Anuran Occupancy and
Abundance Sub-project

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It is generally acknowledged that habitat modification is one of the primary threats to amphibian populations. We therefore decided to focus part of our research on examining factors that might affect the occupancy (presence or absence) or abundance of anurans (frogs and toads) in river habitats. Specifically, we researched the effects that distance to dam and level of urbanization had on these population metrics. 

Dams are known to regulate the flow rate of rivers through alteration of many natural riverine processes such as flooding patterns. Because many anurans use wetland habitats bordering rivers, damming may ultimately influence the quality of habitats available to frogs and toads. We hypothesized that anuran populations close to dams would be negatively affected by these habitat changes. We also investigated whether up- or downstream distances from dams had a larger impact on anuran populations. Finally, much research has shown that urbanization negatively impacts amphibians, and we therefore expected that urban areas would have reduced occupancy rates and abundance of anurans when compared with undisturbed areas.




Data for this project were collected through anuran calling surveys conducted at all 42 study sites along the Pacolet and Broad Rivers. Surveys lasted for five minutes during which all species of frogs and toads heard calling were recorded. Sites were surveyed three times in during two different calling windows (April 13 – May 8 and June 8 – 24) for a total of six surveys at each site in 2010. Using geographic mapping computer software, we identified all dams (16 total) located along the river reaches of our study sites. We then measured distances up and downstream to dams for each site as well as the percentage of urbanization within a one km radius circle surrounding each site. These measurements were then used to create mathematical models in the computer program PRESENCE that evaluated their effects on occupancy and abundance of anurans. 
Signs near dammed areas warn vistors of the dangers associated with rapid changes in water level due to flow regulation. We investigated whether these sporadic changes in habitat quality affect anurans using habitats near rivers.


Results:

Model sets were generated for a total of seven anuran species: cricket frogs, Fowler’s toads (only abundance models), Cope’s gray treefrogs, green treefrogs, spring peepers, bullfrogs, and green frogs.

Model sets for three anuran species (cricket frogs, green treefrogs, and spring peepers) showed that distance downstream from dam had a significant effect on occupancy, with frogs being more likely to occupy sites farther downstream from dams.  

Model sets for four anuran species (cricket frogs, Fowler’s toads, green treefrogs, and green frogs) showed that distance downstream from dam had a significant effect on abundance, with frogs being more abundant at sites farther downstream from dams for all species except for Fowler’s toads.       

Model sets for one anuran species (bullfrogs) showed that urbanization levels had a significant effect on both occupancy and abundance, with bullfrog presence and abundance being greater at sites with less urbanization.  

A student investigator collecting calling survey data at night.
The effect of downstream distance from dam on occupancy of cricket frogs. The red line represents the mathematical model generated using Program PRESENCE and the dashed lines show the 95% confidence interval surrounding the model. Note that cricket frogs are more likely to occupy sites farther downstream of dams.


Summary and Conclusions:

Distance downstream from dams had more influence on occupancy and abundance than distance upstream from dams, suggesting that stress on anurans may be greatest downstream from dams.

For most species, occupancy and abundance is higher at sites farther downstream from dams.

Distance from dam was a stronger predictor of occupancy and abundance than urbanization levels for all species except for bullfrogs, indicating that while urbanization may be an important stressor on amphibians, the effects of dams appear to more strongly impact many anuran species occupying riverine habitats.

Many anurans are likely negatively influenced by the altered river habitat conditions just downstream from dams which often result in reduced flooding events, which in turn reduce the availability of riparian wetlands that serve as important breeding habitat for anurans.

99 Islands Dam, a large hydroelectric dam on the Broad River located near Blacksburg, SC.


   
For questions or comments regarding this site, contact Dr. Michael Dorcas.