Plant community invasibility describes a community’s susceptibility to colonization and establishment by individuals of non-resident species. Niche-based theory predicts greater competition between species more similar in their functional traits and resource use; hence community invasibility should decrease if a potential invader is functionally similar to a resident. We wanted to identify factors that determine how easily a particular community can be invaded by a non-resident species. Our expectation was that invader success would be positively correlated with functional differences to residents and therefore higher if they belonged to a functional group which was not already present among the residents. A nutrient treatment also allowed us to examine the premise that a community is more susceptible to invasion if there is an increase in unused resources. We added seeds into established communities of monocultures and three-species mixtures, where a particular functional group was already among the residents (“home”), or not (“away”). Our resident communities derived from 48 species common to European grasslands, belonging to three functional groups: grasses, forbs, and legumes. Each seed addition strip was further split into four nutrient treatments: control, N, P, and N+P. Invader success was measured by seedling counts and biomass.
Results/Conclusions
Invader biomass was on average double that in the “away” plots versus the “home” plots, and this was consistent for both monocultures and three-species mixtures. Seven out of eight legume and 13 out of 16 grass species were more productive as invaders in “away” communities, compared to only ten out of 22 forb species. Legumes on average produced six times more biomass in “away” communities, and grasses almost double. The mixed forb group response was likely because of too much functional variation related to the seven families it comprised, versus the single families for the grass and legume functional groups. This indicates that commonly-used functional groups may not be better than taxonomic groups in predicting niche overlap. Regardless, our results show that species dissimilarity and thus niche complementarity can be a strong structuring process during plant community assembly. With regard to nutrients, resident species increased their biomass in the N addition treatments, whereas the invaders responded most to P addition. The invader increase with P addition met a corresponding decrease in resident biomass, suggesting that invader success does not simply rely on unused resources, rather, invaders can successfully compete for resources with residents if they are sufficiently different.
