FORAGING RESPONSE TO RISKS OF PREDATION AND COMPETITION IN ARTIFICIAL POOLS

Although ecologists have learned much about the influence of competitors and perceived risk of predation on foraging in terrestrial systems by measuring giving-up density (GUD, the amount of food left behind in a resource patch following exploitation), GUDs have rarely been used in aquatic environments. Here we use foraging activity (proportion foraging) and GUDs to assess the effects that two periphyton consumers and potential competitors, green toad (Bufo viridis) tadpoles and mosquito (Culiseta longiareolata) larvae, have on each other. We also examine the effects of perceived risk of predation imposed by a dragonfly nymph (Anax imperator). To do so, we conducted an artificial pool experiment and developed a food patch appropriate for measuring GUDs for periphyton grazers. More Culiseta individuals foraged in rich food patches than in poor patches. Bufo showed a similar tendency. Fewer Bufo foraged in both patch types in the presence of caged Anax. Culiseta showed a similar tendency. However, in the rich patches, only Bufo reduced foraging activity when the caged predator was present. Both Bufo and Culiseta depleted food patches through exploitation, resulting in lower GUDs. Both competitors together resulted in lower GUDs than did food depletion of each species alone. However, the presence of caged Anax had little or no effects on GUDs. Overall, both Bufo and Culiseta respond to food and safety. They are able to direct foraging effort to richer patches and devote more time to those patches, and they respond to predation risk by choosing whether or not to exploit resource patches.