Ripton Corridors
Where Is It Safe to Cross the Road?:
Habitat Connectivity in Ripton, VT
Analyzing wildlife corridors and roadway crossings between
vital habitats in the Green Mountains
Wildlife corridors are a key strategy in the maintenance of wildlife biodiversity (Saunders et al., 1991). Roads play a large role in fragmenting habitats and cutting off corridors between populations, as many species avoid road networks (Forman and Alexander, 1998; see diagram). Efforts to provide crossing structures have been demonstrated to have positive effects such as reducing wildlife-automobile collisions (Glista et al., 2009) and can aid wildlife movement, even for large predators (Gloyne and Clevenger, 2001).
Vermont’s expanding road network can therefore have many possible negative impacts on wildlife in the state, making the management roadway crossing a necessary conservation practice (Leoniak, 2009). As detailed in the “Critical Paths” report, the National Wildlife Federation and other state organizations are working to maintain and restore habitat connectivity across roads in Vermont, especially north-south linkages in the Green Mountains (Leoniak, 2009).
One of the “Critical Paths” priority zones (Zone 21) lies in Ripton, VT between the town center and Middlebury College’s Breadloaf campus. This zone is notable for a salt lick that attracts surrounding wildlife, several moose collisions along the roadway, as well as anecdotal reports of bobcat and fisher sightings (Leoniak, 2009). This study seeks to better describe the wildlife corridors and corridor usage in this priority zone, with a focus on bobcats.
Diagram of how road construction can cause habitat fragmentation. Species that need large areas of uninterrupted habitat are most affected, and groups of wildlife can be split into separate populations across the roadway.
Methods
The study area was the town of Ripton, VT. Bobcat habitat patches (derived from the USGS Protected Areas Database) lie in the northwest and southeast corners of the town and were represented with two polygons. Other data include the roads in Ripton and a habitat suitability index (HSI). These layers were used to calculate a wildlife crossing value (WCV) for all roads and cost surface, on which the least cost path and corridor analysis were based.
1. HSI =
(.4 * RoadDensity score) +
(.2 * LULC score) +
(.1 * Elevation score) +
(.1 * EdgeDistance score) +
(.1 * SnowDepth score) +
(.1 * RoadDistance score)
Green = more suitable habitat
Red = less suitable habitat
Ground Truthing
The accuracy of the GIS analysis was tested using ground truthing in the field. The corridor tries to predict where wildlife might most often travel between areas of habitat. Observations made in the field can evaluate if the corridor is actually being used by bobcats and other types of wildlife. The ground truthing was conducted on the afternoon of October 30, 2017 by the GEOG 326 class. Two study areas were selected based on their location in the corridor, proximity to least cost paths, and access to trails continuing off of the roadway for ease of sampling. Study area 1 is off of Route 125 near the Robert Frost trailhead and salt lick location. Study area 2 is off of Steam Mill Road, roughly 0.5 miles east of the intersection with Route 125.
Simplified diagram of the multiple transect sampling method. Instead of a randomly located first transect, the transects were centered around an existing trail.
A transect sampling approach was used, with multiple groups walking transects parallel to the paths and roughly perpendicular to the road, with a set of transects on both sides of the road. The transects were spaced roughly 20 meters apart and were sampled every 10 meters. A point was recorded at each sample spot using the Avenza Maps mobile application indicating the presence or absence of any sign of wildlife. If there was a sign of wildlife, the type of sign and possible species were noted. This sampling method is appropriate given the limited time and resources available for the study. It can detect presence of species on both sides of the roadway, which could indicate if species are crossing the roadway. It seeks to systematically categorize a fairly large area, and representatively sample animal signs which are typically on a centimeter scale. Possible ways to improve the sampling are to pick transects independent of existing paths, sample a wider area, and have increased sample density along each transect. Transects were also not perfectly followed, and the distance between sample was measured by pacing and was not always uniform. However, the data gathered is likely representative enough to make broad conclusions about the presence of wildlife in the corridors.
Sample points within the two study areas. Black points indicate a presence of wildlife, while white points indicate absence.
Note the very rough clustering of points along parallel transects.
