EFFECTS OF VARIATION IN BODY SIZE ON INTRASPECIFIC COMPETITION AMONG LARVAL SALAMANDERS

Abstract

I examined the effects of body size on competition among larvae of the salamander Atmbystoma opacuin. Two opposing hypotheses were tested: (1) size differences among competing individuals reduce the negative effects of competition on larval growth, and (2) size differences reflect a hierarchy in competitive ability in which larger larvae are superior competitors. When larvae preyed on macrozooplankton in the laboratory, a twofold difference in larval size had no effect on prey capture rate. In two other laboratory experiments I reared larvae of three size classes in pairs, and showed that the growth of small larvae was independent of the size of co-occurring individuals when the larvae competed exploitatively for macrozooplankton. Larger individuals negatively affected the growth of smaller indi- viduals only if the larvae physically interacted. The physical interactions included attempts at cannibalism. I also manipulated larval density and the sizes of co-occurring larvae in two field experiments. Although in both experiments I found adverse effects of high density on larval growth, larval growth was independent of the size of co-occurring nonspecific larvae. These results suggest: (1) variation in body size does not lead to a reduction in the negative effects of competition; (2) under purely exploitative competition for small prey, larger larvae are not superior competitors; (3) larger larvae do have a size advantage in physical interactions, but physical interactions may be minimized under field conditions even when density adversely affects larval growth.