Effects of Habitat Size on Tadpole Populations

While density dependence has been well documented in many animal species, the effects of absolute size of habitat have not been thoroughly explored. This study investigates the effects of habitat size by examining the response of the distribution of several fitness correlates. I used populations of tadpoles of Bufo americanus and Rana clamitans in artificial ponds as a model system. The natural histories of these species suggested that they might respond differently to variation in habitat size, which was defined in terms of pond surface area and depth. These dimensions were factors in two completely crossed factorial experiments, one for each species. Ponds were of three areas: 529 cm², 2116 cm², and 8464 cm² and three depths: 10 cm, 20 cm, and 40 cm. Tadpole populations were established at a single density (number per unit volume). The use of artificial ponds controlled variation in nutrient content, shape, and vegetation structure that may be confounded with pond size in nature. Increases in pond area resulted in Bufo populations that had lower survival, growth rates, and mass at metamorphosis. Populations in ponds of large area showed greater within—population variation in larval period, although the mean of this parameter was only weakly affected. Size distributions of individuals showed positive skewness in small—area ponds. Increasing depth led to lower survival but greater mass at metamorphosis. In experiments with Rana, increased area and depth both led to lower survival but greater growth by survivors. There were statistically significant interactions between area and depth in their effects on early Bufo growth and on Rana survival. The fitness of Bufo showed a significant positive relationship with increasing surface—area—to—volume ratio while Rana showed a mixed response. The results suggest that competition among Bufo tadpoles intensifies with habitat size under conditions of constant initial density by volume because of a decreasing ratio of edge to interior habitat. The form of competition appears to shift towards stronger interference competition in smaller habitats. Individuals may compensate for early, negative effects of habitat size. The variability of the several fitness correlates, while different among populations, showed no consistent pattern with population size. Theory based on assumptions of equal levels of variation in fitness in large and small populations may not be widely applicable to populations in nature. Evidence of changes in competition and fitness distributions should be sought in other systems characterized by variation in habitat size.