A behavioral trade-off and its consequences for the distribution of Pseudacris treefrog larvae

Ecology, Jan, 1995 by David K. Skelly

INTRODUCTION

One of the primary goals of community ecology is understanding the causes for changes along geographic or environmental gradients. Although experiments have often been used to identify the processes maintaining structure at particular locales, manipulations have been used less often to determine patterns of distribution along gradients (notable exceptions include Connell 1961, Paine 1966, Simberloff and Wilson 1969, Lubchenco 1978, Sousa 1979, Tilman et al. 1981). Among larval anurans, distributions have been found to covary strongly with permanence of aquatic habitats (Collins and Wilbur 1979, Dale et al. 1985). In this study I focus on two species, chorus frogs (Pseudacris triseriata) and spring peepers (P. crucifer), which are quite similar in most aspects of their life histories but show striking differences in their distributions across the pond permanence gradient (see Methods: System).

There have been numerous experiments on competition and predation in communities of larval anurans and their predators (reviewed by Wilbur 1980, 1984, Gurevitch et al. 1992). These studies suggest that the interaction between the intensities of competition and predation contributes to the sorting of amphibian species along the pond permanence gradient. This is because (1) more permanent ponds tend to have more predators (Smith 1983, Woodward 1983, Skelly 1992b), (2) predation tends to reduce the impact of interspecific competition (Morin 1983, Wilbur 1987), and (3) there appears to be an inverse relationship between competitive ability and susceptibility to predators among larval anuran species (Woodward 1982, Morin 1983, Wilbur 1987). However, relatively few anuran studies have been conducted in natural ponds, and of those, almost none have been conducted across a gradient of pond types (for exceptions see Smith 1983, Smith and Van Buskirk, in press). Consequently, the relevance of ideas developed from artificial pond studies to natural communities remains an open question (Jaeger and Walls 1989, Morin 1989, Wilbur 1989). For this reason I conducted a field manipulation to evaluate the roles of competition and predation in determining larval performance of spring peepers and chorus frogs across a set of natural ponds that varied in permanence.

A second novel component of my research concerns an exploration of a behavioral trade-off that potentially underlies patterns of tadpole performance and distribution. Foraging animals can be faced with a conflict between procuring resources and surviving; this trade-off has been found for a diversity of organisms because behavior associated with acquisition of resources often engenders higher rates of mortality from predators (reviewed by Lima and Dill 1990). For anuran larvae this conflict may be mediated through activity (proportion of time spent moving) since there is evidence that activity of larval anurans is related to both growth rate (e.g., Skelly and Werner 1990) and predation risk (e.g., Woodward 1983). If this is true, then interspecific differences in activity may have important ramifications for patterns of larval performance and distribution of species among ponds that vary in permanence and composition of predators.

The aim of this research is to evaluate the possibility that mechanisms operating at the level of the individual explain differences in the impacts of competition, pre-dation, and abiotic factors (e.g., pond drying) on the success of different anurans and, ultimately, explain distribution patterns.

METHODS

System

In southeastern Michigan, chorus frogs and spring peepers are the only spring breeding treefrogs. In many important respects the two species are quite similar. Breeding typically commences in late March and development of the aquatic larvae lasts 2-3 mo (Collins 1975). After metamorphosis into a terrestrial form, individuals reach maturity within 1-2 yr and the species are comparable in adult body size, egg size, and clutch size (Collins 1975).

Chorus frogs and spring peepers typically breed in ponds that range from those that dry in early summer each year, to those that are permanent on the order of decades (Collins and Wilbur 1979). Across this gradient of pond permanence the density of tadpole predators can triple (Skelly 1992b). Composition of the predator fauna also changes, with ephemeral ponds containing primarily small invertebrate predators, and more permanent ponds containing larger predators and even fish (Skelly 1992b; D. K. Skelly, unpublished data). Although larvae of the two Pseudacris species co-occur in many ponds, the overall distributions of the species show strong differences. In samples from southeastern Michigan collected in 22 ponds for up to 4 yr (Skelly 1992b; D. K. Skelly, unpublished data), larval chorus frogs were more abundant than spring peepers in ponds that dried each year and completely absent from permanent ponds where spring peepers were usually present and often abundant. The tendency for spring peeper larvae to be distributed in more permanent and/or more predator rich environments than chorus frogs has also been reported in North Carolina (Alexander 1965), Indiana (Whitaker 1971), and northern Michigan (Smith 1983).