We used two field experiments to examine the nature and effects of size-dependent interactions between two terrestrial salamanders, Plethodon cinereus and Plethodon glutinosus. We reared juveniles of P. cinereus in an enclosure alone, with a similarly-sized conspecific, with a similarly-sized P. glutinosus, and with a larger P. glutinosus. We also reared juveniles of Plethodon glutinosus alone. We found that both intraspecific and interspecific competition reduced growth in juveniles of P. cinereus, but the effect of intraspecific competition did not differ from that of interspecific competition. Interspecific competition reduced growth in juveniles of P. glutinosus. Size asymmetry did not appear to affect competition between juveniles. Size, not species identity, determined the use of the space beneath wooden cover objects placed in the enclosures.
In the second experiment, we housed adults of P. cinereus and juveniles of P. glutinosus with similarly-sized conspecific individuals to simulate intraspecific competition. Adults of P. cinereus were also paired with similarly-sized, smaller, and larger individuals of P. glutinosus. Interspecific competition did not affect growth differently than intraspecific competition for P. cinereus, and the size of the competitor did not affect the growth of this species. Interspecific competition reduced the growth of P. glutinosus significantly more than intraspecific competition and had the greatest effect when competing individuals were similarly-sized.