Competition is stronger between than within species in two coexisting hermit crab species

Abstract

Both intra- and interspecific competition influence the partitioning of limited resources; the relative importance of each type of competition in resource distribution is influenced by several factors, including resource availability and population abundance. Several hermit crab species of the genera Calcinus and Clibanarius coexist, competing for the shells of the same gastropod species. The relative abundance of these species varies along the intertidal gradient, from a bias toward Clibanarius in the upper intertidal to a tendency toward Calcinus in the lower intertidal. We assessed the strength of intra- versus interspecific competition in Cli. albidigitus and Cal. californiensis by comparing the number of successful usurpations of preferred shells in single-species versus mixed-species trials. We performed these comparisons under four scenarios, varying the total individuals and their relative abundance (1.7:1, 1:1, or 1:1.7 ratio) to simulate demographic conditions along the intertidal gradient. There were more shell usurpations in mixed-species trials under all scenarios except for the 1:1 ratio at high total abundance. Calcinus californiensis usurped more shells than Cli. albidigitus in all mixed-species treatments, even in cases where Cli. albidigitus was more abundant. Our results show that interspecific competition is stronger than intraspecific competition, which could drive the competitive exclusion of Cli. albidigitus, the comparatively weaker species.