Predator-induced behavioral indirect effects: Consequences to competitive interactions in anuran larvae

This study examines the non-lethal effects of an odonate predator (Anax junius) on the competitive interactions among several size classes of anuran larvae, In an outdoor experiment using cattle watering tanks, we estimated the effects of both large and small bullfrogs (Rana catesbeiana) on themselves, on each other, and on small green frogs (R. clamitans) in the absence and non-lethal (caged) presence of Anax. The presence of Anax depressed both growth and survivorship of small bullfrogs and green frogs. In contrast, the presence of Anax had positive effects on growth rates and size at metamorphosis of the large bullfrogs. Increasing density of competitors also decreased survivorship of small classes, and growth rates of all classes. The per-unit-biomass competitive effects of the small bullfrogs on target classes were much greater than those of large bullfrogs. The presence of Anax significantly altered the per-unit-biomass competitive effects of small bullfrogs but not large bullfrogs, presumably because individuals in the small class reduced their activity rates in the presence of Anax. Overall production of new tadpole biomass was quite similar across experimental units, with decreases in production of small size classes in the presence of Anax compensated for by increases in production of the large size class. Thus the non-lethal presence of Anax had substantial effects on the nature of competitive interactions in this system, and we discuss the implications of such behavioral indirect effects in the study of ecological communities; Our results also illustrate the futility of attempting to partition the effects of competitors and predators, as both competitors and the non-lethal presence of predators significantly affected growth rates and death by starvation of small larvae. Finally, our results illustrate how individual behavioral responses may be translated to community and ecosystem properties.