Contrast metrics

Background.--Contrast refers to the magnitude of difference between adjacent patch types with respect to one or more ecological attributes at a given scale that are relevant to the organism or process under consideration. The contrast between a patch and its neighborhood can influence a number of important ecological processes (Forman and Godron 1986). The 'edge effects' described elsewhere (see Area and Edge Metrics), for example, are influenced by the degree of contrast between patches. Microclimatic changes (e.g., wind, light intensity and quality, etc.) are likely to extend farther into a patch along an edge with high structural contrast than along an edge with low structural contrast (Ranney et al. 1981). Similarly, the adverse affects of brown-headed cowbird nest parasitism on some forest-dwelling neotropical migratory bird species are likely to be greatest along high-contrast forest edges (e.g., between mature forest patches and grassland), because cowbirds prefer to forage in early-seral habitats and parasitize nests in late-seral habitats (Brittingham and Temple 1983). In addition, patch isolation may be a function of the contrast between a patch and its ecological neighborhood. In particular, the degree of contrast between a habitat patch and the surrounding landscape may influence dispersal patterns and survival, and thus indirectly affect the degree of patch isolation. Similarly, an organism's ability to use the resources in adjacent patches, as in the process of landscape supplementation (Dunning et al. 1992), may depend on the nature of the boundary between the patches. The boundary between patches can function as a barrier to movement, a differentially-permeable membrane that facilitates some ecological flows but impedes others, or as a semipermeable membrane that partially impairs flows (Wiens et al. 1985, Hansen and di Castri 1992). The contrast along an edge may influence its function in this regard. For example, high-contrast edges may prohibit or inhibit some organisms from seeking supplementary resources in surrounding patches. Conversely, some species (e.g., great horned owl, Bubo virginianus) seem to prefer the juxtaposition of patch types with high contrast, as in the process of landscape complementation (Dunning et al. 1992).

Clearly, edge contrast can assume a variety of meanings for different ecological processes. Therefore, contrast can be defined in a variety of ways, but it always reflects the magnitude of difference between patches with respect to one or more ecological attributes at a given scale that are important to the phenomenon under investigation (Kotliar and Wiens 1990, Wiens et al. 1985). Similar to Romme (1982), FRAGSTATS employs weights to represent the magnitude of edge contrast between adjacent patch types; weights must range between 0 (no contrast) and 1 (maximum contrast). Under most circumstances, it is probably not valid to assume that all edges function similarly. Often there will not be a strong empirical basis for establishing a weighting scheme, but a reasoned guess based on a theoretical understanding of the phenomenon is probably better than assuming all edges are alike. For example, from an avian habitat use standpoint, we might weight edges somewhat subjectively according to the degree of structural and floristic contrast between adjacent patches, because a number of studies have shown these features to be important to many bird species (Thomas et al. 1978 and 1979, Logan et al. 1985).

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. Weights must range between 0 (no contrast) and 1 (maximum contrast). 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.

Limitations.--Edge contrast indices are limited by the considerations discussed elsewhere for metrics based on total edge length (see Area and Edge Metrics). These indices are only calculated if an edge contrast weight file is specified. More importantly, the usefulness of these indices is directly related to the meaningfulness of the weighting scheme used to quantify edge contrast. Clearly, edge contrast can assume a variety of meanings for different ecological processes. Therefore, contrast can be defined in a variety of ways, but it always reflects the magnitude of difference between patches with respect to one or more ecological attributes at a given scale that are important to the phenomenon under investigation. Under most circumstances, it is probably not valid to assume that all edges function similarly. Often there will not be a strong empirical basis for establishing a weighting scheme, but a reasoned guess based on a theoretical understanding of the phenomenon is probably better than assuming all edges are alike. For example, from an avian habitat use standpoint, we might weight edges somewhat subjectively according to the degree of structural and floristic contrast between adjacent patches, because a number of studies have shown these features to be important to many bird species. Careful consideration should be given to devising weights that reflect any empirical and theoretical knowledge and understanding of the phenomenon under consideration. If the weighting scheme does not accurately represent the phenomenon under investigation, then the results will be spurious.