Effects of increasing impervious surface cover on macroinvertebrate taxonomic and trait diversity

Background/Question/Methods Macroinvertebrates in streams can be very sensitive to catchment urbanization, particularly to the presence of impervious surfaces.  Recent research shows that many macroinvertebrate taxa do not persist in streams in catchments with >2% impervious surface. However, some studies suggest that alpha diversity does not decline with increasing impervious surface coverage, suggesting species immigrate into the disturbed sites. The loss of macroinvertebrate taxa that are sensitive to impervious surface could result in the loss of taxa with unique traits, causing a decline in trait diversity from stream communities. We examined the relationship of functional trait richness as well as alpha diversity of the macroinvertebrate community to the percentage of impervious surface near the sampling site for stream reaches sampled by the Maryland Biological Stream Survey between 2007 and 2013. Alpha diversity was calculated for each site by counting the number of macroinvertebrate genera present. Functional richness of the macroinvertebrate community were characterized by five traits, which included size class, trophic habit, maximum crawling speed, voltinism, and rheophily, that were coded as ordered categorical variables from which the functional richness was then calculated. Regressions were then used to compare the relationships between alpha diversity and functional trait richness to impervious surfaces.     

Results/Conclusions Our results showed that there was no relationship between alpha diversity and impervious surface (P = 0.52), but that there was a negative relationship between functional trait richness and impervious surface (P = 0.025). Other metrics of functional trait diversity, functional evenness (P = 0.81), a measure of trait evenness in trait space, and divergence (P = 0.42), a measure for extreme traits in the community, did not significantly change with impervious surface. Combined, these results suggest that increasing percentages of impervious surface can drive declines in trait diversity within a community, even as alpha diversity remains unchanged. This is an important finding as it suggests communities may lose functional traits faster than they lose species diversity. Furthermore, these results also suggest that species that invade a community following a perturbation may possess traits similar to those present in the community.