Using an index of habitat patch proximity for landscape design

Abstract

A proximity index (PX) inspired by island biogeography theory is described which quantifies the spatial context of a habitat patch in relation to its neighbors. The index distinguishes sparse distributions of small habitat patches from clusters of large patches. An evaluation of the relationship between PX and variation in the spatial characteristics of clusters of patches showed that reduction in the isolation of patches within a cluster produced exponential increases in PX, and that increase in the size of those patches produced a more modest linear increase in PX. Simulations using neutral model landscapes were used to determine the effect of the scale of analysis on PX. Increased size of the neighorhood considered around a habitat patch (proximity buffer) produced linear increases in PX, the slope being dependent on the proportion of the habitat of interest on the landscape. The proximity index was used to evaluate three alternative conservation reserve designs in an agricultural landscape, and comparisons were made among designs consisting of the same area of forest habitat added to the landscape. The ‘single, large’ reserve design produced the greatest increase in mean PX values among forest patches on the landscape when the total area of forest added was ⩾ 842 ha, and the ‘several, small’ reserve design produced the greatest increase when the total area of forest added was ⩽ 716 ha. The ‘string-of-pearls’ reserve design produced mean PX values approximately equal to those of the ‘single, large’ configuration when the total area of forest added was ⩽ 716 ha and the proximity buffer was ⩾ 2.1 km, since the ‘string-of-pearls’ configuration produced a higher total number of neighbors around the added reserves, which helped offset the smaller size of each reserve. Large reserves have more area, but their influence is limited to fewer neighbors. There appears to be a size threshold where the increased area of single, large reserves produced higher PX values than ith neighbors. Visualization of the spatial distribution of PX values across the landscape can reveal how organisms with specific movement scales might perceive the effective fragmentation of the landscape (spatial variability of PX), further aiding conservation reserve planning and design.