Residential development encroachment on U.S. protected areas.

Conservation of ecological processes and biodiversity may require development of a conservation system consisting of protected "cores" surrounded by "buffer zones" that effectively expand and connect the cores. Nevertheless, residential development near protected areas may threaten de facto protected areas and hinder development of an official conservation system in the United States. We identified potential conservation cores based on existing protected areas, and using a spatially explicit model of housing densities, we quantified how residential development has altered the structural context around cores nationally from 1970 to 2000 and forecasted changes from 2000 to 2030. We found that residential housing development has likely occurred preferentially near some cores, and if encroachment near cores continues at projected rates, the amount of buffer zone will have been reduced by a total of 12% by 2030, with much of this change occurring directly at core edges. Furthermore, development will have reduced the average connectedness (valence) of cores by 6% from 1970 to 2030. Although patterns of encroachment roughly increased west to east, our results painted a more complex picture of the difficulties that would be faced if establishment of an official conservation system was ever attempted. At a minimum, prioritizing future conservation action must consider adjacent land uses, and a key conservation strategy will be to work cooperatively across land-ownership boundaries, particularly for smaller protected areas, which will tend to dominate future conservation activities.