Roads and Road Kill


Quantifying the Impacts of Roads on Wildlife


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The United States has 6.2 million kilometers of public roads that are used by approximately 200 million vehicles (National Research Council 1997).  Road corridors cover approximately 1% of the United States (Forman and Alexander 1998).  Ecological impacts extend well beyond the road surfaces themselves.  A minimum of 19% of the terrestrial United States is directly affected by roads and 22% of the United States appears to be ecologically altered by our road network (Forman 2000).  Roads alter species interactions, animal behavior, soil characteristics, hydrology, and vegetative cover (Forman et al. 2003).  The magnitude of these effects is correlated with proximity to the road surface, but extends throughout a “road effect zone” (defined as a region with an outer limit of significant ecological effects; Forman et al. 1997, Forman 2000).

Road-associated Mortality

Roads and vehicles can have profound impacts on the abundance and distribution of organisms.  Road kill is the most obvious direct effect of roads upon wildlife.  Wildlife-vehicle collisions date back to the origin of vehicles and continue to be an issue of concern to this day (Forman et al. 2003).  Forman and Alexander (1998) imply that this problem is growing in scope, with an estimated one million vertebrates per day killed on roads in the United States.  This puts vehicle-animal collisions above hunting as a mortality source for terrestrial vertebrates.

Most road kill datasets are compiled from accident reports or (as with a recent study by UCSB researchers in Ventura County) from road maintenance crews responsible for clearing large animal carcasses from roadways (Lindsey et al. 2005).  Such data collections ignore the fact many animals initially survive a collision, move away from the roadway, then expire in a concealed location.  Additionally, larger animal carcasses are simply more conspicuous than those of smaller squirrels, snakes, etc.  Collisions with larger animals are more likely to lead to noteworthy outcomes (broken headlights, insurance claims, human fatalities, etc.).  Consequently, statistics regarding the number of animals killed on roads are likely skewed and should be taken to be minimum values.

No previous data existed documenting the number of animals killed on our roadways within the Santa Monica Mountains National Recreation Area (SMMNRA) or Ventura County, although some limited data has previously been collected for western Ventura County (Casterline, et al. 2003, Lindsey et al. 2005).  For the past four years, my students and I have been surveying roads from Santa Barbara to Los Angeles counties.  This data is beginning to help us better understand wildlife corridors, important “pinch points,” and where we might direct our efforts to minimize both animal mortality and auto accidents.