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Factors Influencing the Road Mortality of Snakes on the Eastern Snake River Plain

Background

Transportation lies at the center of our society, linking destinations, and is ever expanding. A vast network of roads stretches across our landscape affecting ecosystem processes in myriad ways. Roads transform existing vegetation into a compacted earthen surface with altered thermal and moisture characteristics, and generate an array of ecological effects that disrupt ecosystem processes and wildlife movement.

Researchers have conducted surveys along roads in attempts to quantify the most conspicuous effect that roads impose on wildlife, mortality inflicted by vehicles. In reviewing the literature, it became apparent that rigorous studies concerning road mortality of snakes are scarce. Furthermore, studies tend to be focused in the southeast and southwestern US, with only three studies conducted in northern latitudes.

However, northern temperate snakes possess several characteristics that increase their susceptibility to road mortality. They migrate seasonally to locate specific resources (Gregory et al. 1987; King and Duvall 1990) such as refuge, mates, prey and egg-laying habitat (for oviparous species). These resources tend to be located in distinct habitats that are patchily distributed across the landscape. Many large-bodied snake species make a loop-like migration from a communal hibernaculum (overwintering den site) to summer foraging habitats (King and Duvall 1990). Seasonal movements are defined by three distinct phases: 1) egress, or rapid movement away from the hibernacula, 2) stationary, or periods of short-distance movements associated with foraging, gestation, or ecdysis, and 3) ingress, or long-distance movements toward the hibernacula as described by Cobb (1994). The overlap of these movement corridors with the road network may result in high mortality. Publications tend to report numbers of fatalities according to species, but rarely explore the relationship of mortality with season, sex, or age of individuals.

Road mortality of snakes is a conservation issue that needs to be addressed. Future research must question if this mortality has the potential to severely reduce snake populations to a level where reproductive output cannot replace road-killed individuals (Rosen and Lowe 1994; Rudolph et al. 1999). The adverse effects of roads can be minimized, but the correct placement of mitigation efforts is critical. Ultimately, this research seeks to identify landscape and road variables that are highly correlated with snake mortality. These correlations could then be used to identify areas that may represent high risks for snake road mortality. Studies suggest that mitigation success is dependent on correct placement of efforts (Jackson 1999) by identifying high-risk sites.

Objectives

This study was designed to address five objectives: (1) quantify the road mortality of snakes on the eastern Snake River Plain; (2) identify any variation of mortality with respect to species, season, sex, age, traffic volume; (3) examine the spatial pattern of mortality across the survey route; (4) evaluate the importance of various landscape factors influencing this pattern; (5) develop a logistic regression model to predict road sections with intense mortality.

Accomplishments

Successful completion of the 2003 and 2004 field seasons including 333 total road observations of snakes along the survey route in over 10,000 kilometers driven.

  • Performed spatial and statistical analyses of the data, as well as identification of important landscape and habitat features that influence where snakes cross roads.
  • Presented general findings of this research at the Intermountain Herpetological Rendezvous in Logan, Utah (2004), Society for Northwest Vertebrate Biology meeting in Corvallis, Oregon (February, 2005), and at the International conference on Ecology and Transportation in San Diego, California (September 2005).
  • Successfully defended a master's thesis based on this work.
  • Generated a poster publication to be used for subsequent presentation.

Results

Road mortality of snakes was quantified by road cruising (driving slowly in a vehicle and recording all snakes observed on a road surface) a 170-kilometer route from May through October of 2003. The survey route is located within the northeastern portion of the Snake River Plain and covers portions of US Highways 20, 26, 20/26, 22/33, Franklin Boulevard, and Lincoln Boulevard. Sampling consisted of 55 total trips along this route, and resulted in 9,350 total kilometers traveled over the 2003 field season (Table 9-4).

A total of 253 snakes were observed on roads along the survey route and across the entire survey period; 93 percent of these animals were found dead on the road surface (kill rate of 0.023 individuals/km surveyed). Spatial visualization and analyses indicate that these observations are clustered along the survey route (Figure 9-8). We documented the road mortality of 4 species belonging to families Colubridae and Viperidae. However, the majority of observations belonged to 2 species, Pituophis catenifer (gophersnake) and Crotalus oreganus lutosus (Great Basin rattlesnake). We observed gophersnakes most often on roads, comprising 74 percent of all road records, and rattlesnakes were observed more frequently than the remaining two species, comprising 18 percent of all road records (Figure 9-9). Furthermore, we observed more adult males dead on roads for both these species than any other sex or age class. Juvenile observations comprised only 28 percent of total gophersnakes, and 17 percent of total rattlesnake road mortality.

