Larval salamanders are flushed downstream more frequently on substrates common to urban streams

Background/Question/Methods The impacts of urbanization on stream systems have been well documented from the perspectives of stream hydrology and geomorphology. For example, it has been shown that urbanization leads to rapid surface runoff, which increases peak stream flows during rain events. Such increases in peak flows typically result in a stream bed with increased width and cross-sectional area. Additionally, stream bed substrates are altered as the availability of large pebbles and cobble, which serve as biotic refugia, decrease either through sedimentation or through high urban stream flows washing such substrates downstream. Studies have documented predictable declines in the abundance and/or diversity of certain stream-dwelling macroinvertebrates and fish in urban centers; however, much less is known about the impacts of urbanization on amphibians. We have been tracking the impact of urbanization on stream-dwelling salamanders in the southern Piedmont for the last three years.

Results/Conclusions While the abundance of Two-Lined Salamanders is similar between urban and reference streams after hatch-out in the spring, we have documented consistent decreases in abundance of the species in urban streams as the larval period progresses. We have also recorded lower substrate diversity and lower stream bed cover availability in these same urban stream habitats. To evaluate the hypothesis that decreased stream bed cover causes salamanders to drift downstream, resulting in decreased larval abundance after hatch, we conducted an experiment. We created a flow-through trough that we lined with one of four substrate types found in Piedmont streams (sand, sand/detritus, gravel/pebble mix, or pebble/cobble mix). We predicted that salamanders would be able to withstand higher water velocity on substrates more similar to the reference condition (i.e, high pebble/cobble composition). After an acclimation period, larval salamanders were subjected to periodically increasing water velocity on each of the substrate types. Salamanders were flushed from the flumes with sand and sand/detritus substrates at significantly lower velocities than salamanders on rocky substrates. This experiment demonstrates the importance of in-stream sediment composition on salamander drift. While there are undoubtedly many factors that contribute to salamander mortality in urban habitats, our results suggest that the absence of refugia from high stream flows is a necessary addition to other factors that negatively impact salamander larvae such as water toxicity or loss of adult habitat. Fortunately, the addition of pebble- and cobble-sized substrate particles to stabilize streambeds is already part of many stream restoration efforts. Our data suggest this practice has biotic, as well as abiotic, benefits.