Introduced exotic species have a tendency to become invasive and impact local communities. Invasions often impact community attributes such as cover and species richness, but these factors may also regulate patterns of invasion. In such cases, impacts may be dependent on the invasion context. We used data from the Buell-Small Succession Study, a long-term permanent plot study in the piedmont region of New Jersey, to document context dependency in invasion. To do this, we analyzed the factors that affected the colonization and growth of four invasive species, Alliaria petiolata, Lonicera japonica, Microstegium vimineum and Rosa multiflora, as well as the impacts of these invasions on the community. For each species invasion, we selected a time (T1) immediately prior to or very early in invasion and a time after the species had become dominant (T2). We first analyzed the conditions at T1 linked with successful plot colonization by T2. Next, we determined which of the conditions at T1 influenced species success (increase in cover by T2). Finally, we ascertained the impact of the invasion on the change in species richness and vegetation cover at T2.
Results/Conclusions
In Lonicera japonica and Microstegium vimineum, it was found that more species rich plots were significantly more likely to become invaded, whereas increased species richness inhibited the likelihood of successful invasion by Alliaria petiolata. The establishment of Rosa multiflora was not effected by variation in species richness. Species richness and total vegetative cover did not significantly affect the growth/spread within successfully invaded plots. Life forms (trees, forbs, lianas, or shrubs) and co-occurring dominant species were linked to the spread of some invasive species. Species richness of the community was significantly affected by each of the invasive species, though the directionality and severity varied by invasion level. Total vegetative cover was similarly affected by species invasion, with the exception of Alliaria petiolata. In each of these case studies, the context of invasion was necessary in understanding their ultimate impacts. The perceived impacts of invasion may be offset or amplified by the factors which regulate invasion. Our data suggest that data from single-time studies may provide inaccurate estimates of invasion impacts.