Examining possible corridors:
We started out with a fairly accurate vegetation data layer that was developed by the Montana GAP Analysis Program. This was based on satellite imagery with about 30 different vegetation classes that were delineated. They've now mapped the entire state of Montana. Similar projects have been done in many other states. California has an excellent GAP project, and so does Idaho, Wyoming, and Utah.
In Montana, the GAP data were mapped at pixels (or cell sizes) of 2 hectares. But for our purposes, because of the large areas we were looking at and because of some data that indicates bears may respond on a larger scale, we merged these units into cells of one square kilometer. We assigned a value, a simple value of 0, 1, 2, or 3 to each vegetation type, depending on its important to Grizzly Bears. This was determined from the scientific literature and from our own experience.
Using the experience of experts is a technique called Expert Opinion. It's slightly subjective but sometimes it's the only way to get any kind of feeling for these things or to come up with any kind of index. In this case, good habitat for bears is light colored. The combinations of all the cells, each with its unique value, defines a cost surface. The cost surface can then be displayed like this using different shades for different values. So you can see dark areas around cities, and areas of agriculture: this is all wheat fields here in north central Montana, all those areas are pretty much out of the picture for bears.
Next, we calculated the amount of forest edge or heterogeneity in each square kilometer. We did this by measuring the length of the forest/meadow interface using the GIS software on the computer and came up with a number that reflected the amount of edge in each cell. Again, there's evidence in the literature that heterogeneous habitat with a mixture of cover and open areas is preferred by Grizzly Bears. Here, high edge areas are light colored.
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Narrative Index
BioForum Index
