Diet comparison in three tadpole species, Rana sylvatica, Bufo americanus, and Pseudacris crucifer, in a northern temperate climate

The natural diet of northern temperate tadpoles is a largely neglected area of study. We investigated the natural diets in three anuran larvae, the wood frog (Rana sylvatica), the American toad (Bufo americanus), and the spring peeper (Pseudacris crucifer), from several ephemeral ponds in northern Minnesota. Previous laboratory studies suggest that these species, as well as others with similar mouthparts, are herbivorous suspension or filter feeders. Our results suggest that all three species are active grazers upon the periphyton (aüfwichs or algal material) found on aquatic vegetation and submerged substrates and detritus. The diet of these tadpoles was primarily composed of detritus: 73.3 % in B. americanus, 73.9 % in R. sylvatica, and 82.1 % in P. crucifer. R. sylvatica consumed a statistically greater proportion (by volume) of invertebrate foods than the other tadpole species, lending support to their suggested predacious feeding behavior. However, this pattern is most likely biologically insignificant, as these foods made up < 2 % of the diet. The remainder of the identifiable organic foodstuffs were counted and analyzed to find possible interspecific trends. Diatoms made up the highest proportion of these identifiable items; diatoms contain large quantities of lipids that tadpoles may utilize as an energy source. The green and bluegreen algae comprised the majority of the remaining identifiable foodstuffs. These data bring into question the nutritional contribution made by detritus and periphyton to the development of tadpoles in the life history of the frog, and the trophic positioning of anuran larvae in ephemeral aquatic ecosystems. Introduction The natural diet of common northern temperate frog and toad (anuran) larvae is a topic that has received little attention. There have been many studies into the feeding strategies of tadpoles under laboratory conditions (e.g., Seale et al 1980); however, few have considered the natural diet of these organisms (e.g., Jenssen 1967, Dickman 1968). The general view has been that anuran larvae are herbivorous filter or suspension feeders (Duellman and Trueb 1986). Diet is especially important in tadpoles because many species are in short-lived aquatic environments (i.e., ephemeral ponds). This places additional constraints on these tadpoles to consume foods that will ensure they reach metamorphosis prior to the drying of the pond. Some tadpoles rely on carnivory of other tadpole species to reach their metamorphic state. Petranka et al (1994) suggest that R. sylvatica tadpoles are facultative predators on B. americanus tadpoles, resulting in differential breeding pond selection by adult B. americanus. More recently, McDiarmid and Altig (1999) have suggested that many tadpoles are grazers, feeding from the substrates in aquatic systems. We studied the natural larval diets of species from three different families: the wood frog (Rana sylvatica Ranidae); the American toad (Bufo americanus Bufonidae); and the spring peeper (Pseudacris crucifer Hylidae). Understanding the diet and natural history of these organisms is crucial for gaining insight into their complex life histories and interactions. We also investigated pond use among tadpole species and whether these species had differing diet compositions. Materials and Methods The following described field methods were carried out at Long Lake Conservation Center (LLCC) in Pallisade, Minnesota in May and June, 2001. LLCC comprises 760 acres of pine forests and peat bogs