Plant diversity, soil biota and resistance to exotic invasion

Aim High species richness at small spatial scales can increase productivity and resist exotic invasion through complementary and selection effects. Recent evidence also suggests that soil biota can drive the increase in productivity caused by high species richness. Here, we take this a step farther and investigate whether soil biota can also affect how high species richness resists invasion. Location Missoula, Montana, USA. Methods Ten native grassland species were used to create plant assemblages with either one species (monocultures) or 10 species (polycultures) in a common garden. Soils cultured by these assemblages were collected and either sterilized or not to examine the combined effects of species richness and soil biota on the growth and competitive ability of the 10 native species against the invader Bromus tectorum. Results Live soil from monocultures inhibited the growth of all native species as a group and native grasses as a functional group more than live soil from polycultures. Sterilization eliminated the negative effects of soil from monocultures but not from polycultures. Native species also competed with B. tectorum more successfully in live soil from polycultures than live soil from monocultures, and sterilization eliminated the competitive advantage of natives in live polyculture soil. Main conclusions We found that local plant species richness can affect soil biota in ways that can increase the competitive effects of natives against an aggressive exotic invader; thus, our results suggest a mechanism by which species diversity might provide resistance to exotic invasion.