The invasion meltdown hypothesis posits a sequential spiral of non-native species invasions where early invaders facilitate later invaders. After several successive invasions, a tipping point is reached where non-native richness substantially alters native communities. However, there are many notable examples of a single, highly abundant, non-native species having substantial negative impacts on native communities. To tease apart the effects of invasive species richness versus abundance in plant communities, we analyzed US Forest Service FIA (Forest Inventory and Analysis) data from 11 contiguous northeastern states, which allowed us to assess invasions on a regional scale. Specifically, we analyzed invasive species data collected between 2007 and 2014 from fixed area plots that were entirely forested. This provided data from a total of 963 plots, 119 of which were sampled twice. Tree seedlings were also measured in 353 of the plots. We created regression models with invasive richness and invasive percent cover (i.e., abundance) as independent variables and tree seedling richness as the dependent variable.
Results/Conclusions
Of the 40 invasive species found in the plots, most were shrubs. An invasive forb (Alliaria petiolata) and grass (Microstegium vimineum) were also in the top five most abundant species. Invasive abundance increased by 26%, while invasive richness only increased by 10% since 2007. Native tree seedling richness was significantly, negatively correlated with invasive abundance, whereas it was not significantly correlated with invasive richness (standard least squared regression; r2 = 0.12, P < 0.0001, P = 0.17, respectively). Tree seedling richness was most strongly correlated with invasive shrub abundance, perhaps because shrubs shade understories more effectively than other invasive life forms. This study demonstrates that at regional scales, invasive abundance may have greater impacts on native communities than invasive richness. Therefore, management that focuses on very abundant invasives may be most impactful.