USGS

Patuxent Wildlife Research Center

whiteshadow

Stream salamanders in Shenandoah National Park:
Movement and survival of stream salamander populations

Description: Pseudotriton ruberResearch in population biology is concerned with factors affecting the change in a population over time, including births, deaths, immigration and emigration. Despite the potential importance of immigration and emigration, empirical data on movement patterns are lacking in many systems. Hence, there is a large body of theory on the causes and consequences of dispersal that remains to be tested in real biological systems. Until recently, research on stream salamanders has not considered the role of dispersal in determining the distribution and abundance of species on the landscape. Recent work suggests that these organisms may be able to move among habitats separated by large distances, despite their diminutive size and physiological reliance on moist microhabitats. Stream ecosystems have been dramatically influenced by land-use change and habitat loss that has resulted in the loss of entire tributaries or the fragmentation of stream reaches within a watershed. We know that the hierarchical nature of stream networks can influence population dynamics and patterns of connectivity in stream-associated organisms and this may have a disproportionate influence on extinction risk for species or life stages that are restricted to movement along riparian corridors.

The goals of this research are to understand 1) rates of interpopulation dispersal in a community of stream-associated salamanders, 2) whether dispersal is related to the spatial arrangement of habitat reaches, population density and demography, and 3) whether the dynamics of a local populations are influenced by dispersal from adjacent stream segments in the river network.

Our study includes an experimental removal and translocation of a population of stream salamanders, combined with repeat visits to capture individually marked animals. Under this design, we will be able to (1) evaluate survival of resident and translocated animals and (2) quantify the probability of dispersal for E. bislineata, D. fuscus, D. monticola and G. porphyriticus to test whether patterns of movement relate to individual characteristics or characteristics of the stream habitat (i.e., a direct measure of dispersal). This work is being conducted in Shenandoah National Park, an ideal place to study movement of stream salamanders in a natural system. Information from this study can be easily translated to other National Parks in the Northeastern United States, and will be useful in managing stream systems within natural areas in the face of increasing urbanization.

We used the observations of marked animals to estimate the dispersal probabilities of two species of lungless salamanders who reside in headwater streams in Virginia’s Shenandoah National Park. While the stream is the best habitat for the salamanders because of the stable temperatures and humidity, both juvenile and adult salamanders can travel over land to forage for food, and occasionally move from one stream to another. This study was the first to track salamanders across all three life stages – larva, juvenile, and adult – because we developed a method for marking the larval salamanders, which are <15 mm long. We used multistate mark-recapture models to estimate the probabilities of a salamander dispersing from one segment to another along multiple pathways. We found that those in the juvenile stage were the most likely to disperse by moving both upstream and overland to the adjacent stream. We then used the observed dispersal probabilities to conduct a computer simulation to show changes in population stability across a range of extinction risk scenarios in the stream networks. We investigated how the combination of dispersal by the three possible movement routes – upstream, downstream, and over land – resulted in changes to predicted extinction times. This modeling showed that when even a small amount of overland movement occurred, it increased the likelihood of salamander population persistence dramatically. This was only the case under low to moderate rates of extinction risk. Under higher extinction probabilities (like we see in stream-breeding frogs in the neotropics), no amount of dispersal could stabilize populations. These results suggest that the specific routes of dispersal play a big role in salamander population stability, and helps to explain why we have not seen declines in headwater stream salamander populations. These data confirm that the terrestrial habitat between streams is important to salamanders and must be maintained and protected.

 

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