Tolerance rather than competition leads to spatial dominance of an Antarctic bryozoan

Understanding the processes that govern species distributions is a fundamental goal of ecology. Species differ in patterns of resource-use due to differences in response to biotic and environmental factors. Some species dominate favourable habitat through competitively superiority, while others have evolved to exploit niches of abiotic stress where competition is low. We observed interspecific differences in the use of horizontal and vertical habitats by Antarctic bryozoa. The most spatially dominant species, Inversiula nutrix, mainly occupied horizontal surfaces, while nine other bryozoan species mainly occupied vertical surfaces. We hypothesized that this spatial segregation may be due to either competitive displacement of other species by I. nutrix, or tolerance of I. nutrix to an environmental stress associated with horizontal orientation. To investigate this we quantified competitive interactions between all species, and conducted a manipulative field experiment to test effects of sedimentation on bryozoan growth. We found that I. nutrix was not a superior competitor, but was more tolerant of sedimentation than were other species. Despite mediocre competitive ability, tolerance to a stressor has allowed I. nutrix to occupy resources unavailable to the other species, and thereby gain spatial dominance within the community.