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A Behavioral Trade‐Off and Its Consequences for the Distribution of Pseudacris Treefrog Larvae

Skelly, David K.
Ecology 1995 v.76 no.1 pp. 150-164
Pseudacris, biogeography, drying, field experimentation, foraging, frogs, interspecific competition, interspecific variation, laboratory experimentation, ponds, predation, predators, risk, spring, stocking rate, survival rate, tadpoles
Larval chorus frogs (Pseudacris triseriata) and spring peepers (P. crucifer) show marked differences in their distribution patterns, with chorus frogs tending to occur in less permanent ponds containing lower predator densities, and spring peepers tending to occur in more permanent ponds with higher predator densities. In order to understand this pattern I performed experiments at two levels. First, at the community level, I evaluated the importance of competition, predation, and pond permanence for patterns of larval performance. Second, at the behavioral level, I asked whether a hypothesized trade—off between gain and risk underlies ecological interactions, and ultimately, patterns of performance and distribution of these species. I employed a field manipulation to asses the contribution of competition and predation within different ponds that varied in permanence. In order to examine the effect of interspecific competition, enclosures contained each tadpole species either alone or in the presence of an equal density of the other species; these enclosures were placed in two temporary ponds, two intermediate ponds, and two permanent ponds. Tadpole stocking densities reflected local natural densities. This competition treatment was crossed with a predator treatment manipulating the absence or presence of predators (with "presence" representing the naturally occurring density within each pond). Patterns of relative performance were consistent with relative distribution patterns across the pond permanence gradient: chorus frogs performed best relative to spring peepers in temporary ponds while spring peepers performed best relative to chorus frogs in permanent ponds (performance was measured as survivorship and growth). The dominant sources of mortality were pond drying in temporary ponds and predation in more permanent ponds. In contrast to most previous experimental studies of larval anurans, competition had small effects on tadpole performance. Results from the field experiment suggest that pond drying and predation may be important determinants of performance, and that the two species are differentially susceptible to these mortality agents. In a set of laboratory experiments I evaluated the hypothesis that a trade—off between foraging gain and predation risk mediated through activity (proportion of time spent moving) might explain patterns of Pseudacris distribution. In both species, individuals facultatively reduced activity in the present of a caged predator with a corresponding drop in growth rate. In general, larval chorus frogs also tend to be more active and faster growing and developing than spring peepers. These results provide support for the existence of the trade—off and suggest that interspecific differences in activity may contribute to patterns of larval performance and distribution among ponds. The general nature of this trade—off suggests that it may also be important in other systems.