Delimiting road-effect zones for threatened species: implications for mitigation fencing

Abstract

Context. Roads are a pernicious form of habitat loss for many wildlife populations because their effects often extend far beyond the roads themselves, giving rise to reduced wildlife abundance in road-effect zones. Quantifying the extent of road-effect zones more accurately portrays their impact on populations and the true extent to which habitat is lost for many species.Aim. The purpose of the present study was to evaluate ways of determining the extent of road-effect zones for a model study species to better quantify the effect of roads on habitat loss.Methods. We conducted road-side surveys for signs of Mojave desert tortoises (Gopherus agassizii) 0, 200, 400, 800 and 1600 m from county roads and interstates, two of the most common road types in critical habitat of this threatened species. Using data from these road-side surveys, we estimated the extent of road-effect zones using piecewise regression and modified von Bertalanffy models.Key results. We found reduced abundances of tortoise sign along both county roads and interstates. Reductions extended farther from the large, high-traffic interstate than from the smaller, lower-traffic county roads (306 m versus 230 m). The increase in the abundance of tortoise signs with distance from roads approximated a negative exponential curve.Conclusions. Interstate and county roads both contribute to habitat loss in road-side areas by making these habitats unsuitable to desert tortoises, presumably by removing animals via mortality from collisions with vehicles. Larger roads with greater traffic have more extensive effects.Implications. Roadside mitigation fencing has been proposed as one way to reduce mortality of desert tortoises and to reclaim habitat by allowing tortoises to recolonise currently depauperate road-effect zones. Immediate mortality is more likely to be prevented by fencing county roads where tortoises occur closer to roads and are more likely to be struck by vehicles and killed. However, fencing interstate should yield more reclaimed habitat than that obtained from fencing county roads. Managers must consider balancing these goals along with other concerns when deciding where to place roadside fencing.

Effects of roads and roadside fencing on movements, space use, and carapace temperatures of a threatened tortoise

“…One site was within the Mojave National Preserve (MNP) along an unfenced, paved 2-lane road (50 vehicles per day; Nafus et al, 2013) and the other site was 11 km north and just to the west of Interstate 15 (I-15; 50,000 vehicles per day; Peaden et al, 2015), where roadside fencing was installed just 3 months before our study began. Both locations had similar habitat with dominant vegetation of creosote (Larrea tridentata) and white bursage (Ambrosia dumosa).…”

“…Even with this lack of understanding, species that are highly sensitive to roads, such as those with increased local extinction probability from road mortality, have been targets for mitigation fencing (Clevenger and Waltho, 2000;Aresco, 2005;Hayward and Kerley, 2009;Peaden et al, 2015). Several studies have documented cases of mortality and altered behavior in response to fencing for several wildlife species (van Dyk and Slotow, 2003;Klar et al, 2009;Gulsby et al, 2011).…”

“…Road mortality can drastically reduce tortoise densities for the first 200-400 m from roads (Boarman and Sazaki, 2006;Nafus et al, 2013;Peaden et al, 2015), and at least one study has suggested reduced abundances may extend up to 1-4 km from a road (von Seckendorff Hoff and Marlow, 2002). In response to the threat that roads pose to desert tortoise populations, roadside fencing has been installed in many areas as a widespread mitigation measure to reduce desert tortoise mortality (US Fish and Wildlife Service, 2011).…”

“…Populations can become fragmented when roads act as barriers to animal movement, either through mortality when animals are killed crossing roads, or because animals avoid crossing roads altogether (Anderson, 2002;Forman et al, 2003;Andrews et al, 2005;Shepard et al, 2008). Roads also contribute to habitat loss and degradation (Forman and Alexander, 1998); not only are paved areas uninhabitable for many species, but many species have reduced abundances that extend for hundreds of meters on either side of roads, resulting in road-effect zones (Forman and Deblinger, 2000;Eigenbrod et al, 2009;Shanley and Pyare, 2011;Peaden et al, 2015). These road-effect zones add to the total extent of habitat that is sometimes lost to roads.…”

“…In contrast, some taxa, such as carnivores, carrion-feeding birds, and small mammals, may occasionally benefit from roads (e.g., Agha et al, 2017), which can increase access to resources, such as prey or carrion, and act as movement corridors (Whittington et al, 2011;Abrahms et al, 2016;Dickie et al, 2016). Current research on the negative effects of roads often focuses on quantifying the extent of habitat lost near roads, the numbers of animals killed on roads, and potential mitigation strategies to limit or reverse road effects (Forman and Deblinger, 2000;van Langevelde et al, 2009;Peaden et al, 2015).…”