The strength of biotic resistance to invasions should vary according to the degree of niche overlap between resident species and potential invaders, which in turn depends on native functional group composition. Likewise, exotic species impact on resident community structure and productivity would be driven by the functional novelty of successful invaders. It remains to be seen, however, whether such patterns hold under varying disturbance levels. We evaluated the interactive effects of functional group composition and defoliation intensity on plant community resistance and response to invasion by two contrasting exotic species. We assembled a series of outdoor grassland microcosms (40 cm x 40 cm x 20 cm) containing the same native plant richness (4 spp) but different functional group compositions: 1) winter grasses, 2) summer grasses, 3) winter+summer grasses, and 4) bare-soil control. Microcosms were exposed to two levels of defoliation (low vs. high intensity) and to three invasion treatments: 1) sowing Lolium multiflorum (annual winter grass), 2) sowing Lotus tenuis(perennial summer legume), or 3) no invader. We set up five blocks, where each replicate of functional composition was represented by a different, randomly chosen species mixture.
Results/Conclusions
Increased defoliation intensity promoted seedling recruitment of both exotic species, regardless of community composition. Recruitment of L. tenuis seedlings was also reduced by summer grass communities, whereas that of L. multiflorum was unaffected by resident functional composition. Overall, native grasses decreased exotic species biomass, and functional group composition interacted with defoliation intensity to influence invader biomass production. L. multiflorum invasion was strongly suppressed by winter grass communities under low defoliation. Conversely, L. tenuis biomass mostly decreased in the presence of summer grasses under high defoliation. Native functional groups exerted additive, rather than multiplicative, effects on community invasibility. On the other hand, L. multiflorum invasion consistently enhanced total aboveground biomass, and only reduced native biomass in mixed winter/summer grass mixtures. Invasion by L. tenuis increased total aboveground biomass of highly defoliated communities. Yet, L. tenuis also reduced winter grasses in both single and mixed grass communities. We conclude that the role of niche overlap in determining invasion resistance varied across life stages, and depended critically on invader identity and disturbance intensity. Nevertheless, we also found that temporal niche overlap did not always predicted which native functional group was negatively affected by a successful invader.