Modeling patch occupancy: Relative performance of ecologically scaled landscape indices
In fragmented landscapes, the likelihood that a species occupies a particular habitat patch is thought to be a function of both patch area and patch isolation. Ecologically scaled landscape indices (ESLIs) combine a species’ ecological profile, i.e., area requirements and dispersal ability, with indices of patch area and connectivity. Since their introduction, ESLIs for area have been modified to incorporate patch quality. ESLIs for connectivity have been modified to incorporate niche breadth, which may influence a species’ ease in crossing the non-habitat matrix between patches. We evaluated the ability of 4 ESLIs, the original and modified indices of area and connectivity, to explain patterns in patch occupancy of 5 forest rodents. Occupancy of eastern gray squirrels (Sciurus carolinensis), North American red squirrels (Tamiasciurus hudsconicus), fox squirrels (Sciurus niger), white-footed mice (Peromyscus leucopus), and eastern chipmunks (Tamias striatus) was modeled at 471 sites in 35 landscapes sampled from the upper Wabash River basin in Indiana. Models containing ESLIs received support for gray squirrels, red squirrels, and chipmunks. Modified ESLIs were important in models for red squirrels. However, none of the models demonstrated high predictive ability. Incorporating habitat quality and using surrogate measures of dispersal can have important effects on model results. Additionally, different responses of species to area, isolation, and habitat quality suggest that generalizing patterns of metapopulation dynamics was not justified, even across closely related species.
KeywordsConnectivity Forest rodent Metapopulation Niche breadth Patch area
We are grateful to hundreds of private landowners who allowed access to their properties. Dozens of field technicians and crew chiefs collected data or digitized GIS layers. J. Crick, T. Preuss, L. Connolly, and N. Engbrecht coordinated field efforts, helped develop field protocols, and collected and managed data. J. Goheen provided data from tree squirrel mobility studies. M. Miller provided the script for ESLI calculation. P. Waser, T. Wiegand, and two anonymous reviewers provided useful comments on the manuscript. Our GIS data sources included the Center for Advanced Applications in GIS at Purdue University, National Land Cover Data, Indiana Unified Watershed Assessment, and the National Water Information System. Funding was provided by the John S. Wright Fund, Department of Forestry and Natural Resources, Purdue University, the U.S. Department of Education Graduate Assistance in Areas of National Need Award P200A030188, and the Cooperative State Research, Education, and Extension Service, U.S. Department of Agriculture, under Agreement No. 2000-04649.
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