FRAGSTATS Metrics.–FRAGSTATS computes several indices based on edge contrast at the patch, class, and landscape levels. At the patch level, the edge contrast index (ECON) measures the degree of contrast between a patch and its immediate neighborhood. Each segment of the patch perimeter is weighted by the degree of contrast with the adjacent patch. Total patch perimeter is reduced proportionate to the degree of contrast in the perimeter and reported as a percentage of the total perimeter. Thus, a patch with a 10% edge contrast index has very little contrast with its neighborhood; it has the equivalent of 10% of its perimeter in maximum-contrast edge. Conversely, a patch with a 90% edge contrast index has high contrast with its neighborhood. Note that this index is a relative measure. Given any amount of edge, it measures the degree of contrast in that edge. In other words, high values of ECON mean that the edge present, regardless of whether it is 10 m or 1,000 m, is of high contrast, and vice versa. At the class and landscape levels, FRAGSTATS computes a total edge contrast index (TECI). Like its patch-level counterpart, this index quantifies edge contrast as a percentage of maximum possible. However, this index ignores patch distinctions; it quantifies edge contrast for the landscape as a whole. FRAGSTATS also computes distribution statistics for the edge contrast index at the class and landscape levels. The mean edge contrast index (ECON_MN), for example, quantifies the average edge contrast for patches of a particular patch type (class level) or for all patches in the landscape.

These edge contrast indices are relative measures. Given any amount or density of edge, they measure the degree of contrast in that edge. High values of these indices mean that the edge present, regardless of whether it is 10 m or 1,000 m, is of high contrast, and vice versa. For this reason, these indices are probably best interpreted in conjunction with total edge or edge density. Because of this, FRAGSTATS also computes an index that incorporates both edge density and edge contrast in a single index. Contrast-weighted edge density (CWED) standardizes edge to a per unit area basis that facilitates comparison among landscapes of varying size. Unlike edge density, however, this index reduces the length of each edge segment proportionate to the degree of contrast. Thus, 100 m/ha of maximum-contrast edge (i.e., weight = 1) is unaffected; but 100 m/ha of edge with a contrast weight of 0.2 is reduced by 80% to 20 m/ha of contrast-weighted edge. This index measures the equivalent maximum-contrast edge density. For example, an edge density of 100 means that there are 100 meters of edge per hectare in the landscape. A contrast-weighted edge density of 80 for the same landscape means that there are an equivalent of 80 meters of maximum-contrast edge per hectare in the landscape. A landscape with 100 m/ha of edge and an average contrast weight of 0.8 would have twice the contrast-weighted edge density (80 m/ha) as a landscape with only 50 m/ha of edge but with the same average contrast weight (40 m/ha). Thus, both edge density and edge contrast are reflected in this index. For many ecological phenomena, edge types function differently. Consequently, comparing total edge density among landscapes may be misleading because of differences in edge types. This contrast-weighted edge density index attempts to quantify edge from the perspective of its functional significance. Thus, landscapes with the same contrast-weighted edge density are presumed to have the same total magnitude of edge effects from a functional perspective.

All edge contrast indices consider landscape boundary and background segments even if they have an edge contrast weight of zero. In the absence of a landscape border, the landscape boundary is assigned as background edge and treated according to the background contrast weight specified in the contrast weight file. In the presence of a landscape border, all landscape boundary edges are made explicit by the information present in the border and are assigned the appropriate contrast weight given in the contrast weight file. Regardless of whether a border is present or not, all background edges, both internal (positively valued) and external (negatively valued), are assigned the background contrast weight specified in the contrast weight file. Assigning a meaningful contrast weight to the boundary and background presents a special challenge because, in practice, background (and the boundary, in the absence of a border) often represents area for which nothing is known. Thus, it can be difficult to assign a single contrast weight that applies equally well to all background/boundary edges. A landscape border is often included to avoid this problem, because all boundary edges are made explicit; however, even a border doesn’t eliminate the problem of assigning a weight to background if it exists. The potential severity of the boundary/background problem depends on the size and heterogeneity of the landscape and the extent of background edge. Larger and more heterogeneous landscapes without little or no background will have proportionately less total edge located along the boundary and/or background.