Resource impacts enable invasion resistance by a native shrub community

The successful invasion of many exotic species in native communities has led to the generalization that exotic invasives are often better resource competitors than native species. Few studies have directly tested R* competition theory in the context of invasions by exotic plants, but R* (measured as resource availability in monoculture) indicates a species’ resource impacts and requirements, and so can lead to useful predictions about invasion success. We hypothesized that 1) successful invaders will have a lower R* than native species, and 2) native species with a lower R* than exotic species should be able to resist invasion. We estimated R* for light, nitrogen, and water in monocultures of native coastal sage scrub shrub (CSS) species and in monocultures of two exotic herbaceous species that are problematic invaders in CSS. Additionally, we measured actual exotic success by experimentally invading exotic species into native and exotic monocultures. Exotic monocultures did not reduce resources below levels found in native monocultures: extractable soil nitrogen and shallow soil moisture were similar in all monocultures, and native monocultures had lower light and lower potential nitrogen mineralization rates than exotic monocultures. Experimental invasions supported the prediction that lower R* should provide invasion resistance, as invaders had lower biomass in native shrub monocultures than exotic monocultures. Rather than being strong competitors, exotics may rely on traits providing an advantage under transient conditions when resource availability is temporarily high. Comparisons of exotic R* with temporal resource variability in native communities may help further pinpoint windows of opportunity for invasives.