Herbaceous species respond differently to increased nutrients and grazing exclusion at sites away from home

Background/Question/Methods

Exotic species have been introduced accidentally and intentionally for ornamental and production purposes to such an extent that recent evidence suggests biological communities are increasing in similarity across continents. Europe is the most frequent source of established exotic species in North America. The global success of European herbaceous species is thought to have been accelerated by the intentional selection of fast growing, and characteristically dominant grasses and forbs; coupled with the simultaneous introduction of intensified agricultural practices that European species are better adapted to. There is, however, little empirical evidence that exotic species respond similarly to perturbations at sites both home and away, and whether these responses are explained by species level traits of strong resource competitors or other drivers such as the release from selective consumer pressure. Here, we compare the cover response of 17 herbaceous species (8 grasses and 9 forbs) to three years of nutrient addition and grazing exclusion treatments at six herbaceous community sites in Europe (native, hereafter home) and 16 sites in North America (introduced, hereafter away).

Results/Conclusions

We find that species generally increased in abundance in response to nutrient addition at away sites but the response was not as consistent at home sites, with forbs decreasing and grasses showing no significant change. The herbaceous species studied showed a similar increase in abundance home and away, when grazing was excluded and nutrients added. Our results suggest a similar growth response to increased resources at sites home and away, but a greater selective grazing pressure at native sites. Our findings provide evidence of interactions between resource competition and consumer pressure promoting opportunities for invasion.  Recently, the ecological justification for the native/exotic species dichotomy was questioned. This criticism was based to some degree on mixed evidence that exotic species behave differently in comparison to native species and to how they behave at home.  Eutrophication and changed consumer pressure can favor the growth and survival of some species over others. Here, we show that the same species can behave differently in response to short-term perturbations at native and introduced sites.