The Effect of Landscape Heterogeneity on the Probability of Patch Colonization

Abstract

The effect of landscape heterogeneity on the dispersal of organisms between habitat islands is poorly understood. Preferred pathways for dispersal (i.e., corridors), as well as dispersal barriers, are difficult to identify when the landscape matrix is composed of a complex mixture of land cover types. We developed an individual—based dispersal model to measure immigration and emigration rates between habitat islands within heterogeneous landscapes. Dispersing individuals of a model organism were simulated as self—avoiding random walkers (SAW) traversing a digital land cover map, with each habitat type assigned a priori a probability that the SAW would enter that habitat type. Each individual began the dispersal process on a random site at the edge of a deciduous forest patch and was allowed to move until it reached a different deciduous forest patch. Visualization of the movement patterns across the landscape was achieved by tabulating the frequency of visitation of successful dispersers to each grid cell on the map. The model was used to estimate the probabilities of disperser transfer between patches by varying the a priori probabilities of movement into each habitat type in order to: (1) estimate the effect of changing landscape heterogeneity on the transfer probabilities, and (2) visualize dispersal corridors and barriers as perceived by model organisms operating by specific movement rules and at specific scales. The results show that 89% of the variability in dispersal success can be accounted for by differences in the size and isolation of forest patches, with closer and larger patches having significantly greater exchange of dispersing organisms. However, changes in the heterogeneity of the landscape matrix could significantly enhance or decrease emigration success from an individual patch, depending on the landscape. Changes in emigration success from an individual patch resulting from changes in matrix heterogeneity were not predictable, and transfer rates between patches were not symmetrical due to differences in the proximity of neighboring patches, and differences in the funneling attributes of certain landscape patterns. Visualizations showed that corridors are often diffuse and difficult to identify from structural features of the landscape. A wide range of organisms with differing movement capabilities can be simulated using the approach presented to increase our understanding of how landscape structure affects organism dispersal.