Competitive Interactions of Two Mobile Prey Species in a Patchy Environment

Abstract

A 6—mo field experiment was conducted to estimate the potential for, and relative strengths of, competition within and between two species of marine reef—dwelling gastropods, Tegula aureotincta and Tegula eiseni. Both intra— and interspecific effects were detected on individuals growth and reproductive status of these snails within the range of densities naturally encountered. Survivorship was only slightly lowered even by prolonged exposure to maximum snail density. Examination of the relative strengths of the interactions revealed that each Tegula species depressed the individual growth of the other to the same degree. However, the relative strengths of intraspecific effects differed. Higher densities of T. aureotincta depressed the growth of individual T. aureotincta more than that of T. eiseni, whereas densities of T. eiseni displayed the opposite pattern: a greater influence on the other species than on conspecifics. The relative impacts of intra— and interspecific competition did not conform to expectations derived from logistic—based models of competitive coexistence. Long—term estimates of population densities of the two Tegula species revealed that both species fluctuated little in abundance over 6 yr, suggesting that the two species indeed coexist, since the span of observation was longer than the longevity of individual snails. However, detailed examination of the microdistribution of Tegula species indicated that they co—occurred within 0.1—m2 quadrats less often than expected by chance. The significant negative correlation in abundance of the Tegula species within the cobble reef is a pattern consistent with the operation of interspecific competition. Previous work has indicated that Tegula—consuming predators influence the among—reef patterns of distribution and abundance of both gastropod species: the mean abundance of both snails were inversely correlated with density of predators and, as a consequence, were positively correlated with each other among reefs. Results presented here suggest that interspecific competition, where it occurs, may lead to a negative correlation in abundance of the two gastropod species within a reef. Both predation and interspecific competition may simultaneously influence the local distribution of the Tegula species, but in opposite manners and at different spatial scales.