The effect of an invasive foundation species on diversity is due to increased habitat availability

Abstract

Both foundation species and introduced species have exceptional influence on community diversity and structure, though they have historically been thought to have opposite effects. However, when introduced species can provide novel habitat within a community, their benefit to the community as a foundation species may outweigh their cost as an invader. The magnitude or direction of species interactions can depend on the environment, thus the net effect of introduced foundation species can vary across different environmental factors. A settlement tile experiment was conducted on the fouling communities of California harbors to determine whether the effects of a widespread invasive bryozoan, Watersipora subtorquata, vary between two locations in its California range, Alamitos Bay and Bodega Harbor. Treatments with live and dead colonies and two sizes of blank tiles were used to evaluate the effects of colony structure and available bare substrate on both mobile and sessile species in the community. Though mobile invertebrate communities in each location across treatments were not significantly dissimilar to each other, communities on large tiles were significantly dissimilar to other communities in each location. Mobile species richness was greatest on live colony and small tiles in Bodega, but greatest on dead colony and large tiles in Alamitos. Both the diversity and community structure of sessile invertebrates across treatments differed significantly between regions. Diversity was greatest on dead colony tiles in Alamitos, where communities were more dissimilar across treatments, but was greatest on live W. subtorquata tiles in Bodega where communities were less dissimilar across treatments. These results highlight the potential for W. subtorquata to have location-dependent net effects on the community throughout its invasive range as a foundation species.