Barrier effects of roads on an endangered forest obligate: influences of traffic, road edges, and gaps
Introduction
Habitat fragmentation and destruction caused by development of infrastructure such as roads and bridges are recognized as major threats to biodiversity (Czech and Krausman, 1997, Forman and Alexander, 1998). To maintain habitat connectivity, genetic variability, and population persistence, the facilitation of movements of animals through landscapes is critical (Frankham, 1996, Hanski and Gilpin, 1991). Roads and traffic can serve as barriers that impede animal movements, decrease accessibility of resources such as food, shelter or mates, lead to reduction in reproductive success and gene flow, and ultimately threaten population persistence (Strasburg, 2006, Trombulak and Frissell, 2000). Barrier effects of roads have been documented in a diversity of terrestrial fauna, including insects (Bhattacharya et al., 2003), reptiles (Shepard et al., 2008), amphibians (Marsh et al., 2005), birds (Laurance et al., 2004) and mammals (Burnett, 1992), but the causes and mechanisms of road avoidance are not fully understood (Bissonette and Rosa, 2009, Chen and Koprowski, 2013, Roedenbeck et al., 2007).
The barrier effects of roads are driven by several distinct but not mutually exclusive mechanisms that include traffic, edge, and gap avoidance (Barber et al., 2010, Forman et al., 2003, Greenberg, 1989, Jaeger et al., 2005). Traffic avoidance includes avoidance of vehicles as well as traffic disturbance that arises from vehicular noise, movements, vibration, exhaust fumes, dust, headlight illumination and human presence, and has been related to reduction in animal abundance at roadside areas (Barber et al., 2010, Goosem, 2002). Edge avoidance results when animals avoid entering roadside areas due to physical and biotic changes caused by an abrupt transition of ground surface or vegetation (Ford and Lenore, 2008, Forman et al., 2003). Edge effects due to roads can affect the distribution, density and abundance of wildlife in adjacent habitat (Goosem, 2000). Yet, how road edges impact animal movements and space use has been assessed less frequently. Gap avoidance occurs when species avoid clearings with low canopy or understory closure such as roads and forest clearcuts, perhaps because of increased predation risk (Greenberg, 1989) and evolutionary constraints (Laurance et al., 2004).
One fundamental question in road ecology is “what is the relative importance of the different mechanisms by which roads affect population persistence?” (Roedenbeck et al., 2007). Effects of roads on animal populations depend on species life history traits as well as behavioral responses to roads (Benítez-López et al., 2010, Jaeger et al., 2005, Rytwinski and Fahrig, 2012). Previous research on barrier effects has focused on one or two of these potential mechanisms contributing to road avoidance. However, to comprehensively understand barrier effects of roads and develop appropriate mitigation, studies that simultaneously address the relative importance of these different mechanisms are needed. For example, barrier effects of roads due to road avoidance should be distinguished clearly from the effects due to road mortality, as both causes lead to reduced individuals cross roads, but the mechanisms are fundamentally different and require different mitigation (Fahrig and Rytwinski, 2009). Both avoidance of vehicles and avoidance of traffic disturbance result in a decreased rate of road crossings, but avoidance of traffic disturbance can also lead to reduction in animal abundance at roadside areas (Forman and Alexander, 1998, Jaeger et al., 2005).
Tree squirrels (Sciurus and Tamiasciurus) are an ideal group for assessing the impacts of roads on forest dependent species. Arboreal squirrels are widespread, common, and are readily sampled and tracked by radio telemetry because of moderate home range size (Gurnell and Pepper, 1994, Koprowski et al., 2008). Previously, barrier effects of roads have been assessed primarily by capture-recapture methods and translocation (e.g. McDonald and St Clair, 2004). Although such techniques increase understanding of road crossing behavior by highly motivated individuals, the pattern of spontaneous movements or the relationship between home range boundaries and roads is difficult to discern (Ford and Lenore, 2008, Laurance et al., 2004). Techniques like radio telemetry that quantify individual movements can alleviate these issues (Clark et al., 2001). Herein, we combine long-term radio telemetry data and traffic monitoring with high-resolution remote sensing data to examine barrier effects of roads and traffic on animal space use and movements. We use an endangered, endemic forest obligate — the Mt. Graham red squirrel (Tamiasciurus hudsonicus grahamensis) as a model to (1) investigate whether forest roads are barriers and assess the relative importance of traffic, edge, and gap avoidance, and (2) examine factors that influence animal movements and identify environmental features and road characteristics that may improve road permeability.
Section snippets
Study area and study species
Our study was conducted in 342 ha of mixed-conifer forest > 3,000 m elevation in the Pinaleño Mountains (Graham Mountains), Graham County, Arizona, USA (32° 42′ 06″ N, 109° 52′ 17″ W). We used bi-directional traffic counters (TRAFx Vehicle Counter Model G3, TRAFx Research Ltd, Canmore, Alberta, Canada) to monitor 6.6 km of 4 graded dirt roads (Fig. 1a): Arizona State Highway 366 also known as Swift Trail (6 to13-m wide, annual average daily traffic [AADT]: 50 vehicles, hereafter, high traffic), the
Results
We included 307 home ranges that estimated each season for 77 squirrels (39 male, 39 female) in our analyses. No mortality of red squirrels duo to wildlife-vehicle collision was detected. Middens were present on both sides of roads along 92.9% of road sections (n = 14), and 64.4% of middens censused (n = 101) were occupied by red squirrels at least one season from 2008 to 2012. Mean distance from middens to roads was 62.2 m (SE 4.4, n = 38) and to random lines was 44.8 m (SE 3.4, n = 64, t100 = − 3.12, p =
Forest roads serve as barriers
By integrating long-term demographic and telemetry data with remotely sensed environmental characteristics, our study directly assesses effects of roads, traffic intensity, and distance to roads simultaneously on space use and movements of small mammals. In addition, we show how environment, seasonal variation in animal activities, and social interactions affect probability of road crossing. We demonstrate that even a narrow (< 10 m), gravel forest road with low traffic volume (< 10 vehicles/day)
Conservation implications
The ecological and genetic consequences of inhibition of movements and population isolation can be serious, particularly in limited habitat, especially for populations of species at the edge of their distribution range like Mt. Graham red squirrels (Fahrig and Paloheimo, 1988, Fitak et al., 2013, Leonard and Koprowski, 2009). Persistence of forest obligates in isolated fragments depends on their physiological and locomotor ability to cross gaps and the connectivity of fragments (Fahrig, 2007,
Acknowledgments
We thank the University of Arizona, Arizona Agriculture Experiment Station, and T&E Incorporated's Grants for Conservation Research for providing funding, the University of Arizona Statistics Consulting Lab in assisting with data analyses, and R. Gimblett, C. Schwalbe, R. Steidl for review and feedback on study design.
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