Effect of plant species richness on invasibility of experimental plant communities

Invasion of unsown species to artificially created assemblages of grassland species was investigated in a 3-year field experiment. In the experiment, assemblages varying in species richness (1, 2, 4, 8, and 16 species) and functional group richness (1–4, grasses, legumes, rosette forbs, and creeping forbs) were grown in control and fertilized plots, without any attempt to prevent the invasion of unsown species or to weed them. The relationship between species and functional group diversity and above-ground biomass was positive for sown species in all study years (2003, 2004 and 2005). In the latter 2 years, weed invader biomass decreased significantly with increasing biomass of sown species and their functional group richness, but not with number of species. However, no suppressive effect of species or functional group richness beyond that by increased biomass of residents was found. In fact, slight but significant positive partial effect of species richness was found, suggesting that the negative effect of the same amount of resident biomass on invaders is stronger when the biomass is composed of fewer species. The negative relationship between the number of functional groups of residents and invader biomass suggested that better coverage of functional trait space could be a mechanism promoting the resistance to invasion. In Addition, species composition of invaders were significantly related to initial composition of sown residents.