

"It's counterintuitive to think that hosts would ever evolve greater susceptibility to virulent parasites¹⁰ during an epidemic, but we found that ecological¹¹ factors determine whether it is better for them to evolve enhanced resistance or susceptibility to infection," said the study's lead author Meghan Duffy, an assistant professor in the School of Biology at Georgia Tech. "There is a trade-off between resistance and reproduction because any resources an animal devotes to defense¹² are not available for reproduction. When ecological factors favor small epidemics, it is better for hoststo invest in reproduction rather than defense."

This study was published in the March 30, 2012 issue of the journal Science. The research was supported by the National Science Foundation and the James S. McDonnell Foundation.

In addition to Duffy, also contributing to this study were Indiana University Department of Biology associate professor Spencer Hall and graduate student David Civitello; Christopher Klausmeier, an associate professor in the Department of Plant Biology and W.K. Kellogg Biological Station at Michigan State University; and Georgia Tech research technician Jessica Housley Ochs and graduate student Rachel Penczykowski.