Effects of Variation Body Size on Intraspecifc Competion among Larval Salamanders

I examined the effects of body size on competition among larvae of the salamander Ambystoma opacum. 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 their 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—occuring individuals when the larvae competed exploitatively for macrozooplankton. Larger individuals negatively affected the growth of smaller individuals 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—occuring conspecific 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 not have a size advantage in physical interactions, but physical interactions may be minimized under field conditions even when density adversely affects larval growth.