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Fall 2005
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The Nature Conservancy's Work Eclipses International Borders

Caribbean Basin Ecological Assessment Uses GIS to Model Connectivity

By Steven R. Schill, Ph.D., Senior Geospatial Scientist, Mesoamerican and Caribbean Region, The Nature Conservancy

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Connectivity and Corridor Module showing preliminary results for remaining terrestrial habitats in the Dominican Republic.

Famous for white beaches and turquoise waters, the Caribbean is also home to some of the world's richest land and marine environments, with more than 14,000 plant and animal species, 50 percent of which are found nowhere else on earth. Ecologically speaking, the Greater Caribbean Basin encompasses 28 countries, most of them islands spread out from Trinidad and Tobago to the Bahamas, but also including the mainland shore that stretches from Venezuela to Belize. Currently, 13 percent of its land has protected status, but these areas are highly threatened by management problems, expanding tourism, introduction of outside species, overfishing, and climate change.

Committed to securing the area's environmental future, The Nature Conservancy (TNC) knew that conservation efforts would have to transcend geographic, political, and cultural borders and should begin with an overall ecological assessment that considered habitats in terrestrial, freshwater, and marine realms. The final product, the set of conservation areas, would have to include examples of more than 85 Caribbean habitats. Not only that, but the protected fragments would also have to be in good enough ecological health to permit establishing realistic conservation goals and biodiversity protection strategies.

"The primary objective of this assessment was to identify a set of significant areas that, if conserved, would ensure long-term survival of the region's biodiversity," says Dr. Richard Jeo, director of the TNC Greater Caribbean Basin Ecoregional Assessment.

Jewel-like landforms separated by the sea, Caribbean islands harbor many endemic species precisely because the islands are isolated from each other. Plants and animals develop unique characteristics over long periods of separation from other populations (hence the term "endemic"). On several islands, impacts from agriculture and urban development threaten to severely impact the terrestrial connections between large intact habitats. Left unchecked, these shrinking corridors may be lost, severing the natural connections between habitats. Protected areas need to be connected by a network of these natural corridors to permit species to move back and forth between their shrinking habitats. Since conservation is generally opportunistic in most countries, corridors are rarely considered for protection, especially because agriculture and urban expansion are not often planned around conservation needs.

Therefore, to prepare an integrated Caribbean ecological assessment, TNC needed to include investigation of habitat isolation and the socioeconomic impacts of agricultural practices and urban sprawl on remaining natural habitats and protected area networks. Consultation with the countries' respective environmental ministries revealed that they are also concerned about the growing changes in the landscape and needed a mechanism to identify important conservation targets, as well as map, maintain, and restore corridors to provide a functional network of protected areas.

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Deforested patches caused by agricultural use encroach on tropical forest in northeast Jamaica. (Photo © Jonathan Kerr).

ArcGIS Desktop (ArcInfo) was chosen as a solution for the overall assessment and specifically for the corridor project, because its comprehensive set of advanced spatial modeling and analysis tools integrated well with existing decision support systems that were designed for evaluating gaps in protected areas based on areas of biodiversity significance. The ArcGIS Spatial Analyst extension was chosen to provide the grid-based modeling function needed to evaluate the spatial relationships between habitat patches.

TNC called on Dr. Dean Urban, professor of landscape ecology at Duke University's Nicholas School of the Environment, to discuss and design the technical aspects of corridor mapping. Urban has pioneered many of the landscape connectivity techniques that employ a graph theory perspective to capture connection measures between habitats.

Microsoft Visual Basic was used to create an ArcGIS extension that linked many of the FORTRAN programs developed by Urban to the data that was needed to perform the connectivity assessments. When calculating how connected the landscape was for each habitat patch, the distance of the least cost paths (LCP) between habitat patches measured over a cost (mobility) surface was used, rather than Euclidian (straight-line) distance. The cost surfaces were created based on expert opinion of how animals respond to different land-cover types and the degree of difficulty for crossing various environments (e.g., urban, agricultural, wetland). LCP distance reflects the difficulty that a species may experience when traversing various kinds of natural, nonnatural (agriculture), or nonvegetated (urban) environments.

To analyze nonhabitat areas that could potentially be used as corridors between habitat patches, the LCP between each habitat type were buffered and overlaid to identify the areas with the highest densities. Habitat patches were also assessed for their degree of isolation based on connection distances within connectivity networks and evaluated on how important each patch contributes to overall island connectivity. Model outputs included quantification of existing connectivity between habitats, assessment of the degree of isolation by habitat patch, and evaluation of the importance of a given habitat patch in the landscape for species with many different dispersal distances.

To collect expert opinions and suggestions, TNC invited country and regional experts to participate in several discussion workshops for presenting and analyzing the data and maps. To estimate ease of species movement through the landscape, custom-tailored cost surfaces representing various animal species' behavior to certain land-cover types were employed based on the opinion of leading animal behaviorists.

The resulting analyses provided a ranking method for determining which habitat patches were more important for maintaining natural connections throughout each island based on current land-cover conditions. By looking at the remaining habitat patches and the accumulated paths that the model produced, scientists were able to identify potential corridors that provided the shortest and most effective path between habitat patches.

"We have completed connectivity assessments for Puerto Rico, Jamaica, and Hispaniola and plan to continue to do the work for the rest of the insular Caribbean," says Dr. Maarten Kappelle, senior scientist for TNC's Mesoamerica and Caribbean region. "Our success so far has been made possible by the participation and contribution of dozens of local and international organizations and government groups that are striving to protect remaining natural habitat throughout this region."

The resulting output of these models is being used to prioritize the importance of the corridors and compare the connectivity of alternative protected area networks to see which scenarios provide better connectivity between habitat patches. This comparison is done by evaluating output, such as minimum distance required to maintain islandwide connectivity and the distance at which no habitat is isolated. Regional management efforts are being focused on those areas identified as high priority and the most important for preserving landscape connectivity.

"Corridor and connectivity modeling is also contributing to the construction of a larger GIS-based tool to support conservation decision making," says Kappelle, adding that TNC plans to hold training sessions throughout the region to teach local GIS users how to use the modeling.

Finally, the results are being used to identify the optimal places for restoration activity. For example, there are many areas where abandoned agriculture land could eventually convert back to primary habitat. In places where restoration may not be feasible, various strategies could be taken that promote more favorable land use to facilitate animal mobility.

"The overall assessment," says Kappelle, "has developed the most comprehensive database of conservation targets and socioeconomic activities for the Caribbean Basin currently available, which allows decision makers to develop, assess, and prioritize multiscale, science-based conservation strategies for this important region."

For more information, contact Steven Schill (e-mail: sschill@tnc.org).

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