Compensatory Microhabitat Selection by Northern Pacific Rattlesnakes (Crotalus oreganus oreganus) in a Cool and Wet Macroclimate
Northern Pacific Rattlesnakes (Crotalus oreganus oreganus) have a range that extends from southern California into British Columbia. This subspecies is common in relatively warm and arid regions, but also occurs in habitats that are neither warm nor arid. We hypothesized that the presence of suitable microhabitat conditions can compensate for a less suitable macroclimate, allowing C. o. oreganus to exist in less suitable regions. We developed environmental niche models for C. o. oreganus at two spatial scales: (1) northern California and western Oregon, and (2) the northern California coast. These models explored macroclimatic suitability of northern, coastal California relative to other regions within the range of C. o. oreganus. The models revealed that the three most northern and coastal counties of California have a significantly less suitable macroclimate relative to the rest of each study area. Next, we used paired resource selection functions to determine microhabitat differences between rocky outcrops used as hibernacula and outcrops that are unoccupied by rattlesnakes despite similarities. Our analysis indicated selection for outcrops with more deep crevices, less vegetative cover, and slopes facing due south (180° from North). Additionally, we mapped landslides near hibernacula, which revealed that hibernacula commonly occurred within the head-scarps of landslides. We suggest that because landslide triggers (e.g., heavy rainfall and earthquakes) occur frequently along California's north coast, more rocky outcrops are created that are suitable as hibernacula. The relatively high abundance of these suitable outcrops compensates for the marginal macroclimate, allowing C. o. oreganus to occur in the region.Abstract
(A) The range of Northern Pacific Rattlesnakes (C. o. oreganus) relative to Western Rattlesnakes (C. oreganus) and (B) the location of the two study areas within the range of Northern Pacific Rattlesnakes. The Northwest Branch (NW Branch) study area encompasses all of the different environments occupied by C. o. oreganus in northern California and western Oregon. The Northern California Coast (NorCal Coast) study area includes California counties that fall within the EPA's “Coastal” eco-region north of the San Francisco Bay (Omernik and Griffith, 2014). We acquired the polygon for the range of the C. oreganus from Natural Earth (http://www.naturalearthdata.com/) and used an estimated range as a guide to clip out the range of C. o. oreganus (Ashton, 2001). The microhabitat study area is where the analysis of rocky outcrops used as hibernaculum occurred.
Map showing all 160 outcrops that were surveyed for presence of C. o. oreganus and suitability as a hibernaculum near Maple Creek, California. We located 22 hibernacula, 64 suitable but unoccupied rocky outcrops, and 74 unsuitable rocky outcrops. Each polygon represents a subregion: (A) Lower Madrone, Upper Madrone, Garcia Ridge; and (B) Hunter Ranch. Subregions were designed so that all the rocky outcrops in each could be surveyed in 1 day. Note that the Hunter Ranch subregion (B) was separated from the others by approximately 9 km.
The climatic suitability map (A) for C. o. oreganus in northern California and western Oregon (NW Branch) produced by the selected environmental niche model (ENM). The outlined California counties are (south to north) Mendocino, Humboldt, and Del Norte. The selected ENM for the NW Branch contained the environmental variables elevation, mean precipitation in March (Precip), maximum temperature in March (Tmax), and minimum temperature in March (Tmin). Results of the jackknife test (B) indicate that the temperature variables had the greatest influence of climactic suitability.
The climatic suitability map (A) for C. o. oreganus within coastal, northern California counties (NorCal Coast) produced by the selected environmental niche model (ENM). The outlined California counties are (south to north) Mendocino, Humboldt, and Del Norte. The selected ENM for the NorCal Coast contained the environmental variables elevation, mean precipitation in March (Precip), maximum temperature in March (Tmax), and minimum temperature in March (Tmin). Results of the jackknife test (B) indicate that the mean precipitation had the greatest influence of climactic suitability.
The climatic suitability values (CSVs) for C. o. oreganus presence locations within Mendocino, Humboldt, and Del Norte counties compared to all other C. o. oreganus presence locations within each study area used for generating environmental niche models: northern California and western Oregon (NW Branch) and northern, coastal California counties (NorCal Coast).
Map of a landslide at a C. o. oreganus hibernaculum near Maple Creek, California overlaid on an aerial image provided by ArcGIS v 10.3.1. The long U-shaped line represents the landslide boundary. The incipient boundary is what we assume to be the landslide boundary (but we cannot be certain based on the local topography). Notice that the hibernaculum is located within the head scarp of this landslide.
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