| Summary: | Fundamental debates persist regarding the ecology of species invasions, the risk posed by exotic species, and the most effective management to diminish invasion and promote native-dominated ecosystems. Using vascular plant surveys at three scales (between small-island and ‘mainland’ patches, among patches, within patches) from 86 patches in a threatened meadow ecosystem, I addressed the following questions arising from such debates: 1) Are latent invasions prevalent among exotics, and if so, among which species? 2) What are the relative roles of latent invasions, competition and environmental response in determining native versus exotic biogeographic patterns? 3) How can native and exotic species distribution and richness models at multiple scales be used to improve conservation management? Species-level analyses demonstrated latent invasions among and within patches for short-dispersing exotics, and a positive relationship between exotic species’ abundances and minimum residence time in the study region, suggesting that population expansion of some exotic species is at an early stage. A mix of scale-dependent concordant and discordant relationships with environmental variables, rather than competition, appeared to be the primary determinant of native versus exotic species richness and composition patterns. While incomplete invasion of exotics did not produce substantially different community-level biogeographic patterns between native and exotic communities, exotics were dominated by long-dispersing ruderal species more abundant on disturbed patches, while dominant natives were often short-dispersing stress-tolerant species more abundant on isolated patches. Such complexities, overlooked in most previous comparative analyses of native and exotic communities, can be used to predict future patterns and prescribe efficient management. In addition, spatially explicit distribution models revealed greater predictability for native species, and greater predictability among than within patches. Environmental variables related to native and exotic distributions were often shared within patches. Thus, management prescriptions applied among patches are likely to be most successful and predictable. Finally, native species at risk were more common on isolated small-island patches, contrary to biogeographic theory. Protection of small-island patches would be the most efficient conservation strategy for the study system. Interventions to control large exotic species populations, especially where propagule pressure from nearby disturbed areas is high, represent a far less efficient strategy.
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