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Ripton Wildlife Corridor

Using Bobcat population data to identify a least-cost path for wildlife movement

   As development of land increases and more roads are built, wildlife habitats become increasingly fragmented. This fragmentation leads to higher rates of human-wildlife conflict. An all too common example of such conflict in Vermont is car accidents, which occur when wildlife attempt to cross roads that divide their habitat range, and often result in wildlife fatalities, human injury and substantial economic costs to repair damaged vehicles (Lab: Least Cost Corridor).

One strategy to help reduce human-wildlife conflict is to create wildlife corridors, or structures that connect important blocks of habitat that have been fragmented by roads.

   The Critical Paths project aims to improve habitat connectivity, and is being used to assist the Vermont State Wildlife Action Plan (SWAP). SWAP specifically emphasizes the need to address road crossing areas, and has identified 21 priority zones across the state. One of those priority zones is located along Route 125 in Ripton, Vermont.

Methods

   In this analysis, I used GIS to identifiy a Least Cost Corridor between the Battell Wilderness area and the Green Mountain National Forest, which are on either side of Route 125 in the priority zone identified by the Critical Paths Project. A Least Cost Corridor refers to the path that represents the most favorable path for wildlife between two habitat areas.

   First, I created a cost surface layer by inverting the Habitat Suitability Index from the Bobcat Habitat project. This involved a simple raster calculator operation to invert the HSI values, producing a layer showing the unsuitability for wildlife habitat. Then, I used the Cost Distance tool in ArcGIS to calculate the cost distance from each habitat patch (ran the tool for both the Battell Wilderness and Green Mountain National Forest, resulting in two raster layers). In a cost distance layer, each pixel has a value representing the lowest possible accumulated cost (based on the cost surface layer) of reaching that point from the source feature (habitat patch). This tool also outputs a cost back link file, which assigns a value to each pixel based on the direction of travel back to the habitat patch that results in lowest accumulated cost.

Ground Truthing

  In an effort to confirm that wildlife are using the identified corridor, and confirm that the area around Route 125 is in need of corridor management, I collected data on site using the Avenza Maps app. We conducted field observation at 5 meter intervals along a section of the Robert Frost trail just to the north and south of Route 125 (from trail head at Route 125) and combined it with field observation data collected near Steam Mill Road, to the north east of Route 125.

  We collected the field data by transecting the Robert Frost trail and the trail head on the opposite side of Route 125, collecting data at 5 meter intervals and noting presence or absence of wildlife. Transect methods of data collection method are standardized and objective, and useful for covering a large amount of area around a road.

Corridor and Cost Path analysis

   To identify a corridor of least-cost travel between the two habitat patches, I used the Corridor tool in ArcGIS. This tool takes in the cost distance layers for each habitat patch and returns a raster with the sums of the cost distance layers. Corridor analysis outputs a layer where each pixel is assigned a value based on total cost of travel between the two habitat patches. The lowest values of that raster layer can be highlighted to identify the least cost corridor based on cost distance from both habitat patches. By changing the display color of the lowest values (8979-8985) to bright turquoise green, I made the resulting corridor between Batell Woods and the National Forest easily identifiable (map at left).

   I also ran a tool called Cost Path, which uses a cost distance raster and back link rasters from one of the habitat features to identify the least cost paths to reach the other habitat feature. I ran the tool using the cost distance and back link for the Battell Wilderness area. The map produced by this tool (above) shows the least cost path options in turquoise, overlaid on the cost surface layer produced in the first step.

Image source: National Wildlife Federation Northeast Regional Center

Limitations

   The cost surface layer is only as good as the habitat suitability index on from which it is created. The limitations of the bobcat suitability index are discussed here.

   The extent of the field data collected was limited to what could be gathered by our lab team in a three hour time block, so while it was sufficient to document the presence of wildlife to the north and south of Route 125 and confirm the likelihood of its use for wildlife crossing, it does not provide much information about where specifically along Route 125 the wildlife usage might be highest. The site sampled was slightly to the west of the corridor identified in the study.

   Another limitation of the data collection is that spotting signs of wildlife presence was difficult due to recent rain.

Results and Implications

   The ground-truthed data show the presence of wildlife to the North and South of Route 125, and the GIS model identified a least-cost corridor suggesting that the area is in need of corridor management. These results support the Critical Paths project's identification of priority zone 21 along Route 125 in the town of Ripton, Vermont. Strategies to help reduce human-wildlife conflict in this area could include:

- Identify wildlife crossing as significant wildlife habitat

-Signage to promote awareness of the importance of wildlife crossing at Robert Frost Memorial trailhead

-Efforts to conserve private land in the area

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