One of the salient ecological characteristics of exotic “invasive” species is that they appear to become far more abundant and have much stronger impacts in their non-native ranges than in their native ranges.  However, despite a substantial amount of effort invested in attempts to understand why invaders undergo strong biogeographic shifts in their ecology there have been very few quantitative studies of the shifts themselves.  I report for a group of international colleagues who compared the impacts of three exotic invasive plant species, Acroptilon repens, Prosopis juliflora, and Ageratina adenphora on other species in the field, and in both native and non-native ranges.  We then followed up these field patterns with simple greenhouse experiments designed to test the potential for volatile chemicals, root exudates, or leaf leachates released from these plants to differ in their effects on other species from the native and non-native ranges of the invaders and correlate with field patterns.

Results/Conclusions

In the field, we found that all three invasive species appeared to be more harmful to other species in their non-native ranges than in their native ranges. Prosopis juliflora and A. adenphora, showed neutral to facilitative effects on natives in their home ranges of Peru and Mexico, respectively, but were associated with strong negative effects in the invaded ranges of India and China. The herbaceous invader, A. repens, produced denser stands and more biomass in its non-native range in the USA than in its native range in Uzbekistan, and had far stronger gram per gram effects on other species in the non-native range. Litter from P. juliflora had far stronger inhibitory effects on species native to India than litter from P. cineraria, another Indian native. Volatiles produced from the litter of A. adenphora had weaker effects in general than P. juliflora litter, but volatiles were significantly more inhibitory against species from the non-native ranges of China and India than species native to Mexico, which is also the native range of A. adenphora. The competitive effects of A. repens, and the effects of its leachates, were more inhibitory to species native to North America than species native to Uzbekistan, the native home of A. repens.  These results are consistent with the idea that invasions are promoted by the mixing of species without evolutionary histories, and in particular species that have not had the opportunity to adapt to the biochemical effects of new neighbors.