Exotic deer diminish post-fire resilience of native shrub communities on Santa Catalina Island, southern California

Browsing by exotic mule deer on Santa Catalina Island (SCI) off the coast of southern California may diminish the post-fire resilience of native shrublands. To assess this, deer exclosures were established following a wildfire to monitor post-fire recovery of three dominant, native shrub species (Heteromeles arbutifolia, Rhus integrifolia, and Rhamnus pirifolia). Post-fire resprout growth, mortality, and tissue water status as well as pre- and post-fire shrub density and cover were measured inside and outside of deer exclosures. We found that deer browsing significantly limited post-fire resprout growth and led to increased mortality of resprouting H. arbutifolia shrubs (88 % mortality outside compared to 11 % inside exclosures). Post-fire resprouts maintained favorable water status during the study despite drought conditions, indicating that water stress was not a proximate cause of resprout mortality. Deer browsing resulted in a >93 % reduction in canopy coverage of dominant shrub species. The dramatic reduction of native shrubs at this site may create opportunities for displacement by exotic species, resulting in eventual vegetation-type conversion. The observed link between intense browsing and post-fire shrub mortality provides much needed information concerning the environmental impact of exotic deer on SCI and illustrates the interaction between exotic herbivores and fire on an island system.