In order to obtain an overall view of possible habitat locations on Blue Mountain we first obtained aerial photographs (US Department of Agriculture Agricultural Stabilization and Conservation Service 1964) and USGS 7.5 minute series topographical maps of Blue Mountain. The photographs were scanned into a computer with an HP-5SE Scanjet scanner and enhanced using Adobe Photoshop. Potential habitat was identified on the photographs. In order to perform our surveys from an airplane, we had to ascertain that good habitat is distinguishable from the air. Suitable habitat was identified as loose rock free of fill, similar to habitat inhabited by woodrats in other areas. We determined the border between good and poor habitat at the powerline west of Sterretts Gap (N 40 degrees 18 minutes, W 77 degrees 8 minutes), then flew over the same area in a Cessna 172 to determine whether this difference is apparent from the air. Next followed a series of aerial reconnaissance flights where we photographed possible habitat patches with a Kodak DC120 digital camera and transferred the pictures into a computer using Kodak Picture Transfer. These photographs were used to delineate the borders of good habitat patches. Using Adobe Photoshop, we created a transparent layer on the aerial photographs containing the habitat borders. These two layered photographs were converted from jpg to tiff format and reduced in size using Graphics Converter. The scale of the pictures was established using GPS coordinates that were obtained with a Garmin 40 hand held GPS receiver of three points on Waggoners Gap (N 40 degrees 16 minutes, W 77 degrees 17 minutes) Road, forming a triangle of known area. These points were used to calibrate the scale of the aerial photographs. Using NIH-image the areas of each patch and the linear distance between the borders of patches was calculated. Microsoft Excel was used to calculate the mean and median patch size and the mean and median distance between patches.

    Based on previous data (Wright 1998), we grouped habitat patches with a linear distance between patches of less than 100 meters. Woodrats can easily travel between these patches during foraging or den movements, so these clusters represent subunits within this metapopulation.

RESULTS

     The area of Blue Mountain, Cumberland County, PA, included in this study stretched from the Susquehanna River 14.75 km west to a distinctive bend in the mountain we designated the elbow. No woodrat habitat was found east of Sterretts Gap, 3.13 km west of the Susquehanna River. All calculations therefore include only the area between Sterretts Gap and the elbow. The total area of the mountain included in the calculations was between Sterretts Gap and the elbow and included an area of 13,219,206 square meters. Within this area, the average rocky woodrat habitat patch size was 31,720 (+/- 51,685 s.d.) square meters. The median patch size was 12,426 square meters. The total habitat on this section of Blue Mountain was 1,586,015 square meters and covered 12% of this area of the mountain. The average distance between patches was 138 (+/- 78 s.d.) meters. The median distance between patches was 113 meters. Click here to view a geographical map of habitat on Blue Mountain, including a table of patch size and distance between patches. This map shows reasonable habitat groupings, with less than 100 meters between each patch.

DISCUSSION

     All suitable woodrat habitat on Blue Mountain was located west of Sterretts Gap, and mostly on the south face of the mountain where the sandstone outcroppings occurred. Patches ran roughly parallel to the ridge of the mountain. Although there seemed to be many habitat patches within this metapopulation, several were located more than 100 meters from the borders of any other patches, making it unlikely that woodrats will colonize many of these patches. Woodrats are capable of moving longer distances, but to establish a colony on a habitat patch within a metapopulation requires free movement of woodrats between patches.

     Thirty of the forty-nine suitable habitat patches were found to be within 100 meters of another patch, allowing them to be grouped. Movement between these patches is likely. According to the metapopulation model some patches in one cluster may be occupied while others are not, and changes in the subpopulation of each patch are frequent, although the population of the cluster remains relatively stable. This cycle of habitation can be disturbed by an increase in stressers, which cause the population in a patch to die out faster than it can be recolonized, which might eventually cause the population in the habitat grouping to die out. More isolated patches tended to be smaller, and are therefore less likely to be colonized. In the event of some form of stress on the woodrats in these colonies, such as heavy rate of infection by parasites or an increase of predators, relative isolation makes these colonies more likely to be extinguished and less likely to be recolonized.

     Only one experimental recolonization of a woodrat population has been done in Pennsylvania (Wright, 1998). Although this experiment included a relatively small sample size, it demonstrated the kind of long distance movement that could recolonize an uninhabited patch. The woodrats were released on patch number 14. One woodrat ended up in patch number 22, while another stayed in patch 14 for a year, then moved to patch number 1 or 2. Future recolonization studies conducted on Blue Mountain should reflect the habitat groupings of this study. Studies should include other metapopulations as well to determine if survival or extinction is based on the layout of the habitat. The likelihood of success of a future recolonization study is greatly increased if the study area coincides with a likely habitat grouping. Allegheny woodrats are known to have lived in the Waggoners Gap area as late as 1998. It is unknown whether woodrats live in any other habitat patches on Blue Mountain, so live trapping is necessary to determine the presence of woodrats in any of the habitat patches on Blue Mountain. If there are woodrats living in one or more patches, radiotelemetry studies should be done to explore the dispersal capabilities of woodrats. These studies would help refine the metapopulation model for woodrats, aiding in the effort to conserve the Allegheny woodrat. A project similar to that suggested has been done on Stephens' Kangaroo Rat (Price 1996). This project utilized actual data on the Kangaroo Rat, making the predictions reflect real circumstances. This project could serve as a model for what types of information and what methods are needed to create a metapopulation model for woodrats.

Acknowledgements

We would like to thank Professor Janet Wright, Dickinson College, PA for her exceptional skill in piloting.

We would like to thank the Natural Lands Trust of Media, PA who provided support for this research.

To view the aerial photographs of Blue Mountain click here

To see the resources used in preparation of this page click here

To see a list of related links click here

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