Ontogenetic Scaling of Competitive Relations: Size‐Dependent Effects and Responses in Two Anuran Larvae

Abstract

This study examines size—dependent competitive interactions between larvae of two species of ranid frogs, the bullfrog (Rana catesbeiana) and the green frog (R. clamitans). Surveys of naturally breeding populations on an experimental pond site indicated that the two species often compete in the larval phase, and because both species may overwinter as larvae, interactions can occur between disparate size (year) classes. Using an additive design, I conducted field experiments manipulating densities of four species—class combinations (year I and II bullfrogs and green frogs) in the presence of low numbers of the other classes. The design permitted estimates of the effect of manipulated species/classes on different target classes and, because the latter were held constant, the effects of manipulated classes on themselves. Strong competitive effects were found within and between species/classes with one important exception. Neither class of green frogs affected small bullfrogs despite the fact that both had strong effects on large bullfrogs. Both classes of bullfrogs, however, effected both classes of green frogs. In general, smaller classes had a greater per unit biomass effect on target classes than larger classes and tolerated competitive effects better. Independent of size effects, bullfrogs appeared to be better competitors than green frogs because of other traits, presumably greater activity levels. I interpret these results in terms of a simple model of the ontogenetic scaling of foraging costs and gains. This model enables a mechanistic interpretation of the above results through the two components of competitive interactions, effect and response. Larger classes have greater per capita effects on resources, but because the gain curve is decelerating, smaller classes have greater per unit biomass effects. On the other hand, smaller classes respond less to competitors because net gain for these classes is higher or affected less by a decline in resources. I argue that to understand size—structured interactions in many taxa, we must first isolate the underlying ontogenetic or size—scaling component of competitive relations, and then add to these relations the effects of unique species traits.