Density‐Dependent Aspects of Metamorphosis in Ambystoma and Rana Sylvatica

Two species of salamander (Ambystoma) and a frog (Rana sylvatica) were raised at a variety of initial densities in enclosures in a pond to study the effect of population density on survival and body size at metamorphosis. The relationship between the probability of survival and the initial density of the population was fit to two alternative models: a linear model which assumes equal competitive ability among individuals in a population and predicts no survivors from populations with very high densities, and a negative exponential model which assumes unequal competitive abilities and predicts that few individuals will dominate the highest density populations and successfully complete metamorphosis. The negative exponential model fits the data better in four out of five experiments but it is significantly better in only one experiment. The prediction of a few survivors even at very high densities is true in all experiments. Mean body size at metamorphosis decreases exponentially as initial density is increased. These data are interpreted as the result of a functional link between body size and competitive ability. As density increases fewer individuals are able to obtain sufficient resources to to successfully complete metamorphosis. These experiments demonstrate very high variances between simultaneous replicates in the same pond. This variance reduces the ability of field experiments to test general models and demonstrates the importance of environmental heterogeneity and historical effects in the outcome of population interactions.