Multiple predator effects on size‐dependent behavior and mortality of two species of anuran larvae

Abstract

This study examined the effects of multiple predators on size‐specific behavior and mortality of two species of anuran larvae. Particularly, we focused on how trait changes in predators and prey may be transmitted to other species in the food web. In laboratory experiments, we examined the effects of bluegill sunfish, Lepomis macrochirus, and the odonate larva Anax junius on behavior and mortality of tadpoles of the bullfrog, Rana catesbeiana, and the green frog R. clamitans. Experiments were conducted with predators alone and together to assess effects on behavior and mortality of the tadpoles. The experiments were replicated on five size classes of the tadpoles to evaluate how responses varied with body size. Predation rates by Anax were higher on bullfrogs than on green frogs, and both bullfrogs and green frogs suffered greater mortality from Anax than from bluegill. Bluegill only consumed green frogs. Predation rates by both predators decreased with increasing tadpole size and decreased in the non‐lethal (caged) presence of the other predator. Both anuran larvae decreased activity when exposed to predators. Bullfrogs, however, decreased activity more in the presence of Anax than in the presence of bluegill, whereas green frogs decreased activity similarly in the presence of both predators. The largest size class of green frogs, but not of bullfrogs, exhibited spatial avoidance of bluegill. These responses were directly related to the risk posed by the different predators to each anuran species. Anax activity (speed and move frequency) also was higher when alone than in the non‐lethal presence of bluegill. We observed decreased predation rate of each predator in the non‐lethal presence of the other, apparently caused by two different mechanisms. Bluegill decreased Anax mortality on tadpoles by restricting the Anax activity. In contrast, Anax decreased bluegill mortality on tadpoles by reducing tadpole activity. We discuss how the activity and spatial responses of the tadpoles interact with palatability and body size to create different mortality patterns in the prey species and the implications of these results to direct and indirect interactions in this system.