Using Landscape Indices to Predict Habitat Connectivity

The ecological consequences of habitat fragmentation include the direct effects of habitat loss and the indirect effects of reduced inter-patch dispersal. In particular, habitat patches that survive the process of fragmentation become increasingly isolated from one another, and this can cause species' declines in excess of predictions based strictly on reductions in habitat area. To quantify the hindrance of dispersal caused by habitat frag- mentation, landscape ecologists have invented the notion of habitat connectivity. Indices of landscape pattern are frequently used to estimate habitat connectivity, but whether they actually do so remains undocumented. If indices of habitat pattern do indeed estimate habitat connectivity, then these indices should correlate well with predictions of dispersal success. To test this possibility, I looked for correlations between nine common indices of habitat pattern and the results of a simulated dispersal process conducted using GIS data on the distribution of old-growth forest through the Pacific Northwest. The nine indices of habitat pattern that I examined were only weakly correlated with the results from the dispersal modeling, but I identified a new pattern index, termed patch cohesion, for which the fit was much better. Moreover, I found patch cohesion to be insensitive to the details and artifacts of the dispersal model. The methodology described here will be useful to inves- tigators using indices of landscape pattern to quantify habitat fragmentation.