FACTORS MODIFYING ANTIPREDATOR BEHAVIOR IN LARVAL TOADS

This study examines the behavioral responses of larvae of the American toad, Bufo americanus, to the presence of an invertebrate predator, l arvae of the dragonfly Anax junius. In particular, we examine whether responses to predators are categorical ones to some threshold level of predation risk or alternatively whether the responses are adjustable depending on the level of predation risk. If the responses are graded, does the response vary according to what cues about predation risk ar e available to prey? Because avoiding predation can incur costs in ter ms of resource acquisition, does food availability affect the response s of toad larvae to their predators? Finally, because predation risk i s a function of body size, we also examined whether the response to pr edator presence was affected by size of toad larvae. We addressed thes e questions by measuring the activity levels and habitat use of toad l arvae in the presence and absence of predators while manipulating pred ator density, whether the predators had fed, food availability, and la rval toad size. In an additional experiment, we also measured activity and movement speed of toad larvae when we manipulated food availabili ty and predator density. We found that toad larvae were less active as predator density increased and as food availability increased. Small toad larvae were more sensitive to the presence of predators than were large larvae. Although toad larvae did reduce activity by 35% in the presence of fed predators as opposed to unfed predators, the result wa s not statistically significant. Toad larvae did avoid predators, but the level of avoidance could not be shown to vary with predator densit y, food availability, or body size. However, there was a borderline ef fect of predator feeding status. Movement speed was not significantly related to either food availability or predator density.