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The Connectivity Analysis Toolkit

Carroll, Carlos; McRae, Brad 2/26/2011

Conservation biologists have long recognized that protecting isolated natural areas can be ineffective for biodiversity conservation, and that linking areas into connected networks is often necessary for achieving conservation goals. Although connectivity conservation was formerly thought of in a static sense (e.g., as protecting conservation corridors) we now recognize that planning for connectivity requires incorporating process as well as pattern into conservation planning, and that connectivity can be conserved by means other than corridors.

However, existing corridor mapping tools — such as those included in commercial GIS software — might not be flexible enough to incorporate new knowledge on patterns of animal movement and ecological processes. Land-use planning for biodiversity conservation necessarily involves decisions that have large economic and social impacts, and thus merits use of the most rigorous and informative tools available. To fill this need, the Connectivity Analysis Toolkit (CAT) was developed by the authors with the assistance of Allen Brookes of the U.S. EPA.

The CAT is a software package that provides new tools for both linkage mapping and landscape-level “centrality analysis.” Centrality analysis ranks the importance of sites as “gatekeepers” for flow across a landscape network. Because centrality metrics can analyze potential linkages between all pairs of nodes (sites) in a network, they avoid the necessity for a priori identification of corridor endpoints. For example, betweenness centrality identifies the shortest paths connecting each pair of nodes, and counts the number of such shortest paths in which a node participates. The loss of an area that lies on a large proportion of the shortest paths in a network would result in a disproportionate loss of connectivity across the network. Computational advances now allow such metrics to be applied to landscapes of continuous habitat gradients rather than patches in a matrix of unsuitable habitat. T

he CAT complements methods commonly available in GIS and allows planners to better evaluate alternate assumptions on how to represent wildlife movement and ecological processes. It allows users to develop and compare three contrasting centrality metrics (shortest path, circuit theory and network flow) based on input data representing habitat suitability or permeability — in order to determine the areas that would be priorities for conservation measures that might facilitate connectivity and dispersal across the landscape as a whole. The CAT also allows application of these approaches to the more common question of mapping the best habitat linkages between a source and a target patch. The newest version of the CAT adds methods for analyzing connectivity across time as habitat shifts under changing climates.

Due to the CAT’s recent development, there are few published examples of its use in planning. However, hundreds of conservation planners around the world have already begun applying the CAT to map habitat linkages and rank sites for their contribution to landscape connectivity. These methods can be applied to both conservation plans focusing on single species (such as recovery plans) and to multi-species planning efforts. The methods can be applied at a range of scales, from local watersheds to large regions.

In the western United States, for instance, planners are using centrality analysis of data on “landscape integrity” to map a multi-state network of multi-species linkages. (Landscape integrity is a metric of landscape naturalness derived from data on land use, roads and human population.) In northern British Columbia, planners are using centrality analysis in a regional land-use plan to evaluate connectivity for caribou and other focal species. In both these examples, the CAT allows planners to represent habitat quality as a gradient rather than mapping a landscape of suitable patches embedded within an inhospitable matrix.

Readers may also be interested in two recent connectivity analysis tools that complement the functionality of the CAT:

  • Circuitscape (www.circuitscape.org; see article in the January 2009 issue of Science Chronicles) applies circuit theory to applications in both connectivity analysis and landscape genetics, and can analyze connectivity on high-resolution raster surfaces (habitat maps) of millions of pixels.
  • Linkage Mapper (due to be released in early 2011) was developed to automate least-cost corridor analyses for Washington’s just-released statewide connectivity assessment (report available at www.waconnected.org).

The Connectivity Analysis Toolkit and a manual with example applications are freely available at www.connectivitytools.org. If you have any questions on how to apply the software to your work, please don’t hesitate to contact Carlos Carroll or Brad McRae.


By Carlos Carroll, Klamath Center for Conservation Research, and Brad McRae, Spatial Ecologist, The Nature Conservancy in Washington