Linking process to pattern: Causes of stream-breeding amphibian decline in urbanized watersheds

Sufficient data have been collected to document negative effects of urbanization on many abiotic parameters, aquatic insects, and fish. Recently, stream-breeding salamanders have been observed to decrease in density in urban areas. We employed a two-step process to determine the mechanism(s) most likely responsible for salamander decline in disturbed streams. First, we conducted a 2-year survey of larval two-lined salamanders (Eurycea cirrigera) and abiotic variables among 12 streams in western Georgia, USA surrounded by land disturbed by urbanization and pasture. From these surveys we constructed hypotheses that might explain differences in observed larval salamander density among streams. A model in which increased watershed impervious surface causes an increase in spate (i.e., high water flow) frequency and magnitude, which then leads to decreased larval density had the most support. This analysis led to a second step in which we conducted an experiment using artificial streams lined with substrates commonly found in urban and forested streams (sand/sand with detritus and gravel-pebble/pebble-cobble, respectively). We placed salamander larvae in the artificial stream channels and subjected them to increasing water velocity. Larvae on sand-based substrates common to urban streams were flushed from the artificial stream at significantly lower velocities than larvae on rock-based substrates. The experimental data were consistent with the hypothesis generated from field sampling that high water velocities in urban streams result in decreased larval retention in streams. Our result emphasizes the need for upland habitat restoration in urban areas, which will reduce run-off and improve stream habitat quality.