Rapid human induced environmental changes may affect ecological community structure by decreasing the number of species and by homogenizing community composition. Long term monitoring efforts can be useful for detecting if these changes are threating communities. However, it has not been appreciated that the detected community responses may be strongly scale dependent. We wanted to determine how the biodiversity of fish communities off the Southeast Atlantic coast of the US have changed over time when scale-dependent effects have been taken into account. We analyzed data provided by the South Carolina DNR’s Southeast Atlantic Monitoring and Assessment Program (SEAMAP) trawl surveys conducted by SEAMAP from Cape Hatteras, NC to Cape Canaveral, FL (1989 to 2016). We used data on the recorded abundance of each fish species caught in trawls conducted from 1990-1995 (historic) and from 2010-2015 (modern) to compute the degree of change in average and total species richness, individual density, degree of evenness, and the estimated size of the species pool. Additionally we compared species accumulation curves to estimate the relative importance of changes in the species abundance distribution (SAD), individual density, and intraspecific aggregation on the detected changes in richness.
In contrast with our expectations, we detected no difference in average species richness between the historic and modern data, and species in the modern communities appeared patchier rather than more homogenous than the historic community. But we did observe that the modern communities had significantly higher numbers of individuals than the historic assemblage which may reflect a shift in fish community size structure, conservation efforts, and increased sampling efficiency. Evenness was significantly higher in the historic community. The species accumulation curves provided greater insight in the community structure changes relative to univariate statistics. Specifically we observed that the shifts in evenness primarily emerged at larger spatial scales. Our results suggest that although species richness has not changed appreciable over the past twenty years in our fish communities, non-trivial but less studied components of fish diversity, namely abundance and evenness, have. Additionally our multiscale approach allowed us to detect at what spatial scales these effects were most prevalent which will be helpful in determining the processes underlying these changes.