Connectivity Is a Vital Element of Landscape Structure

In a recent FORUM article, Dunning et al. (1992) put forward a framework of landscape processes that stimulated considerable discussion amongst us. We are in general agreement with their ideas, but feel that a more explicit recognition of the importance of movement as a component of landscape structure would add clarity and utility to the framework. We wish to build on their foundation by presenting some additional ideas. Dunning et al. (1992) outline four ecological processes acting at the landscape scale: 1) landscape complementation; 2) landscape supplementation; 3) sources and sinks; and 4) neighbourhood effects. Each process depends upon the distribution of resources in the landscape. Resources are distributed in patches. Implicit in the definition of each process is that animals move among the resource patches. In the first two, animals move among resource patches to: 1) acquire a full complement of resources to meet their needs; and 2) supplement their existing resources with those in additional patches. In the third, movement from sources to sinks is required for the maintenance of sink populations. The fourth process, neighbourhood effects, implies that individuals move between patches, but focuses on the permeability of the boundaries between contiguous patches. Dunning et al. (1992) assume thaf the ability of an organism to complement or supplement its resource requirements depends only on the distance to those resource patches. In a related paper, Pulliam et al. (1992) state: When both landscape physiognomy and composition are incorporated into a population model, the dispersal of organisms across the landscape can be followed.... However, an animal's ability to utilize a resource patch will also be dependent upon its ability to get there. This ability will be determined not only by the distance between patches (i.e. physiognomy), but also by the biophysical nature of the route(s) between two patches and the biology and behaviour of the organism (Henein and Merriam 1990). Some routes facilitate or allow unimpeded movement among patches; others impede to varying degrees the amount, or success, of movement. Because movement is so critical to animal population survival, we recognize a third measure of landscape structure (sensu Dunning et al. 1992) namely landscape 'connectivity' (Merriam 1984, Baudry and Merriam 1988, Merriam 1991). Landscape physiognomy and landscape composition measure the distribution of resource patches in a landscape (Dunning et al. 1992). Landscape connectivity is the degree to which the landscape facilitates or impedes movement among resource patches. Each of the components of landscape structure (landscape physiognomy, landscape composition and landscape connectivity) can be measured. Turner (1989) pre-