Viewpoint: The ecological value of shrub islands on disturbed sagebrush rangelands

Abstract

Undisturbed plant communities dominated by shrubs or trees are often left isolated within landscapes otherwise devoid of woody vegetation following large-scale disturbances such as wildfires. We discuss potential ecological benefits associated with these terrestrial vegetation "islands", giving special attention to islands in disturbed shrub systems dominated by big sagebrush (Artemisia tridentata Nutt.). Shrub habitat islands provide important refugia for plant and animal species that are associates of shrubsf?rom those that generally require shrub cover to those that have evolved obligate symbioses with a particular shrub species. Even if islands are not able to support breeding populations, they may provide essential temporary habitat for maintaining a plant or animal metapopulation or for dispersing animals. Habitat islands are likely to enhance local biological diversity of plants and animals, because they harbor species that are lacking in disturbed areas, and because abrupt structural changes from disturbed to undisturbed vegetation provide a habitat mosaic that facilitates high levels of species turnover. A previous study confirmed that small mammal species richness in sagebrush islands is intermediate to the high species richness in undisturbed sagebrush "mainlands" and the low richness associated with burned sagebrush habitats. In re-analyzing some of the data from the latter study, we found that small mammal richness in sagebrush islands increases with time since the surrounding habitat burned. Finally, habitat islands provide more evenly dispersed seed sources for re-establishment of decimated vegetation within disturbed areas, and they may harbor animal species that provide seed dispersal services. Thus, they should accelerate vegetation recovery after disturbance. Managers, fire crews, and others who may influence how disturbance patterns affect habitat heterogeneity should be aware of these ecological benefits of habitat islands.