"There is a trade-off between resistance and reproduction because any resources an animal devotes to defense are not available for reproduction. When ecological factors favor small epidemics, it is better for hosts to invest in reproduction rather than defense."
In addition to Duffy, also contributing to this study are Indiana University biologists Spencer Hall and David Civitello; Christopher Klausmeier, a plant biologist at Michigan State University; and Georgia Tech researchers Jessica Housley Ochs and Rachel Penczykowski.
For the study, the researchers monitored the levels of nutritional resources, predation and parasitic infection in seven Indiana lakes on a weekly basis for a period of four months.
They calculated infection prevalence on live hosts using established survey methods, estimated resources by measuring the levels of phosphorus and nitrogen in the water, and assessed predation by measuring the size of uninfected adult Daphnia.
The researchers also conducted infection assays in the laboratory on Daphnia collected from each of the seven lake populations at two time points: in late July before epidemics began and in mid-November as epidemics waned
The assays measured the zooplankton's uptake of Metschnikowia bicuspidata and the infectivity of the yeast once consumed.
The infection assays showed a significant evolutionary response of Daphnia to epidemics in six of the seven lake populations.
The Daphnia population became significantly more resistant to infection in three lakes and significantly more susceptible to infection in three other lakes.
The hosts in the seventh lake did not show a significant change in susceptibility, but trended toward increased resistance.
In the six lake populations that showed an evolutionary response, epidemics were larger when lakes had lower predation and hig
|Contact: Cheryl Dybas|
National Science Foundation