Canopy structure, microclimate, and habitat selection by a nocturnal snake, Hoplocephalus bungaroides

Forest structure strongly influences ambient environmental conditions such as light and temperature, but most studies on habitat selection by mobile organisms have either ignored canopy structure or treated it as a dichotomous variable (e.g., "shady" or "sunny"). Furthermore, the predominance of active diurnal species as model organisms in such studies has left many unanswered questions about the importance of vegetation-related variables for nocturnal and sedentary species (e.g., what does "shade" mean to an organism that moves at night and sits in a cave all day?). We used hemispherical photography to quantify canopy structure and examine its role in determining the thermal microenvironments available to a rock-dwelling nocturnal snake (Hoplocephalus bungaroides) across two different spatial scales. The narrow plateaus inhabited by the snakes in southeastern Australia are highly heterogeneous with respect to vegetation: east-facing aspects are densely covered whereas west-facing aspects are patchy mosaics. We found that temperatures of potential retreat sites increased with increasing canopy openness, but the definitive determinant of retreat-site temperature was incident radiation intensity, which depended upon the location of canopy gaps relative to the sun path. This factor restricted the snakes to west-facing cliff tops, and there only to an optimal subset of rocks that received adequate irradiance. Moreover, thermal regimes of retreat sites displayed higher maxima and were evening shifted relative to randomly sampled rocks in the same area. Our results suggest that thermally suitable retreat sites are a limiting resource, and that local increases in vegetation density might contribute to the decline of this endangered species.