For the past two and a half years I have been studying habitat suitability of northern Pacific rattlesnakes. On a regional scale I have developed heat maps that describe varying levels of suitable habitat. On a microhabitat scale I examined the physical make up of the rattlesnake's hibernaculum (i.e., den) to get a better understanding of a key piece of the rattlesnake's ecology. Throughout my research I have been seeking to create the most comprehensive understanding of northern Pacific rattlesnake habitat suitability in northern California and eastern Oregon. This is a part of the rattlesake's range that has not been studied.
Datasets of environmental data such as elevation, slope, aspect, distance from rivers, mean monthly precipitation, and minimum, mean, and maximum temperatures were used to generate models of habitat suitability for northern Pacific rattlesnakes. The maximum entropy modeling package MaxEnt (Pillips, 2005) was utilized to generate these models. The models were compared using the Akiake Information Criteria (AIC) adjusted for small sample sizes(AICc) to to compare the models and influence the selection of the best model(Hurvich &Tsai, 1989). The modeling is sill ongoing, but early results are indicating that northern Pacific rattlesnakes prefer warmer habitats that are relatively close to rivers, and with open canopy vegetative structures.
On a microhabitat scale, the hibernaculum of a northern Pacific rattlesnakes is arguably the most important aspect of its habitat. The hibernaculum is typically a rocky outcrop where rattlesnakes will retreat in to deep crevices to avoid harsh winter conditions. Two important life history characteristics have emerged from the use of hibernaculum: communal denning and philopatry. Hibernaculum can be rare within a rattlesnakes home range leading to large congregations of rattlesnakes at the hibernaculum, in particular during the spring emergence period. The northern Pacific rattlesnake also exhibits high levels of philopatry, meaning rattlesnakes that are born near a hibernaculum will return to the same hibernaculum to hibernate annually (Geinger and Beck 2011). I have used a paired Resource Selection Function (pRSF) to compare rocky outcrops that are used as hibernaculum by northern pacific rattlesnakes and unoccupied rocky outcrops. Each hibernaculum is paired with the nearest unoccupied rocky outcrop microhabitat measurements of various habitat characteristics were taken at all sites and compared. I was looking for consistent differences in covariates between pairs. Again models were compared using AICc values. The best model selected showed that northern Pacific rattlesnakes prefer rocky outcrops that face due South, have high densities of deep crevices, and low amount of covering vegetation.
- Huizar, C. M. and Tsai, C. L. (1989). Regression and Time Series Model Selection in Small Samples. Biometrika. 76, 297-307
- Gienger, C.M. and Daniel D. Beck. 2011. Northern Pacific rattlesnakes (Crotalus oreganus) use thermal and structural cues to choose overwintering hibernacula. Canadian Journal of Zoology. 89, 1084-1090
- Phllips, S.A., R.P. Anderson, and R.E. Schapire. 2005. Maximum entropy modeling of species geographic distributions. Ecological Modeling. 190, 231-259