How Many is Too Many: A Review of Road Density Thresholds for Wildlife

The negative impacts of wildland roads on wildlife have been well documented (e.g., Wisdom et al. 2000, Trombulak and Frissell 2000). In addition to road-kill, roads increase animals’ vulnerability to over-hunting, poaching, and the effects of degraded habitat. Despite this qualitative understanding, however, the actual road mileage that would significantly reduce wildlife populations is still under review. A common measure used to gauge the impact of roads on wildlife is road density, measured as kilometers of roads per square kilometer of land area. For example, if there were 2 kilometers of road in a 1 square kilometer area, the road density would be 2 km/km2. In this article, I review studies that have measured road density thresholds for wildlife and report on research that examined the benefits of reducing road density.

The majority of studies on this topic have identified road density thresholds for large endangered carnivores or hunted game species. Gray wolves (Canis lupus) in the Great Lakes region and elk (Cervus elaphus) in Montana and Idaho have had the most long-term and in depth analysis. Forman and Hersperger (1996) were the first to review road density thresholds at the Transportation Related Wildlife Mortality Seminar (FL). By comparing previous studies of wolves and mountain lions (Felis concolor), they found that in order to maintain a naturally functioning landscape with sustained populations of large mammals, road density must be below 0.6 km/km2 (1.0 mi/mi2). Several studies have since substantiated their claim.

Elk
Elk are one of the most well studied animals in the U.S., probably because of their popularity as a game animal and their sensitivity to disturbance. Other game species have been linked to road density, including moose (Alces alces, Crete et al. 1981, Timmermann and Gallath 1982) and white-tailed deer (Odocoileus virginianus, Sage et al. 1983), but the amount of data is limited. Lyon (1983) was the first study to report the impact of road density on elk populations. He states, “habitat effectiveness can be expected to decline by at least 25 percent with a density of 1 mile of road per square mile and by at least 50 percent with two miles of road per square mile…..As road densities increased to five to six miles per square mile, elk use declined to less than 25 percent of potential” (Figure 1). A comprehensive review of the impacts of roads on elk was recently published by Rowland et al. (2005) and provides dozens of citations.

In addition to extensive documentation of the impacts of roads on elk, studies have shown that closing roads has benefited elk. Irwin and Peek (1979) found that road closures allowed elk to stay in preferred habitat longer while elk in roaded areas were displaced. More recently, Leptich and Zager (1991) found that closing roads extended the age structure and doubled the bulls per cow sex ratio. Gratson et al. (2000) measured elk hunter success in relation to road density. They found that hunter success almost doubled when open road density was reduced from 2.54 km/km2 to 0.56 km/km2. Rowland et al. (2005) reported that road closures may improve the animals’ performance, increase the amount of effective habitat, increase hunting opportunities, decrease damage to crops, improve diet quality, increase hunter satisfaction, and decrease vulnerability of elk during the hunting season.

Wolves
Several studies have also measured road density thresholds for wolves (Table 1). One of the first studies to identify a road density threshold for wolves was in Wisconsin. Thiel (1985) reported that wolves could not survive in areas with road densities higher that 0.6 km/km2. The following year, Jensen et al. (1986) documented a maximum road density of 0.6 km/km2 on the Ontario-Michigan border. Mech et al. (1988) found similar findings in northern Minnesota. They observed that wolves were absent if road densities exceeded 0.58 km/km2. Mech (1989) later reported that wolves persisted in areas with road densities greater than 0.58 km/km2 if they were adjacent to extensive roadless areas.

Fuller et al. (1992) was the first study to incorporate human density into thresholds. They found a maximum threshold of 0.7 km/roads/km2 with 4 humans/km2 or a maximum of 0.5 km/roads/km2 with 8 humans/km2 in northern Minnesota. Thus, the higher the density of humans, the lower the threshold for persistence of wolves would be. More recently in the northern Great Lakes region, Mladenoff et al. (1995) found few portions of any pack territory were located in areas of road density greater than 0.45 km/km.2 Core areas (defined as 40 percent use) did not exceed road densities of 0.23 km/km2 and no portion of any pack area was in an area of road density greater than 1.0 km/km2. Wydeven et al. (2001) most recently observed that changing attitudes towards wolves has allowed them to persist in areas with road densities as high as 0.63 km/km2 in Wisconsin.

There have also been studies that suggested closing roads to improve survival of wolves. Thurber et al. (1994) examined the impact of roads on wolves in Alaska and found that wolves were using closed roads while avoiding open roads. They recommend gating or seasonally closing roads to increase the amount of available habitat for wolves. Carroll et al. (2006) developed a spatially explicit model of wolf habitat in the continental U.S. They found that the amount of wolf habitat could increase by 24 percent if you removed one percent of the roads each year for 20 years.

Bears, lynx, and wolverine
Other wildlife have also been found to have road density thresholds. Black bear (Ursus americanus) populations were shown to be inversely related to road density in the Adirondacks, New York (Brocke et al. 1988). There was a strong negative relationship between road density and population fitness of grizzly bears (Ursus arctos horribilis) in the U.S. Rocky Mountains (Mace et al. 1996, Mattson et al. 1996). Similar relationships have also been hypothesized for wolverine (Gulo gulo) and lynx (Felis lynx, ICBEMP 1996b, 1996c, and Terra-Berns et al. 1997 cited in Wisdom et al. 2000). Road closure has been recommended to reduce black bear (Powell et al. 1996) and grizzly bear mortality risk (Frederick 1991, USFWS 1993, Boone and Hunter 1996, Mace et al. 1999), and has been suggested to benefit rare forest carnivores (Bull et al. 2001).

Conclusion
Road density thresholds have been identified for several species, and when exceeded, many species cannot persist. Closure and removal of roads has been found to effectively provide wildlife security and increase the amount of available wildlife habitat. Future research is needed to determine thresholds for other sensitive species such as lynx and wolverine. There is also little peer-reviewed data on road density thresholds for aquatic species such as fish. Wildland managers should strive to keep roaded lands below 0.6 km/km2 (1.0 mi/mi2) to ensure healthy wildlife populations.

— Adam Switalski is Wildlands CPR’s Science Coordinator. He is currently studying the extent that road removal restores wildlife habitat in Idaho.

Table 1: Road density thresholds for wolves in the northern Great Lakes region.

*Adjacent roadless area allowed higher road density threshold
**Changing attitudes allowed higher road density threshold

References

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