Applicability of Viewshed Analysis to Wildlife Population Estimation
Maichak, Eric J., Schuler, Krysten L., The American Midland Naturalist
Population estimations and indices based on roadside or spotlight counts typically require investigators to determine the route with maximum viewing area and calculate area visible from the census route. Viewshed analysis may provide a simpler and more accurate method for determining route, area and population estimate. From September 2000 to August 2001 we observed adult bison from a driving route at The Nature Conservancy's Tallgrass Prairie Preserve. We recorded group locations to field test the ability of viewshed analysis to predict the area where bison were visible. We developed viewsheds based on digitized layers of the driving route and for high, low and random points within the Preserve to determine the viewshed with maximum visible area. We developed a population estimate based on number of bison observed and visible area calculated from our driving route. We compared population estimates to known bison populations before and after 1 May 2001 and for our entire study period. Ninety-nine percent (P < 0.001) of bison locations occurred within the viewshed. Our driving route provided the most visible area (81%), followed by viewsheds of high (77%), random (61%) and low (31%) points. Estimated adult bison populations before 1 May 2001 were different (P < 0.001), but similar on and after 1 May 2001 (P = 0.10) to known populations. The estimated population for the entire study was different (P < 0.001) from the mean population size. Viewshed analysis is an efficient method to predict locations of easily observed ungulates and has utility in planning census routes with maximum visible area a priori. We suggest that viewshed analysis provides a more accurate estimation of area visible from census routes and resulting population estimations than conventional surveys.
Population indices based on road counts (Norton-Griffiths, 1978) and nocturnal spotlight counts are widely used because of their low cost and simplicity in design, use and comprehension (Whipple et al, 1994). Such surveys have been conducted to estimate population densities of brown hares (Lepus capensisL., Barncs and Tapper, 1985) and trends of population abundance of San Joaquin kit foxes (Vulpes macrotis mutica, Rails and Eberhardt, 1997). Spotlight surveys have been used also to determine population densities, trends and herd compositions of white-tailed deer (Odocoileus virginianus, Progulske and Duerre, 1964; Fafarman and DeYoung, 1986; Mitchell, 1986; Storm et al, 1992), herd compositions of black-tailed cleer (O. hemionus colubianus, McCullough et al, 1994) and population densities and trends of raccoons (Procyon lotor, Gehrt, 2002). Population estimates and indices based on road and spotlight counts rely on observers sighting animals from a travel route and extrapolating population size for a given area (visible and not visible) based on number of animals counted in the area visible from the travel route (Ratti and Garton, 1996). Methods for determining area visible from the travel route require researchers take distance measurements perpendicular to the route every 0.16 km (Whipple et al., 1994) to 0.8 km (Mitchell, 1986). These methods can be tedious and average area derived from perpendicular measurements may be under or overestimated (Whipple et al., 1994) resulting in an inaccurate estimate, index and trend of population size and/or density.
Geographic Information Systems (GIS) software has applications to various wildlife management objectives (Naugle et al., 1998). Viewshed analysis within the 3D Analyst extension of Arcvicw GIS (Ormsby and Alvi, 1999) determines the area an observer can or can not see from a given point and has been used to locate optimal sites for observation towers (Lee, 1991) and communication relays (De Floriani et al., 1994). Viewshed analysis may facilitate locating population census routes with maximal visibility (e.g., area visible from a driving route or transect, Berry, 2001). …