Universal drivers of exotic species dominance in terrestrial ecosystems: The origin of species does matter

Background/Question/Methods: Exotic species currently dominate many communities and can impact ecosystem processes. Nevertheless, it is still a matter of debate whether a species’ origin has functional significance, or if conservation concerns about invasive species merely reflect our value systems. At the core of this debate is whether exotic species function differently from the native, resident community. We examined patterns of abundance in native and exotic species in terrestrial, herbaceous plant communities at 54 sites in nine countries and experimentally tested the effects of two fundamental drivers of invasion – resources and consumers at 45 of these sites.

Results/Conclusions: We found that exotic species are six times more likely to play a dominant role in the community (cover >80% or more) of a surveyed area than are native species (8.3% versus 1.4% of total species). In addition, native and exotic species responded differently to experimental nutrient additions across the wide array of sites included in the study; nutrient enrichment (e.g. addition of nitrogen or phosphorous) led to declines in native richness while leaving exotic richness unaffected. Exotic cover also increased in response to phosphorus while native cover was unaffected. Exotic cover increased twice as fast in response to nitrogen than did native cover. The differential distribution of abundances and response to nutrients between native and exotic species suggests that species origin affects species’ functional role in communities, and that eutrophication will lead to general increases in exotic dominance. The effect of nutrients on exotic dominance is particularly problematic, as human endeavors have greatly increased nutrient loading to ecosystems worldwide.