A species-addition experiment showed that prairie grasslands have a
structured, nonneutral assembly process in which resident species inhibit, via
resource consumption, the establishment and growth of species with similar
resource use patterns and in which the success of invaders decreases as
diversity increases. In our experiment, species in each of four functional
guilds were introduced, as seed, into 147 prairie–grassland plots that
previously had been established and maintained to have different compositions
and diversities. Established species most strongly inhibited introduced
species from their own functional guild. Introduced species attained lower
abundances when functionally similar species were abundant and when
established species left lower levels of resources unconsumed, which occurred
at lower species richness. Residents of the C4 grass functional guild, the
dominant guild in nearby native grasslands, reduced the major limiting
resource, soil nitrate, to the lowest levels in midsummer and exhibited the
greatest inhibitory effect on introduced species. This simple mechanism of
greater competitive inhibition of invaders that are similar to established
abundant species could, in theory, explain many of the patterns observed in
plant communities.