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Factors influencing the road mortality of snakes on the Upper Snake River Plain, Idaho

  • Author(s): Jochimsen, Denim M.
  • et al.
Abstract

This study documents the magnitude of road mortality on snake species that occur in sagebrush steppe habitat, provides insight into how susceptibility to this mortality differs among species as well as by sex and age class of individuals, and examines how different landscape variables influence road-kill aggregations using a logistic regression model. I collected data by road cruising a 183-km road loop on the upper Snake River Plain in southeastern Idaho from May through October of 2003. I conducted 56 total routes, traveling 10,248 km and encountering a total of 253 snakes (0.025 snakes/km) over the six-month survey period; 93 percent of these animals were found dead on the road surface (DOR). The majority of observations belonged to two species, with gophersnakes (Pituophis catenifer) comprising 75 percent of all road records, and western rattlesnakes (Crotalus oreganus) comprising 18 percent of all road records. Monitoring data from three of the largest snake hibernacula on the site indicate that rattlesnakes are the most abundant snake species, comprising 50 percent of all captures at trapping arrays since 1994. This suggests that gophersnakes may be more susceptible to road mortality due to higher vagility, or that our monitoring efforts do not effectively estimate their populations; this question remains to be explored. Overall, I documented more traffic casualties of adults than any other age class, the majority of which were males (64%). Road mortality varied seasonally by age and sex classes for both gophersnakes and rattlesnakes. More adult male gophersnakes were discovered DOR in May and June, while the death of adult females did not exhibit a trend. I documented a significant pulse of subadult mortality during the month of September. The seasonal trends in mortality of rattlesnakes differed from gophersnakes, but were not significant. This indicates that individuals may be more susceptible to road mortality during specific movements, such as mating or migration. The logistic regression indicated that increased cover of grass along roadsides, basalt piles, and mean distance to den were positively associated with gophersnake occurrence on roads. As most grasses on the site are invasive, this result implies that habitat change due to invasive species may be increasing susceptibility of gophersnakes to mortality.

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