Background.–Contagion refers to the tendency of patch types to be spatially aggregated; that is, to occur in large, aggregated or “contagious” distributions. Interspersion, on the other hand, refers to the intermixing of patches of different types and is based solely on patch (as opposed to cell) adjacencies. Contagion and interspersion are both aspects of landscape texture; they both reflect the adjacency of patch types, but do so in a different manner. Contagion reflects both the dispersion (i.e., the spatial distribution) and intermixing of patch types, whereas interspersion reflects only the latter. Thus, as a measure of landscape texture, contagion subsumes interspersion.

Contagion is also closely allied to the concept of subdivision. In its narrowest sense, contagion ignores patches per se and measures the extent to which cells of similar class are aggregated. In other words, contagion reflects the overall clumpiness of the landscape without explicit reference to the patches. Subdivision, on the other hand, refers explicitly to the degree to which patch types are broken up (i.e., subdivided) into separate patches (i.e., fragments), not the shape, relative location, or spatial arrangement of those patches. These differences are subtle, but important–at least computationally, if not conceptually. Contagion (at the landscape level) deals with both the dispersion and interspersion of patch types, while subdivision deals only with the dispersion of patch types, not interspersion. This distinction is relatively straightforward. The confusion between contagion and subdivision lies in how they handle dispersion. Contagion deals specifically with the ‘aggregation’ of patch types; it is affected only by the clumpiness of cells of the same class. Computationally, contagion is computed from the proportion of cell adjacencies that involve the same class (i.e., like-adjacencies); it doesn’t matter what patch a cell belongs to or how many patches there are, only how many of the cell sides are like-adjacencies. Large, compact patches have a high proportion of like-adjacencies and therefore produce high contagion. Conversely, large, but highly convoluted (e.g., linear) patches have a low proportion of like-adjacencies and therefore produce low contagion–despite the similarity in patch sizes. Accordingly, the number and size of disjunct patches–that is, the subdivision of the landscape–affects contagion only indirectly by affecting the proportion of like adjacencies. Thus, contagion reflects the ‘compactness’ of patches, not the number and size of patches per se–although in real landscapes compactness and size are often highly correlated. Subdivision, on the other hand, deals with the aggregation of patch types, like contagion, but deals explicitly with the number and size of patches as well. Indeed, the subdivision metrics computed by FRAGSTATS (described below) are based on the cumulative patch size distribution, not cell adjacencies. Large, contiguous patches, even if they are highly elongated or convoluted, are undivided and therefore produce low subdivision. Despite the theoretical and conceptual differences between contagion and subdivision, in practice these two aspects of landscape texture are often highly confounded. For this reason, it is easier to consider contagion and subdivision together, as we have done here.

Contagion and interspersion broadly refer to the overall texture of the entire landscape mosaic, as described above. However, contagion and interspersion can be applied at the class level as well, although their meaning changes somewhat. At the class level, interspersion has basically the same interpretation; it refers to the intermixing of the focal patch type with the other patch types. The distinction here is the focus on a single patch type and its adjacencies to other patch types, as opposed to the intermixing of all patches. Similarly, contagion at the class level refers to the tendency of a single focal patch type to be spatially aggregated; that is, to occur in large, aggregated or “contagious” distributions. Here, the distinction between class and landscape levels is important. Recall that at the landscape level, contagion refers to both the dispersion and interspersion of patch types. At the class level, however, contagion refers to the spatial aggregation of the focal patch type without reference to its interspersion. Consequently, measures of class-level contagion are quite different computationally from measures of contagion computed at the landscape level. At the class level, contagion is more closely akin to the concept of subdivision because it deals exclusively with the aggregation or disaggregation (i.e., fragmentation) of the focal class–although the subtle distinction between contagion and subdivision regarding dispersion described above still applies.

The texture of a landscape is a fundamental aspect of landscape pattern and is important in many ecological processes. The subdivision of a patch type of course plays a crucial role in the process of habitat fragmentation. Specifically, habitat fragmentation involves the disaggregation and subdivision of contiguous habitat into disaggregated and/or disjunct patches. As habitat fragmentation proceeds, habitat contagion decreases, habitat subdivision increases, and eventually ecological function is impaired (Saunders et al.1991). Specifically, the subdivision and isolation of populations caused by this fragmentation can lead to reduced dispersal success and patch colonization rates which may result in a decline in the persistence of individual populations and an enhanced probability of regional extinction for entire populations across the landscape (e.g., Lande 1987; With and King 1999a,b; With 1999). In addition, the subdivision and interspersion of patch types may affect the propagation of disturbances across a landscape (Franklin and Forman 1987). Specifically, a patch type that is highly disaggregated and/or subdivided may be more resistant to the propagation of some disturbances (e.g., disease, fire, etc.), and thus more likely to persist in a landscape than a patch type that is highly aggregated and/or contiguous. Conversely, highly disaggregated and/or subdivided patch types may suffer higher rates of disturbance for some disturbance types (e.g. windthrow) than more aggregated and /or contiguous distributions. Similarly, interspersion is presumed to affect the quality of habitat for many species that require different patch types to meet different life history requisites, as in the process of landscape complementation (Dunning et al. 1992). Indeed, the notion of habitat interspersion has had a preeminent role in wildlife management during the past century. Wildlife management efforts are often focused on maximizing habitat interspersion because it is believed that the juxtaposition of different habitats will increase species diversity (Leopold 1933).