The role of behavior in influencing headwater salamander responses to disturbance

Background/Question/Methods

Changes in behavior can alter movement patterns of animals, which can impact patterns of occupancy among habitats and fragment populations. Evolutionary theory predicts that animals have evolved to move in response to a suite of cues that maximize animal survival and growth and minimize risk of injury or mortality.  Behavioral plasticity allows these animals to adjust their behavior to appropriately respond to variable cues.  When humans modify environments, new conditions can suddenly yield maladaptive responses to cues that were adaptive in the undisturbed environment. Headwater streams generally have dense over- and mid-story canopies that limit direct light penetration and moderate values and ranges of light intensity, temperature, and humidity known to affect salamander behavior.  We tested whether canopy gaps in otherwise natural systems could 1) alter salamander responses to light cues, and 2) fragment previously contiguous populations. We performed controlled laboratory tests to determine salamander responses to light in the presence of sand, cobble, and leaves as refugia. Alternatively, a landscape level experiment was performed by translocating individuals above and below canopy gaps and monitoring their homing behavior.

Results/Conclusions

Laboratory tests indicated that salamander larvae exhibited strong, negative phototaxis that was slightly mediated by the presence of cobble substrates.  Alternatively, when individuals were collected from canopy gaps, they exhibited lower levels of negative phototaxis, but this response was mediated by the presence of any type of refugia.  Secondly, reciprocal transfers across a canopy gap and within forested areas demonstrated that salamanders were approximately 50% less likely to home to their capture location if a gap was present.  This study demonstrates that behavioral plasticity allowed salamanders to adapt to high light environments by taking advantage of all available refugia.  Field experiments demonstrated that despite this plasticity, even small canopy gaps (< 15 m) were capable of inhibiting homing behavior.  Canopy gaps are a ubiquitous consequence of anthropogenic activity, often precede more intensive development, and may yield unforeseen consequences for stream populations.  Lastly, this study demonstrates the importance of considering the consequences of individual behavior in determining how disturbance influences populations.