Ecotypic diversity of a dominant grassland species resists exotic invasion

Many species are characterized by high levels of intraspecific or ecotypic diversity, yet we know little about how diversity within species influences ecosystem processes. Using a common garden experiment, we studied how intraspecific diversity within the widespread and often dominant North American native Pseudoroegneria spicata (Pursh) A. Love. affected invasion by Centaurea stoebe L. We experimentally manipulated Pseudoroegneria intraspecific diversity by changing the number of Pseudoroegneria ecotypes in common garden plots, using ecotypes collected throughout western North America. Invader biomass was 46% lower in mono-ecotype Pseudoroegneria plots than in control plots without any plants prior to invasion, and plots with 3-12 Pseudoroegneria ecotypes were 44% less invaded by Centaurea than the mono-ecotype plots. Across all plots, the total biomass of invading Centaurea plants was negatively correlated with total Pseudoroegneria biomass, but biotic resistance provided by high ecotypic diversity of Pseudoroegneria was not explained only by the increase in productivity that occurred with ecotypic diversity. Relative to Pseudoroegneria yield, Centaurea yield was lowest when Pseudoroegneria overyielded due to size-independent complementarity effects. This was not observed when overyielding was due to size-dependent effects. Our results suggest that the intraspecific diversity of a widespread and dominant species has the potential to impact invasion outcomes beyond its effects on native plant productivity and that mechanisms of biotic resistance to invaders may be to some degree independent of plant size.