Density‐Dependent Aspects of Growth and Metamorphosis in Bufo Americanus

Larvae of Bufo americanus (Anura: Bufonidae) were reared at controlled densities and food levels in laboratory populations and at controlled densities in enclosures in a farm pond in one— and two—species populations. Survival during the larval period was independent of population density however, the proportion of the population that successfully metamorphosed was a negative, exponential function of density. This outcome is interpreted as the result of the effect of density on the growth rate of larvae. In high density populations, a few individuals grow at the expense of smaller members of the cohort, which then have a lowered probability of metamorphosis. In spite of the strong effect of environmental heterogeneity in field experiments, the mean size at metamorphosis was significantly decreased as the initial density of either Bufo (—0.00031 g—body wt per unit increase in density) or Rana palustris (—0.00020 g—body wt per unit increase) was increased. Rana palustris frequently breeds at the same time and in the same ponds as B. americanus in the Piedmont of the Carolinas. At low densities there is an Allee effect in which body size increases with population densities between 1 and 20 individuals. As food becomes limiting, body size decreases with density in crowded populations.