Density-Dependent Fitness Attributes and Carry-Over Effects in Crawfish Frogs (Rana areolata), a Species of Conservation Concern

Abstract

Fitness attributes acquired in aquatic habitats by amphibians exhibiting complex life histories have been shown to cascade through terrestrial juveniles into adulthood, a phenomenon termed carry-over effects. We explored density-dependent fitness attributes and carry-over effects in Crawfish Frogs (Rana areolata) using a set of field enclosure experiments and a series of field data. Using field enclosures, we hypothesized that 1) at high densities, intraspecific competition would produce smaller Crawfish Frog juveniles that took longer to metamorphose; 2) at high densities, interspecific competition would also produce smaller Crawfish Frog juveniles that took longer to metamorphose; and 3) vertebrate (ambystomatid salamander) predation on Crawfish Frog larvae would reduce survivorship, but by releasing competition pressure would produce relatively larger tadpoles that metamorphosed earlier. Further, we hypothesized 4) that these enclosure results would apply to field data, and that fitness attributes in newly metamorphosed Crawfish Frogs would carry over to first-time breeding adults. Our results confirmed all four hypotheses. Specifically, in Crawfish Frogs, at high densities, both intra- and interspecific competition reduced size (length and mass) at metamorphosis (hypotheses 1 and 2), and predation reduced survivorship and increased size at metamorphosis (hypothesis 3). Finally, we observed density-dependent fitness effects on newly metamorphosed Crawfish Frog juvenile size (length and mass), and carry-over effects from the larval stage on juvenile survival, adult size, and breeding adult numbers (hypothesis 4). In the absence of predators, high densities of intra- and interspecific competitors had no effect on Crawfish Frog larval survivorship. We also present suggestive evidence for compensatory effects. We discuss the potential mechanisms underlying the patterns of these interactions, as well as the role of these relationships in informing management guidelines intended to ensure the future of this species of conservation concern.