Differential Effects of Urbanization and Non-natives on Imperiled Stream Species

The distribution of imperiled stream fish, crayfish, salamander, and freshwater mussel species of Maryland streams in relation to urban land cover and non-native species was investigated. Over the last 30 years, extinction or extirpation of 13 stream animal species (including the endemic Etheostoma sellare [Maryland Darter]) was observed within the Piedmont region of Maryland, where urbanization has spread extensively outward from Baltimore and Washington, DC, and many non-native species have become established. The presence of imperiled species in this area was correlated with urbanization and non-native species occurrence. However, correlations with land-cover data were stronger than with non-native occurrence. The majority of sites with imperiled species contained less than 10% urban land cover and less than 5% impervious land cover in their catchments. In contrast, stream reaches with non-native species spanned the entire gradient of urban, agriculture, and forested land cover, with the majority of non-native species persisting in streams with over 60% urban and 40% impervious land cover. The persistence of rare species coincident with introduced species in rural portions of Maryland indicates that habitat degradation (like that typically accompanying urbanization) may be the most important threat limiting the distributions of the rarest species that remain in these streams. Limits on urbanization in areas with rare species are needed to maintain regional and global biological diversity. This is particularly important in areas like Maryland that are anticipating extensive human population and urban growth over the next 30 years.