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ForestERA Data Layer Overview - Pronghorn Habitat Suitability

Description

This is a layer representing predicted habitat suitability for Pronghorn Antelope (Antelocapra americana) across the assessment area. The layer was built using simple rules along with slope and canopy cover as predictor variables. This layer has a resolution of 90m (0.8 ha or 2 acres). Values for the layer range from zero to one and are unitless. Higher values indicate a higher degree of habitat suitability.

Purpose

This data layer was created as part of the ForestERA project to support landscape-scale forest restoration planning efforts by a broad group of stakeholders including federal and state agencies, academic institutions, and non-governmental entities. These data are intended for regional analyses over spatial extents on the order of tens to hundreds of thousands of acres, and were not developed for use at finer spatial scales, although they may be useful for some applications at finer scales.

Development

The Pronghorn habitat suitability model was created using a set of simple rules based on literature review.

In Arizona, Pronghorn typically avoid areas of high slope (> 20%; Lee et al. 1998; Schuetze & Miller 1992). We created a map of suitable habitat using slope. As areas of slope < 10% have been identified as “preferred” habitat (Schuetze & Miller 1992), we gave these areas a value of 1, areas between 10% and 20% slope were given a value of between 1 and 0 (1 - (slope - 10 / 10)), and areas with slope above 20% were given a value of zero. Pronghorn also typically avoid areas of high woody vegetation cover (Ockenfels et al. 1994). In Arizona, pronghorn frequently use areas with canopy cover 20% or less (Ockenfels et al. 1994), and will use areas of higher cover when traveling between preferred rangelands (Lee et al. 1998). As with slope, we created a map of suitable habitat areas using canopy cover. Areas with canopy cover < 20% were given a value of 1, areas with cover between 20 and 60% were given a value between 1 and 0 (1 - (cover - 20 / 40)), and areas with cover > 60% were given a value of zero.

We combined the two habitat maps into the final habitat suitability map using another set of simple rules. First if either habitat map had a value of zero, the final map was given a value of zero. Otherwise the values for the two maps were added together and divided by two. Thus, the final habitat suitability map has a continuous range from zero to one, with higher values representing habitat that should be more suitable for pronghorn (lower cover and slope).

Accuracy Assessment

A limited accuracy assessment was undertaken using 435 pronghorn locations obtained from Rick Ockenfels of the Arizona Game and Fish Department. These data represent locations of radiocollared Pronghorn that were obtained using GPS triangulation from a flyover airplane. All of the locations come from west of I-17 and south of I-40, primarily in the Garland Prairie area. Of the 435 locations, 336 (77.2%) fall in areas with habitat suitability values of 0.9 or greater. An additional 72 locations (16.5%) fall in areas with values between 0.5 and 0.9. The remaining 47 values (6.3%) fall in areas with values of 0.5 or less.

Sources of errors

It is important to note that this is a model that provides a measure of habitat suitability, not habitat quality, as we have no measure of pronghorn survival or reproduction that we could use to assess habitat quality. In addition, many of the factors that influence habitat quality, such as rangeland quality and herbaceous vegetation composition (Lee et al. 1998; DeVos 1999) cannot be measured easily using remote-sensing imagery.

Pronghorn are unlikely to occupy some areas of apparently suitable habitat due to human-related factors. Fences and highways are barriers that pronghorn will not cross, and thus there may be areas of suitable habitat that the pronghorn cannot reach (Lee et al. 1998; DeVos 1999; Ticer et al. 1999). In addition, Pronghorn are likely to avoid areas of high human activity (Lee et al. 1998; DeVos 1999).

The accuracy assessment uses data from a limited area and also represents sampling from a limited number of pronghorn. Many of the 435 locations used in the accuracy assessment came from the same animals, and are thus not independent samples. A much larger group of samples, covering a larger spatial extent, and including more animals, would be necessary to fully assess this model.

Recommendations

As stated above, this is not a layer representing habitat quality, which would require inclusion of some measure of survivability or reproductive success. We recommend that this layer be used as an overall indicator of how favorable conditions are for the presence of pronghorn. However, finer-scale details such as forage quality, accessibility, and disturbance will also affect pronghorn occupation. Thus more detailed models are needed to predict exactly how Pronghorn will use areas identified as suitable habitat.

Literature Cited

DeVos, J. C. 1999. Status and management needs of pronghorn habitat in Arizona. Pp. 7-15 in Proceedings of the 18th Biennial Pronghorn Antelope Workshop.

Lee, R. M., J. D. Yoakum, B. W. O’Gara, T. M. Polar, and R. A. Ockenfels (eds.). 1998. Pronghorn Management Guides. Arizona Game and Fish Department and Arizona Antelope Foundation. 110 pp.

Ockenfels, R. A., A. Alexander, C. L. D. Ticer, and W. K. Carrel. 1994. Home ranges, movement patterns, and habitat selection of pronghorn in central Arizona. Arizona Game and Fish Department Technical Report #13. 80 pp.

Scheutze, S. M. and W. H. Miller. 1992. Seasonal habitat preferences of pronghorn antelope in central Arizona. Pp. 30-39 in Porceedings of the 15th Biennial Pronghorn Antelope Workshop.

Ticer, C. L. D., S. R. Boe, R. A. Ockenfels, and J. C. DeVos. 1999. Factors affecting home ranges and movements of pronghorn on a shortgrass prairie in northern Arizona. . Pp. 84-90 in Proceedings of the 18th Biennial Pronghorn Antelope Workshop.

Last updated February 11, 2005