No abstract

Interactions among competition, predation, and disturbance in determining the abundances of four species of anurans were studied in a factorial experiment using 36 replicated experimental ponds. Hatchlings of the four species (Rana utricularia, Scaphiopus holbrooki, Bufo americanus, and Hyla chrysoscelis) were introduced at the same relative abundances at a low or a high initial density. Competition determined survival, body size at metamorphosis, and length of larval period in high-density communities, which were food-limited. The predatory salamander Notophthalmus viridescens did not alter either the total number of metamorphs or their combined biomass in the high-density communities, but the relative abundance of metamorphs was shifted as Scaphiopus holbrooki, the competitive dominant, was selectively eaten. In low-density communities, predation decreased survival and biomass production of tadpoles, often to zero, in all species. For each of the four combinations of tadpole density and presence or absence of predators, ponds were drained at three rates to simulate different drying regimes. Competition slowed growth and thus increased the risk of desiccation in high-density populations in drying ponds. Predation ameliorated the effects of competition, allowing survivors to grow rapidly enough to metamorphose before ponds dried.Survival of tadpoles to metamorphosis, body size at metamorphosis, and the timing of metamorphosis were determined in a complex way by interactions among all of the treatment effects and the life history characteristics of the four species studied. Scaphiopus was the species least sensitive to tadpole density and was the competitive dominant in ponds without newts. It has a rapid growth rate and metamorphosed soon enough to escape desiccation. It suffered the greatest risk of predation and was eliminated from even some of the high-density communities. Rana was most successful in low-density communities without predators. No Rana survived in rapidly drying (50-d) ponds because of insufficient time to obtain a minimum size for metamorphosis. Rana were also eliminated from most populations exposed to predation. The effect of competition on Bufo in high-density populations, few or no survivors, was reversed by predation as newts selectively fed on Scaphiopus and Rana. This result was most striking in the tanks that dried most rapidly. Hyla did very poorly in all slowly drying ( 1 00-d) tanks compared with controls because of intense competition. It had moderate success in high-density communities where newts had removed most competitors.These results show that biological and environmental factors interact to determine the structure of anuran communities. Neither competition nor predation is the single unifying force, but rather they interact to determine the different consequences of the date of drying of a pond to the success of each species.

Populations of six species of amphibians were manipulated in field enclosures to study the biological tractability of current concepts of . the organization of natural communities. Experimental communities with a known composition of mature eggs were introduced into screen enclosures in a pond to assay the importance of competition and predation to the ecology of amphibian larvae in temporary ponds. The competitive ability of each population was measured by its survivorship, mean length of its larval period, and mean weight at metamorphosis. Three simultaneous experiments (requiring 70 enclosures and 137 populations) were replicated in a randomized complete-block design for variance analysis.The assumptions of the classical Lotka-Volterra model of competition were tested by raising Ambystoma laterale, Ambystoma tremblayi, and Ambystoma maculatum in all combinations of three initial densities (0, 32, and 64). All three measures of competitive ability were affected by competition with other species. Higher-order interactions decreased the variance of the outcomes of the experiments as species were added to the communities. The statistical effects of these higher-order interactions between the densities of competing species often exceeded the simple effects of competition. The increase in community stability with the addition of species to the community is not predicted by the classical models of community ecology.The second experiment tested the effects of adjacent trophic levels on the structure of the three-species community. Eggs of Ambystoma tigrinum, a predator, and Rana sylvatica, an alternate prey of Ambystoma tigrinum, were added singly and together into systems with 16 eggs of species in the Maculatum species-group. Ambystoma tigrinum was a predator if it acquired an initial size advantage by preying on Rana sylvatica tadpoles; otherwise it was principally a competitor. Rana sylvatica adversely affected the Maculatum group by competing with invertebrate prey for periphyton and photoplankton. The three species in the Maculatum group had nearly the same response to the addition of both A. tigrinum and R. sylvatica.Ambystoma texanum, which occurs sporadically in southern Michigan at the northern limit of its range but not on the study area, was introduced as a test for community saturation. Ambystoma texanum was successfully raised alone. When mixed with the Maculatum group, Ambystoma texanum had a low survivorship, a small body size, and a long larval period. The native species were affected equally by the introduction of Ambystoma texanum, demonstrating the complexity of the food web and the ecological pliability of salamander larvae.The uncertainty of the temporary pond environment precludes extreme ecological specialization among these species of salamanders. Coexistence is a consequence of the relative advantages of the species in different years and the long adult life spans. The complexity of the food web and "predator switching" are probably important elements of the densitydependent interactions that...

The role of predators and competitors in the choice of oviposition site by the treefrog Hyla chrysoscelis was examined in a randomized complete block experiment using 90 replicated experimental ponds. Control ponds containing neither predators nor competitors were contrasted with treatment ponds into which one of four species ofpredtors (Ambystoma maculatum larvae, Enneacanthus chaetodon adults, Notophthalmus virzdescens adults, Tramea carolina larvae) or one oftwo species of competitors (Rana catesbeiana, Hyla chrysoscelis) was added.Treatments had significant effects on the mean number of eggs deposited in ponds.Fewer eggs were laid in ponds with Ambystoma, Enneacanthus, or Hyla, as a result of fewer females laying eggs and fewer eggs laid per visit, compared with control ponds.Notophthalmus, Rana, and Tramea had no effect on the number of eggs laid.Ovipositing Hyla discriminated among potential oviposition sites based on the species present. Choice ~f oviposition site can determine the success of a female's reproductive mvestment, and It can be a mechanism affecting the structure of ecological communities as well. Our results emphasize the importance of oviposition site choice in the evolution of~epr<_>ductive patterns and implicate species avoidance by ovipositing females as a mechamsm Important in generating variability in ecological communities.

A synthetic theory of the ecology of amphibian metamorphosis is founded on the observation that the large variation in length of larval period and body size at metamorphosis typical of a particular species of amphibian cannot be directly explained by differences in dates of hatching or egg sizes. It is proposed that as development proceeds, variation in exponential growth coefficients causes a trend from a normal distribution to a skewed distribution of body sizes. The degree of skewing increases and the median of the distribution decreases with increasing initial densities of populations. The relative advantages of the largest members of a cohort may arise from a variety of mechanisms including the production of growth inhibitors, interference competition, and size-selective feeding behavior. These mechanisms result in a nonnormal distribution of competitive ability, a possible source of the density-dependent competition coefficient found in systems with many species (1). In our model the ranges of body sizes and dates of metamorphosis are determined by a minimum body size that must be obtained and a maximum body size that will not be exceeded at metamorphosis. Between these two size thresholds the endocrinological initiation of metamorphosis is expected to be related to the recent growth history of the individual larva. Species that exploit uncertain environments will have a wide range of possible sizes at metamorphosis. Species exploiting relatively certain environments will have a narrower range. The evolution of neoteny and direct development logically follow from the application of these ideas to the ecological context of the evolution of amphibian life histories. Species that live in constant aquatic habitats surrounded by hostile environments (desert ponds, caves, high-altitude lakes) may evolve permanent larvae genetically incapable of metamorphosis. Other populations may evolve a facultative metamorphosis such that populations are a mixture of neotenes and terrestrial adults. Direct development results from selection to escape the competition, predation, and environmental uncertainty characteristic of some aquatic habitats and is usually accompanied by parental care. The relation between our ecological model and the physiological mechanisms that initiate metamorphosis can only be suggested and it remains an open problem for developmental biologists.