Plant community diversity and invasibility by exotics: invasion of Mediterranean old fields by Conyza bonariensis and Conyza canadensis

A series of communities were established in situ to differentiate the effec ts of species richness, functional richness and functional group identity o n invasibility of Mediterranean annual old fields. We monitored the demogra phic and vegetative parameters of two exotic annuals introduced as seedling s, Conyza bonariensis and C. canadensis. Community species richness and fun ctional composition determined resistance to invasion by Conyza. Conyza bon ariensis biomass decreased with increasing species richness. Legumes increa sed the biomass and consequently the net fecundity of both Conyza, while su rvival was favoured by Asteraceae. Communities with fewer Asteraceae and gr asses increased the reproductive effort of C. bonariensis. A separate glass house experiment using the same species mises revealed chat establishment o f Conyza decreased with increasing species richness or when grasses were pr esent. Patterns of Conyza performance are interpreted in the light of measu rements of ecosystem functional parameters, making it possible to formulate hypotheses about mechanisms limiting community invasibility.