There are at least two different perspectives on categorical map patterns (or patch mosaics) that have profoundly influenced the development of landscape metrics and have important implications for the choice and interpretation of individual landscape metrics.
- Island Biogeographic Model -- In the island biogeographic model, the emphasis is on a single patch type; disjunct patches (e.g., habitat fragments) are viewed as analogues of oceanic islands embedded in an inhospitable or ecologically neutral background (matrix). This perspective emerged from the theory of island biogeography (MacArthur and Wilson 1967) and subsequent interest in habitat fragmentation (Saunders et al. 1991). Under this perspective, there is a binary patch structure in which the focal patches (fragments) are embedded in a neutral matrix. Here, the emphasis is on the extent, spatial character, and distribution of the focal patch type without explicitly considering the role of the matrix. Under this perspective, for example, connectivity may be assessed by the spatial aggregation of the focal patch type without consideration of how intervening patches affect the functional connectedness among patches of the focal class. The island biogeography perspective has been the dominant perspective since inception of the theory. The major advantage of the island model is its simplicity. Given a focal patch type, it is quite simple to represent the structure of the landscape in terms of focal patches contrasted sharply against a uniform matrix, and it is relatively simple to devise metrics that quantify this structure. Moreover, by considering the matrix as ecologically neutral, it invites ecologists to focus on those patch attributes, such as size and isolation, that have the strongest effect on species persistence at the patch level. The major disadvantage of the strict island model is that it assumes a uniform and neutral matrix, which in most real-world cases is a drastic over-simplification of how organisms interact with landscape patterns.
- Landscape Mosaic Model -- In the landscape mosaic model, landscapes are viewed as spatially complex, heterogeneous assemblages of patch types, which can not be simply categorized into discrete elements such as patches, matrix, and corridors (With 2000). Rather, the landscape is viewed from the perspective of the organism or process of interest. Patches are bounded by patches of other patch types that may be more or less similar to the focal patch type, as opposed to highly contrasting and often hostile habitats, as in the case of the island model. Connectivity, for example, may be assessed by the extent to which movement is facilitated or impeded through different patch types across the landscape. The landscape mosaic perspective derives from landscape ecology itself (Forman 1995) and has emerged as a viable alternative to the island biogeographic model. The major advantage of the landscape mosaic model is its more realistic representation of how organisms perceive and interact with landscape patterns. Few organisms, for example, exhibit a binary (all or none) response to habitats (patch types), but rather use habitats proportionate to the fitness they confer to the organism. Moreover, movement among suitable habitat patches usually is a function of the character of the intervening habitats, which usually vary in the resistance they offer to movement. The major disadvantage of the landscape mosaic model is that it requires detailed understanding of how organisms interact with landscape pattern, and this has complicated the development of additional metrics that adopt this perspective.