Interactions of Food Level and Population Density in Rana Sylvatica

Tadpoles of the wood frog, Rana sylvatica were reared in the laboratory at four densities (1, 2, 4, and 8/dish) and at four food levels (1, 2, 4 and 6 multiples of a ration) in a randomized 4—block design for a factorial analysis of variance. Both food and density had significant effects on growth rate and body size at metamorphosis. The length of the larval period was significantly correlated (r=.81) between pairs of larvae reared together but was not significantly associated with either food or density. At low ratios of supply (food level) to demand (density level) body weight increased with food availability. At high supply: demand ratios the excess food caused a pollution effect which reduced growth, body size at metamorphosis and survival. At different densities maximum growth was achieved at different supply: demand ratios. In natural populations, body size decreases with density as if food were in short supply.