187 points were sampled, with 46 points (25%) containing some presence of wildlife. Of those 46 points, the signs included live sightings, scratch marks, tracks, scat, shelters/burrows, and browse. 7 of the signs were attributed to birds, 13 to deer, 1 to porcupine, 1 to squirrel, 10 to some small mammal, and 14 to some unknown source. All species were roughly evenly scattered between the two study areas, although the sample size is too small to make meaningful conclusions comparing the two study areas. The presences had roughly equal frequency on both sides of the road. Study area 1 had 4 points north of the road, 2 near the road, and 6 south of the road. Study area 2 had 13 presences northwest of the road, and 21 southeast of the road.
No definitive signs of bobcat were observed, however given the elusive nature of bobcat and scope of the field study, this is a reasonable result. However, the presence of wildlife in the area was observed with roughly equal frequency on both sides of the two roads, indicating that wildlife likely cross the roadway along the corridor. The lack of control study areas outside of the corridor prevents further conclusions about the relative use of roadway crossing compared to other areas in Ripton, but the ground truthing results somewhat support that these corridors are being used by wildlife to cross the road.
Management Implications
The “Critical Paths” report suggests several possible management solutions to improve the condition of Zone 21: slower speed limits and better signage to reduce collisions with moose, fill the salt lick to reduce animal activity in the area, replace an existing culvert in the center of the zone, and include wildlife crossings as a priority in the Ripton town conservation policies for the zone (Leoniak, 2009).
The lack of traffic on the roads and generally undeveloped nature of the area makes the construction of over- or underpasses built for the specific purpose of habitat connectivity totally unnecessary, although they have been proven to be effective in more heavily trafficked areas (Gloyne and Clevenger, 2001). The lack of traffic also lowers the incentive to actively enforce speed limits. While the ground truthing data does not provide any clear conclusions, I would suggest that management efforts be focused on Route 125, as opposed to Steam Mill Road or other dirt roads in the area.
Bobcats generally avoid roads (VT Fish and Wildlife, 2017), but have been shown to use culverts to cross four-lane highways in Texas (Cain et al., 2003). Therefore the widening and removal of the grate covering the culvert near the Robert Frost trailhead could aid bobcat road crossing, and be a practical step that improves road crossing without negatively impacting wildlife in the area, like the closure of the salt lick would. Increasing awareness in Ripton about the importance of wildlife road crossings in the area is also a useful strategy. Local drivers may be more cautious around the Robert Frost trailhead, and citizens may be willing to help out by regularly reporting wildlife presence in the area. While it is difficult to tailor a management solution specifically to bobcat, general awareness and culvert repair could lead to an overall safer wildlife crossing zone in Ripton.
Works Cited
Austin, J. M., Viani, K., Hammond, F., & Slesar, C. (2005). A GIS-based identification of potentially significant wildlife habitats associated with roads in Vermont. Road Ecology Center.
Cain, A. T., Tuovila, V. R., Hewitt, D. G., & Tewes, M. E. (2003). Effects of a highway and mitigation projects on bobcats in Southern Texas. Biological Conservation, 114(2), 189–197.
Forman, R. T. T., & Alexander, L. E. (1998). Roads and their major ecological effects. Annual Review of Ecology and Systematics; Palo Alto, 29, 207.
Glista, D. J., DeVault, T. L., & DeWoody, J. A. (2009). A review of mitigation measures for reducing wildlife mortality on roadways. Landscape and Urban Planning, 91(1), 1–7.
Gloyne, C. C., & Clevenger, A. P. (2001). Cougar Puma concolor use of wildlife crossing structures on the Trans-Canada highway in Banff National Park, Alberta. Wildlife Biology, 7(2), 117–124.
Leoniak, G. (2009). Critical Paths: Enhancing Road Permeability for Wildlife in Vermont.
Saunders, D. A., Hobbs, R. J., & Margules, C. R. (1991). Biological Consequences of Ecosystem Fragmentation: A Review. Conservation Biology, 5(1), 18–32.
VT Fish and Wildlife. (2017). Bobcat Study in Vermont. Retrieved from http://www.vtfishandwildlife.com/