Species invasions present a conservation paradox—native invaders help maintain species diversity, yet non-native invasive species can substantially alter communities and ecosystems. To address this issue, we investigated patterns of native and non-native colonization and the role each group played in long-term changes in native understory diversity and composition. We used a 55 year-old baseline dataset from 94 upland forest stands in southern Wisconsin to evaluate the utility of initial diversity, successional state, local environmental variables and surrounding landscape conditions as predictors of native and non-native species colonization.

Results/Conclusions

For native species, both early successional stands and large stands in non-fragmented landscapes were more likely to gain new native species than were late-successional stands or those in highly fragmented or urbanized landscapes. Native colonization was twice as important as extinction in explaining diversity changes over the last half century in Wisconsin forests, which supports the idea that stand invasibility is critical to diversity maintenance. For non-native species invasions, sites with initially high richness were less likely to be invaded, but urbanization strongly favored non-native invasion and overwhelmed the protective effects of high richness. Overall, exotic species invasion had no significant correlation with native species losses, and thus suggests that exotic invasion is more a symptom than a cause of declines in native species richness and floristic quality.