Habitat connectivity and spotted owl population dynamics

Abstract

The ecological consequences of habitat fragmentation include the direct effects of habitat loss and the indirect effects of reduced inter-patch dispersal. I examine these consequences of habitat fragmentation through two separate studies. The first study measures habitat connectivity through the success of a simulated dispersal process, and then asks if connectivity can be estimated from measures of habitat pattern alone. The second study examines the effect of past habitat loss, and of potential future habitat gains, on an isolated population of northern spotted owls on the Olympic Peninsula of Washington state.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 for old-growth forest throughout 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.I constructed a spatially explicit simulation model for the population of spotted owls on the Olympic Peninsula of Washington state, and used this model to examine the future of this population, and how patterns of occupancy might differ in habitats of varying quality. Results from model simulations that incorporate habitat loss over the last three quarters of a century lend support to suspicions that the population of spotted owls on the Olympic Peninsula may presently be in a sharp decline. But simulations that account for regeneration of owl habitat over the next 100 years show that the potential exists for this trend to reverse, and for the population to stabilize. The simulated owl population showed a two-fold response to the addition of new habitat over what would have been expected based on estimates of habitat area alone.