Core area metrics

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 and 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.

FRAGSTATS Metrics.--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 very 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 of disjunct core areas (NDCA) in each class and the landscape as a whole. 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 as the 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 two 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 one observation, regardless of whether the core area is contiguous or divided into two 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 two 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 the Area-weighted mean core area index (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.

Limitations.--All core area indices are affected by the interaction of patch size, patch shape, and the specified edge depths. In particular, increasing edge depths or shape complexity, or decreasing patch size will decrease core area, and vice versa. On the one hand, this may be desirable as an integrative measure that has explicit functional relevance to the organism or process under consideration. On the other hand, there are potential pitfalls associated with integrative measures like core area. In particular, the confounding of patch area and configuration effects can complicate interpretation. For example, if the core area is small, it indicates that very little core area is available, but it does not discriminate between a small patch (area effect) and a large patch with a complex shape (configuration effect). In addition, core area is meaningful only if the specified depth-of-edge distance is meaningful to the phenomenon under investigation. Unfortunately, in many cases there is no empirical basis for specifying any particular depth-of-edge effect and so it must be chosen somewhat arbitrarily. The usefulness of core area as a metric is directly related to the arbitrariness in the specified edge depths, and this should be clearly understood when using these metrics. Ultimately, the utility of core area metrics compared to their patch area counterparts depends on the resolution, minimum patch dimensions, and edge influence distance(s) employed. For example, given a landscape with a resolution of 1 m2 and minimum patch dimensions of 100 x 100 m, if an edge influence distance of 1 m is specified, then core area and patch area will be nearly identical and core area will be relatively insensitive to differences in patch size and shape. In this case, core area offers little over its patch area counterpart.