Do stream mayflies exhibit trade-offs between food acquisition and predator avoidance behaviors?

Abstract

Organisms often experience trade-offs between behaviors necessary for food acquisition and predator avoidance. In mesocosms and microcosms, we manipulated the risk and resource environments of grazing mayflies (Baetis bicaudatus) common in streams near the Rocky Mountain Biological Laboratory (RMBL) in western Colorado, USA. We exposed mayflies to treatments with or without chemical cues from predatory brook trout (Salvelinus fontinalis) and high or low levels of benthic algal resources to test the nonconsumptive effects of predators and food availability on grazer foraging behavior, predator-avoidance behavior, growth rates, and the effect of grazers on accrual of algal food resources. Total drift (daytime + nighttime) of Baetis was significantly reduced under conditions of fish cues and high food levels. However, daytime drift, a risky food-search behavior, was negligible in all cases except the no-fish-cue/low-food treatment. Therefore, predator-induced nocturnal periodicity of drift behavior was influenced by variation in food levels. At low-food levels, Baetis growth rates and grazer effect on algal accrual were reduced, but neither measure of grazer performance (growth rates nor grazer impacts) was affected by fish cues or their interaction with food levels. We conclude that mayflies faced with potential trade-offs between food acquisition and predator-avoidance behaviors exhibit risky behavior (high daytime drift) only when predator-risk cues are absent and resources are low. Moreover, we observed no short-term costs of predator-induced suppression of foraging movements under low-resource conditions. These results imply that previously observed negative effects of trout on size and fecundity of mature Baetis are not explained by slower growth rates.