Wetland Habitat Suitability Analysis for the North American Beaver in Hadley and Amherst, Massachusetts Mark Salhany Department of Environmental Conservation Habitat Analysis
Introduction The North American Beaver is an aquatic rodent that is endemic to North America, found in close proximity to bodies of water. While beaver populations were once extremely high prior to European colonization of North America, the subsequent fur trade throughout the growing colonies reduced beaver populations to dangerously low numbers. Eventually, the North American beaver was nearly wiped out from the northeast entirely and trappers and traders ventured westward to seek out their desired furs. Protective measures arose in the late 19th century effectively bringing this animal back from the brink of extinction and over the course of the 20th century, beaver populations have rebounded drastically. However, with modern day urbanization, beaver populations in New England face severe habitat Figure 1: North American destruction and fragmentation. Beaver
While some consider beavers to be pests due to the potential flooding caused by the building of their dams, they are actually a highly important keystone species and they bring a wide range of beneficial impacts on their local ecosystems. The building of their dams leads to the creation of beaver ponds which enable a greater area of water to exist in an aquatic environment. This helps to generate new aquatic habitat for riparian plants (as aquatic nutrients collect in the water) which ultimately leads to local increases in biodiversity of insects, invertebrates, fish, mammals, and birds. Their unique behavior has also been shown to increase stream flow and water quality (Managing beaver ponds. North Carolina A & T State University, Greensboro, NC, Retrieved from http://www.ces.ncsu.edu/forestry/pdf/www/www23.pdf).
Methods MassGIS provides many of the required datalayers for conducting this study. From MassGIS, I first obtained a shapefile representing the towns in the state of Massachusetts from which I clipped out my specific study area. I then obtained a Massachusetts wetlands index layer as well as a DEM from which I was able to extrapolate slope and stream gradient. I obtained the necessary road and landcover datalayers that included information on deciduous and coniferous tree cover. To perform the analysis, I underwent a series of raster stacking operations for each of my data layers using the raster calculator tool in ArcMAP. To begin, I first needed to create a series of rasters that represented the preferred factors contributing to highly suitable beaver habitat as defined earlier. Making use of the Euclidian distance tool, I generated two new rasters, one portraying a continuous distance grid from sources of water, and the other showing distance from roads. Then, using the raster calculator, I set up my conditions as needed to create two new rasters, one that was set equal to 1 at all locations that are greater than 150m from roads (figure 4), and another that set all locations equal to 1 that fall within 30m of wetlands (figure 5).
Regarding the habitats of beavers and the locations of their preferred habitats, a study conducted by the U.S. Fish and Wildlife Service found that beavers prefer the leaves and bark of deciduous plants over that of coniferous plants and that they require a permanent supply of water such as a stream, pond, or lake (rarely residing more than 30 meters from this water source) (Alan, 1983). It's also known that beavers prefer inhabiting streams that have a low degree of incline (channel gradient of 15% or less). A study in Colorado showed that 68% of all beaver lodges were found on streams with a channel gradient less than 6%. Lastly, roads and railways passing near waterways can have a disruptive effect on beaver habitat suitability. With this information, a highly suitable beaver habitat could be defined as one with an abundance of deciduous foliage, within a very close proximity of a water source (30m), at a sufficient distance from roadways (150m), and located on streams with very slight incline (stream gradient less than 6%). This poses a key research question: how much of this suitable habitat actually remains intact in Hadley and Amherst, Massachusetts and its surrounding wetland areas?
Figure 2: Study Area
Figure 3: North American Beaver Lodge
This GIS effectively locates the areas of both highly favorable beaver habitat (areas in which beavers are most likely to reside) and areas of less favorable (but still viable) beaver habitat in Hadley and Amherst, Massachusetts for the ultimate purpose of determining the level of threat that beavers face due to local habitat destruction and fragmentation. The GIS calculates both the precise locations of potential beaver habitat based on the above factors as well as the total remaining area of these habitats.
Results Figure 8 displays the results of the Boolean And operations as detailed in the methods section. Green cells represent locations that satisfied all of the criteria for favorable beaver habitat and so these locations were deemed as “ideal” or highly suitable areas of beaver habitat. Yellow cells represent locations that satisfied three of four of the specified criteria and these were deemed as “good” or viable locations for beaver habitat. And finally, the orange cells represent locations that satisfied just two of four of the specified criteria and were deemed as “moderate” or semi-suitable locations for beaver habitat. According to this GIS analysis, there is approximately 4,000m2 of highly suitable (green) beaver habitat throughout Amherst and Hadley, 6,400m2 of good habitat (yellow), and 800m2 of semi-suitable habitat. Thus, Amherst and Hadley have an estimated total of 11,200m2 of suitable or highly favored beaver habitat, which is about 2.8 acres.
Figure 4: Ideal distance from roads
Figure 5: Ideal proximity to wetlands
Then, to approximate slope gradient for streams and water bodies, I used the spatial analysis slope tool to create a raster representing changes in elevation over each cell. Using this raster, I classified areas with rapidly changing values of elevation to be unsuitable for beaver habitat and I classified areas with small elevation gradients (less than 6%) to be highly suitable, giving these areas a value of 1 (figure 6). The final necessary raster, obtained from the USGS Land Cover Institute (LCI), provides a full range of land cover data. Using this data, I created three new rasters: a grid of deciduous tree cover (ideal for habitat), coniferous tree cover (unfavorable), and mixed coniferous-deciduous land cover (good), as shown in figure 7. With all of these datalayers, I now performed a series of Boolean And operations using the raster calculator, stacking together the ideal conditions for beaver habitat, as seen in figure 8. Rasters representing differing levels of habitat viability were similarly stacked together, showing a broader spectrum of available beaver habitat within the study area.
Figure 6: Preferred slope
As seen through the results, both Amherst and Hadley are not abundant in highly suitable beaver habitat with Hadley containing slightly more highly suitable habitat than Amherst, (by about 1000m2). This difference is of course likely a result of the vastly greater population density as seen in Amherst as well as the way in which roads, bike paths, and railways intersect and fragment the wetland areas in Amherst to a much higher degree. A relevant follow up study might seek to analyze the specific impacts brought about by roadway intersection throughout wetland areas and its subsequent effect on endemic species of wetland fauna, such as but not limited to the North American beaver. Again, beavers often come in conflict with human settlements and infrastructure and are purposely removed in some municipalities. For the case of Amherst and Hadley, a series of guidelines have been published to educate the public in these areas about the protection of and interaction with beavers.
Figure 7: Tree cover data