Abstract
Abstract Background/Question/Methods As urbanization increases across the globe, more ecologists have taken note of its consequences to stream systems. Sufficient data have been collected to document repeated patterns in urbanized streams for many abiotic parameters, aquatic insects, and fish. For example, we now know that urbanized streams experience more frequent and intense spates as a result of increased runoff form impervious surfaces in the urban watershed. The spates eventually lead to a more incised and wider stream bed. Such abiotic shifts consistently result in decreased aquatic invertebrate and fish diversity. More recently, stream-breeding salamanders have been observed to decrease in density in urban areas. We monitored the density of southern two-lined salamanders for the duration of two cohorts in 12 streams in western Georgia, USA. We then used path analysis to determine the relationships between land-use change, abiotic shifts in the stream environment, and larval salamander density. Results/Conclusions We found that southern two-lined salamanders in the streams we monitored exhibited no change in reproductive output between urban and reference habitats. However, repeated sampling throughout the larval period revealed a large decline in density of larvae in urban areas prior to metamorphosis, while a similar decline was not seen in reference habitats. We evaluated several hypotheses that might explain the observed decline in urban areas, and a model in which increased impervious surface causes an increase in spate frequency and magnitude, which then leads to decreased larval density had the most support. This is the first attempt to compare multiple plausible hypotheses as to why salamander density and diversity decreases in urban habitats. By describing larval density at the beginning and end of the larval period, and by identifying a likely mechanism for the observed decline in density, species-specific and stream restoration efforts can be enhanced.
Highlights
Urbanization has been correlated with the loss of many streamdwelling species, including salamanders, but mechanisms are not well studied
We documented a decline in salamander larval density from hatch to metamorphosis in urban streams, relative to reference streams
Using data from streams representing a range of abiotic conditions we compared a priori models explaining low salamander density in urban streams
Summary
Calculated salamander density during spring (pre-metamorphic larvae from the previous year’s cohort) and summer (hatchling larvae) for the 2006 and 2007 cohorts. At each watershed several abiotic variables assessed (Table 1). Created 7 a priori models to explain observed density patterns in pre-metamorphic larvae (see Bonus Table). Path analysis was used to evaluate individual models and AIC was used to compare among models. We designed an experiment to test the hypothesis that salamander density decreases because larvae are washed from streams. Experiment supported the conclusions from field data described above
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