Richness, not evenness, of invasive plant species promotes invasion success into native plant communities via selection effects

Native plant communities are often invaded by multiple alien species. It
is still unclear how increasing diversity of alien invasive species
suppresses the growth of native species and thus contributes to invasion
success. In the subtropical monsoon region of Southeast China, we
experimentally created a native plant community with 18 herbaceous
species. One week later, we let it be invaded by either zero (controls
without invasion), one, two, four or eight alien plant species, with
either high or low species evenness. After a four-month growth period we
harvested the aboveground biomass of each species. We found that
increasing invasive species richness significantly increased invasive
plant biomass, the biomass of all invasive and native plant species within
the community, and invasion success (the ratio of invasive plant biomass
to the biomass of all native and invasive plants), but it did not
significantly reduce native plant biomass. Experimentally manipulating
invasive species evenness did not influence invasion success and did not
show any differential suppression effects on native plants. One invasive
species, Sesbania cannabina, became dominant in terms of plant biomass,
irrespective of its proportion in the alien plant mixtures. Throughout
this experiment, effects of invasive species richness on invasion success
were mainly due to such selection effects among the invasive species. On
the other hand, the unchanged biomass of native species under increasing
invasive plant richness suggests the presence of at least partly
complementary resource niches between invasive and native species.