FRAGSTATS Metics.--FRAGSTATS computes several metrics based on core area at the patch, class, and landscape levels. Most of the indices dealing with number or density of patches, size of patches, and variability in patch size have corresponding core area indices computed in the same manner after eliminating the specified edge from all patches. For example, patch area, class area, total landscape area, and the percentage of landscape in each patch type all have counterparts computed after eliminating edge area defined by the specified edge depth; these are core area (CORE) at the patch level, total core area (TCA) at the class and landscape levels, and core area percent of landscape (CPLAND) at the class level. The latter index quantifies the core area in each patch type as a percentage of total landscape area. For organisms strongly associated with patch interiors, this index may provide a better measure of habitat availability than its counterpart, percentage of landscape (PLAND). In contrast to their counterparts, these core area indices integrate into a single measure the affects of patch area, patch shape, and edge effect distance. Therefore, although they quantify landscape composition, they are affected by landscape configuration. For this reason, these metrics at the class level may be useful in the study of habitat loss and fragmentation.

From an organism-centered perspective, a single patch may actually contain several disjunct patches of suitable interior habitat, and it may be more appropriate to consider disjunct core areas as separate patches. For this reason, FRAGSTATS computes the number of core areas (NCORE) in each patch, as well as the number in each class and the landscape as a whole (NDCA). If core area is deemed more important than total area, then these indices may be more applicable than their counterparts, but they are subject to the same limitations as their counterparts (number of patches) because they are not standardized with respect to area. For this reason, number of core areas can be reported on a per unit area basis (disjunct core area density, DCAD) that has the same ecological applicability as its counterpart (patch density), except that all edge area is eliminated from consideration. Conversely, this information can be represented as mean core area (CORE_MN). Like their counterparts, note the difference between core area density and mean core area at the class level. Specifically, core area density is based on total landscape area; whereas, mean core area is based on total core area for the class. In contrast, at the landscape level, they are both based on total landscape area and are therefore completely redundant (at least if the landscape contains no background). Furthermore, mean core area can be defined in 2 ways. First, mean core area can be defined as the mean core area per patch (CORE_MN). Thus, patches with no core area are included in the average, and the total core area in a patch is considered together as 1 observation, regardless of whether the core area is contiguous or divided into 2 or more disjunct areas within the patch. Alternatively, mean core area can be defined as the mean area per disjunct core (DCORE_MN). The distinction between these 2 ways of defining mean core area should be noted.

FRAGSTATS also computes an index that quantifies core area as a percentage of total area. The core area index (CAI) at the patch level quantifies the percentage of the patch that is comprised of core area. Similarly, at the class and landscape levels core area index area-weighted mean (CAI_AM) quantifies core area for the entire class or landscape as a percentage of total class or landscape area, respectively. Note, that this is equivalent to the total core area index reported in FRAGSTATS 2.0. The core area index is basically an edge-to-interior ratio like many of the shape indices (see Shape Metrics), the main difference being that the core area index treats edge as an area of varying width and not as a line (perimeter) around each patch. In addition, the core area index is a relative measure; it does not reflect patch size, class area, or total landscape area; it merely quantifies the percentage of available area, regardless of whether it is 10 ha or 1,000 ha, comprised of core. This index does not confound area and configuration like the previous core area indices; rather, it isolates the configuration effect. For this reason, the core area index is probably best interpreted in conjunction with total area at the corresponding scale. For example, in conjunction with total class area, this index could serve as an effective fragmentation index for a particular class.

An alternative method of assessing core area is based is based on the medial axis transformation (MAT) of the patch (Gustafson and Parker 1992). The MAT skeleton is derived from a depth map of the patch, where each pixel value represents the distance (in pixels) to the nearest edge. The MAT skeleton is then produced by removing all pixels from the depth map except local maxima (pixels with no neighbors having greater values). The resulting MAT skeleton gives the depth to the extreme core of the patch. As such, it provides explicit information on how far the ‘core’ of the patch is from the nearest edge. The average depth index (ADEPTH) and maximum depth index (MDEPTH) provide two different ways to summarize the depth of the MAT skeleton. Like most other core area metrics, indices based on the MAT skeleton integrate the effects of patch area and shape. Holding area constant, more convoluted shapes will tend to have MAT skeletons closer to the perimeter. Similarly, holding shape constant, larger patches will have MAT skeletons farther from the perimeter. However, in contrast to all other core area metrics, metrics based on the MAT skeleton do not depend on user-specified edge depths. Thus, the ecological interpretation of these metrics is done after-the-fact based on the ecological phenomena under consideration; whereas the functional relevance of edge effects is explictly incorporated into the edge depths used in all other core area metrics.