Predators influence prey population dynamics in two basic ways. Predators can alter prey densities by killing and consuming prey. Predators can also induce behavioral, morphological, or life historical changes in prey. Because these trait shifts may entail significant costs, their induction may indirectly influence population growth rates. It has been speculated that the magnitude of these nonconsumptive effects of predators on prey population dynamics may rival the direct, lethal effects of predators on prey populations. Although there are a number of studies assessing the short-term costs of predation risk on prey growth or reproductive effort, there are virtually no multigenerational studies of predation risk and prey population growth. Using outdoor mesocosms, we investigated the long term effects of predation risk on population dynamics of Helisoma trivolvis, a freshwater pulmonate snail, in a variety of environmental conditions. We intimidated H. trivolvis with crayfish and pumpkinseed sunfish and manipulated the snail’s environment by altering competition and resource levels in this large scale, long term study.
Results from the first year of study show that predators had a strong influence on prey populations via non-consumptive mechanisms. Predation risk significantly increased refuge use by H. trivolvis, decreased growth rate, and delayed reproduction. At the end of the reproductive season, most populations in no-predator treatments had reproduced, but there was no evidence of offspring in any of the predation risk treatments. Surprisingly, mortality in the parental generation of Helisoma trivolvis was significantly elevated in predation risk treatments. These preliminary results suggest that predation risk not only induces trait shifts in H. trivolvis, but also has a substantial effect on population growth. Because we are focusing on the long term effects of predation risk on prey populations, this experiment will be run for a second reproductive season.