Landscape connectivity is, broadly, "the degree to which the landscape facilitates or impedes movement among resource patches". Connectivity includes both structural connectivity (the physical arrangements of patches) and functional connectivity (the movement of individuals among patches). The degree to which a landscape is connected determines the amount of dispersal there is among patches, which influences gene flow, local adaptation, extinction risk, colonization probability, and the potential for organisms to move as they cope with climate change.
Although connectivity is an intuitive concept, there is no single consistently-used metric of connectivity.
Generally, connectivity metrics fall into three categories:
- Structural connectivity metrics are based on the physical properties of landscape patches (size, number of patches, average distance to each other).
- Potential connectivity metrics are based on the landscape structure as well as some basic information about the study organism's dispersal ability such as average dispersal distance, or dispersal kernel.
- Actual (also called realized, or functional) connectivity metrics are measured based on the actual movements of individuals between patches. This takes into account the actual number of individuals born at different sites, their and mortality during dispersal. Some authors make a further distinction based on the number of individuals that not only disperse between sites, but that also survive to reproduce.
Circuitscape is an open source program that uses circuit theory to predict connectivity in heterogeneous landscapes for individual movement, gene flow, and conservation planning. Circuit theory offers several advantages over common analytic connectivity models, including a theoretical basis in random walk theory and an ability to evaluate contributions of multiple dispersal pathways. Landscapes are represented as conductive surfaces, with low resistances assigned to habitats that are most permeable to movement or best promote gene flow, and high resistances assigned to poor dispersal habitat or to movement barriers. Effective resistances, current densities, and voltages calculated across the landscapes can then be related to ecological processes, such as individual movement and gene flow.
Graphab is a software application devoted to the modelling of landscape networks. It is composed of four main modules: graph building, including loading the initial landscape data and identification of the patches and the links; computation of the connectivity metrics from the graph; connection between the graph and exogenous point data set; visual and cartographical interface. Graphab runs on any computer supporting Java 1.6 or later (PC under Linux, Windows, Mac...). It is distributed free of charge for non-commercial use.
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