Why do urbanized watersheds lead to amphibian declines?

Two-lined salamander (Eurycea cirrigera). Credit Paul Sattler.Numerous studies over the years have found that increasing urbanization leads to population declines for certain species of wildlife. However, it is much less common for a study to specifically determine why urbanization results in these problems.

In this vein, a new study led by Kyle Barrett from the University of Georgia makes a valuable contribution to science by identifying the specific mechanism through which the urbanization of watersheds leads to the decline of the two-lined salamander and possibly other amphibian species.

Using a combination of field sampling from western Georgia and laboratory experiments the researchers determined that an increase in extreme flow events in streams fed by urban runoff washes away the larvae of the salamanders leading to lower population density in urbanized areas.

This is an important finding since resource managers typically pay more attention to water quality as an impact on amphibian habitat.

Laboratory experiments shed additional light on the mechanism for decline. Researchers placed salamander larvae in artificial rivers made from cut PVC pipe and natural substrates and exposed them to increasing velocities of water flow.

Larvae on sand-based substrates typically found in urbanized streams were washed away at lower water velocities than salamanders on gravel-pebble and pebble-cobble mixes.

The authors hypothesize that since salamander larvae primarily inhabit low-order streams "they presumably have not evolved mechanisms, such as those present in many fish species, which allow them to survive and/or re-colonize after extreme flow events encountered in urban stream systems."

This may help explain why 4 of the 5 salamander species native to forested streams in the region have largely disappeared from urban areas, although other factors could be at work as well. Given the study findings, the authors write,

" Our work suggests in-stream restoration efforts will have a minimal effect on the biota if upland processes such as increased overland flow due to impervious surfaces are not addressed"

--by Rob Goldstein

Barrett, K., Helms, B., Guyer, C., & Schoonover, J. (2010). Linking process to pattern: Causes of stream-breeding amphibian decline in urbanized watersheds Biological Conservation DOI: 10.1016/j.biocon.2010.05.001