Estimating Annual Vertebrate Mortality on Roads at Saguaro National Park, Arizona

Road-killed vertebrates are a conspicuous effect of roads on animals, particularly in natural preserves where wildlife is protected. Knowledge of the number of vertebrates killed by vehicles in a national park or other natural area is important for managers, but these numbers are difficult to estimate because such mortality patterns vary greatly in space and time and by taxonomic group. Additionally, animals killed by vehicles may be difficult to observe, particularly during driving surveys, and carcasses may not persist between surveys due to scavenging and other factors. We modified an estimator previously developed for determining bird mortality at wind turbines to estimate the average annual number of vertebrates killed by cars on roads within and along the boundary of Saguaro National Park, Tucson, Arizona. Our model incorporates estimates of carcass (hereafter, roadkill) persistence and detectability (determined, respectively, by conducting surveys on 8 consecutive days and by comparing simultaneous walking and driving surveys) with data from regular roadkill surveys conducted throughout the park over a 6-year period. Using this model, we estimated that an average of 29,377 (SE 6,807) vertebrates (approximately 1.1/km/day; SE 0.24) were killed annually during 1994 to 1999 on 76.6 km of roads associated with the park. The majority of killed animals were amphibians and small reptiles, but birds and mammals also were killed in large numbers. The amount of wildlife killed in and near reserves by vehicles may be higher than appreciated by many managers and should be factored into within-park and regional transportation planning