Elevated habitat quality reduces variance in fish community composition

Understanding the intrinsic variability of habitat-specific faunal communities is important to species conservation and ecosystem management. Community variability is driven by many environmental factors, including density-dependent habitat selection. Extensions of MacCall's Basin Model of density dependent habitat selection indicate that variance of a species' density is generally expected to be independent of habitat quality while variance in community composition is expected to decrease with habitat quality. We used these expectations to investigate variability of nearshore fish communities. Fish collections were conducted biweekly in eelgrass (high quality) and unvegetated (low quality) habitats from early summer to late autumn from 1996 to 2009. Higher fish density and biomass, taxa richness, and elevated variability of these measures were associated with the higher quality habitat. Seasonality explained the greatest variance in the fish community followed by habitat quality and year. Consistent with expectation, variability in community composition was less in the high quality habitat. In addition to differences in variability, cyclical seasonal transitions were less predictable and directional multi-year shifts in community structure were more pronounced in the low quality habitat. Our results show that community variability is associated with suboptimal or degraded habitats; a finding of significance given the increased prevalence of anthropogenic disturbance in ecosystems globally.