Monitoring data indicate that rattlesnakes are the most abundant species based on hand and drift fence captures at dens. In fact, rattlesnakes made up 85 percent of captured snakes (n=2,459), with gophersnakes representing most of the remaining percentage of snakes (n=372) over a ten-year sampling period. This raises an interesting question, are gophersnakes more susceptible to road mortality on the Eastern Snake River Plain? This species is a habitat generalist and is perhaps more vagile than rattlesnakes, indicating that individuals would encounter roads more often, exposing them to the risk of road mortality.

The road mortality of snakes was documented in all months surveyed and seasonal patterns were evident. The mean number of snakes observed per route while road cruising was highest during the fall season, with a secondary peak in spring. These differences were significant (analysis of variance [ANOVA], F = 3.638, P = 0.033). The total number of sampling days without snake observations (11 total) was highest in late July and early August. There were also significant differences across season based on sex and age in gophersnakes (Figure 9-10).

Specifically, more males were killed in spring, whereas more subadults were dead in fall (results based on Kruskal-Wallis test). The higher numbers of certain age and sex classes with respect to seasons indicates that individuals may be more susceptible to road mortality during specific movements. Methods designed to ameliorate the road mortality of snakes should therefore coincide with these activity periods to be effective.

In addition to the systematic surveys, a 10 km segment of the route along State Highway 22/33 (running north/south) located on the western most edge of the study area was road-cruised in 2004. These surveys were designed to assess the probability of a snake successfully crossing the road. In attempts to address this, the shortened segment was driven between June and October in 2004, during periods of peak snake activity. Twelve of these routes were surveyed, covering 746 km and 80 snakes were observed (rate 0.107 snakes/km surveyed). Of these 80 snakes, 59 were observed killed. Similar to the 2003 surveys, 74 percent of observations were gophersnakes and 23 percent were rattlesnakes. This high mortality rate occurred in low traffic, and it appears that a traffic volume of less than ten vehicles per hour was sufficient to cause 100 percent mortality on some nights.

To assess the effect of road and landscape variables on snake mortality, several relevant variables were measured at each observation location from 2003, as well as an equal number of randomly chosen non-crossing points along the route. The variables measured were road slope, percent vegetation and major cover type within 10 meters of the road, distance to nearest vegetation, distance to nearest shrub, presence of burrows, presence of basalt, mean distance to dens (including those identified in this research), solar radiation, and major cover type at 50, 100, and 500 meters from the road (based on a geographic information system [GIS] coverage). A multiple logistic regression analysis was used to determine those variables significantly associated with road crossing locations. Finally, only gophersnake locations were used in this analysis as this was the only species with enough sample locations. Four variables were consistently included in each significant model. These were the grass major cover type (positively associated with snake crossing), percent vegetation cover within 10 meters (positively associated), presence of basalt (positively associated), and mean distance to den (positively associated). These results were surprising because it was expected that snake presence would be correlated with shrubs and would be negatively associated with den distance (i.e. the closer a den was to the road, the more mortality would be expected). A possible explanation for the positive association of grass cover type with snakes on roads is that this habitat is less suitable. If this is unsuitable habitat, individual snakes may move more to find more suitable habitat, and this increased movement would increase the likelihood of encountering a road. Second, if increased proximity of dens to a road actually reduces the probability of a snake encounter, then this likely indicates a population effect of road mortality. Specifically, this means that dens near roads either have reduced numbers of individuals or that snakes from those dens are not moving towards roads. Either way, this could influence population connectivity and ultimately, population persistence.

Investigators and Affiliations

Denim M. Jochimsen, Graduate Student, Herpetology Laboratory, Department of Biological Sciences, Idaho State University, Pocatello, ID.

Charles R. Peterson, Professor, Herpetology Laboratory, Department of Biological Sciences, Idaho State University, Pocatello, ID.

Funding Sources

ISU Biological Sciences Department
ISU Graduate Student Research Committee
BBW Bechtel and the INEEL - ISU Education Outreach Program
ISU Biology Youth Research Program
National Science Foundation (NSF) GK-12 project

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