Background.– Core area is defined as the area within a patch beyond some specified depth-of-edge influence (i.e., edge distance) or buffer width. Like patch shape, the primary significance of core area in determining the character and function of patches in a landscape appears to be related to the ‘edge effect.’ As discussed elsewhere (see Area/Density/Edge Metrics), edge effects result from a combination of biotic and abiotic factors that alter environmental conditions along patch edges compared to patch interiors. The nature of the edge effect differs among organisms and ecological processes (Hansen and di Castri 1992). For example, some bird species are adversely affected by predation, competition, brood parasitism, and perhaps other factors along forest edges. Core area has been found to be a much better predictor of habitat quality than patch area for these forest interior specialists (Temple 1986). Unlike patch area, core area is affected by patch shape. Thus, while a patch may be large enough to support a given species, it still may not contain enough suitable core area to support the species. In some cases, it seems likely that edge effects would vary in relation to the type and nature of the edge (e.g., the degree of floristic and structural contrast and orientation). Thus, FRAGSTATS allows the user to specified an edge depth file that contains edge influence distances for every pairwise combination of patch types. In the absence of such information, the user can specify a single edge depth for all edge types.

In raster images, there are different ways to determine core area. FRAGSTATS employs a method involving the use of a variably-sized masked placed on cells on the perimeter of a patch, where the mask size varies depending the specified edge depth associated with the corresponding combination of patch types. Actually, the mask is placed over cells just outside the patch perimeter; referred to here as ‘bounding’ cells. Briefly, a mask is placed over each bounding cell. The mask itself is near circular in shape (as circular as you can get in the raster world) and sized according to the specified edge depth. Note, the resolution of the mask is constrained by cell size; thus, the mask is rounded up or down to the nearest cell given the specified edge depth. For example, given a 30 m cell size and a specified edge depth of 50 m, the mask will be rounded up to 2 cells (60 m) wide in the orthogonal directions. The non-orthogonal directions will be rounded similarly, producing a near circular mask. Cells within the mask are eliminated from the ‘core’ of the patch. After all bounding cells are treated in this manner, the remaining cells not masked constitute the ‘core’ of the patch.