Biotic and Abiotic Limits to the Spread of Exotic Revegetation Species

Natural habitats vary in the degree to which they are invaded by exotic species, but it is unclear whether they differ in the mechanisms underlying the spatial spread of a given exotic species. To compare the dynamics of invasion in highly invaded nonserpentine oak woodlands and less-invaded serpentine habitats, we used an historical ‘‘experiment’’ consisting of the introduction of several nonnative species for post-mining revegetation, supplemented by a pot experiment and a factorial field experiment. Three species showed significant declines in abundance on transects from revegetated zones into natural habitats, indicating that these species had spread into the natural habitats from revegetated zones. Dactylis glomerata and Trifolium hirtum were found up to 95 m into oak woodland, 35 m into serpentine meadows, and 0–25 m into serpentine seeps and chaparral, while Elytrigia pontica was found up to 45 m into serpentine seeps. The pot experiment showed that this pattern of distribution for Dactylis was not caused or limited by variation in soil properties. The field experiment showed that Dactylis invasion in both oak woodland and serpentine meadow habitats was limited by disturbance and seed supply. Dactylis success was negatively correlated with species richness in oak woodlands, but positively correlated with richness in serpentine meadows, suggesting that the relationships between diversity, invasion, and underlying habitat suitability differed between these habitats. Our results show that in harsh serpentine soils, the spread of recently introduced exotic species is slower than in more fertile and more invaded oak woodlands. However, disturbance and propagule addition are equally important in promoting the spread of invaders in both environments.