Roadkills are good indicators of wildlife populations
Roadkills are easily and usually overlooked, but they can be important indicators of abundance or scarcity of various wild species, as Ohio wildlife biologists have learned.
They have been conducting regular counts of roadkills since 1979 and now have amassed a significant string of numbers that are telling indicators about the status of raccoons, skunks, opossums, and woodchucks.
“Roadkill surveys provide the most reliable indices to population change for relatively abundant species,” said Suzie Prange, furbearer specialist with the Ohio Division of Wildlife. She noted that until 2003, muskrats, gray and red foxes, mink, rabbits, and squirrels also were included in spring and fall roadkill surveys.
But it was determined that spring surveys provided adequate information regarding population trends for the four species of note. Forty-nine 40-mile survey routes were established across the state, with each being checked six times. “Roadkill surveys are an efficient way to index population size of medium-to-large-bodied mammals that are frequently found on roads,” said Prange.
For spring, 2016, overall roadkill rates (roadkills/1,000 miles) were 41.4 for raccoons, 43.7 for opossums, 11.5 for skunks, and 7.2 for woodchucks. But the results varied by region (Table 1). During 1989–2016, roadkill rates varied from 20.4 to 63.8 for raccoons, 33.6 to 89.5 for opossums, 11.5 to 31.7 for skunks, and 6.9 to 27.5 for woodchucks.
“Raccoons exhibited a slight increasing trend in all regions, except the glaciated region (southeast), where the annual percent change was negative but near zero,” said Prange. Opossum trends were declining in all regions, though the annual percent change was relatively small. Skunks likewise exhibited negative trends in all regions, with the most decline in the glaciated region. Woodchucks exhibited the strongest declines, with significant declines occurred in all regions, with the greatest declines in the till plain and unglaciated regions of western Ohio.
Prange said that raccoon numbers and range have been increasing throughout North America since the 1940s. Declines in raccoon trapping because of low pelt prices are partially responsible for the increase, but raccoons also have expanded in urban and suburban areas and they readily adapt to a wide range of food and habitats.
Opossums and skunks are also generalists, but raccoons are more adaptable to settled areas. Too, noted Prange, both opossums and skunks are prone to intermittent fluctuations in population size. Skunks are typically may succumb to diseases and/or parasites, and severe winters also may take a toll. Opossum populations are even more likely to be affected by winter weather, with substantial deaths in more severe winters.
In any case, more than 28 years of roadkill surveys show a clear declining trend for both species. The reason for this is unknown, though the take by nuisance trappers and competition and/or disease transmission caused by the more numerous raccoon may be factors. Thus, said Prange, continued surveillance of these populations is warranted.
Most perplexing are the continuing low roadkill rates and high rates of decline for woodchucks. Woodchucks are considered nuisances by many people and declines may be at least in part due to increased pest control, the biologist said. Increasing coyote numbers, which have expanded their range in recent decades and prey readily on woodchucks, are another suspected factor. Woodchucks also are sometimes affected by the raccoon roundworm, which is carried by raccoons but is not fatal to them. Given the rate of decline and the lack of information concerning the cause, additional observation is also warranted for woodchucks, Prange said.
She noted that roadkill surveys are reliable tools but should be interpreted with caution, because such factors as annual variation in weather or changes in traffic volume can influence yearly roadkill rates.
Bowhunter surveys provide an additional, independent source of information regarding annual changes in the relative abundance of raccoons, opossums, and skunks. And bowhunters surveys are in agreement in the overall trends for these three species. Roadkill surveys are the only monitoring tool available woodchucks at present.
So there is something to be learned from those flattened, easily ignored carcasses by the roadside. To wildlife researchers, they count.