Context-dependency and the effects of species diversity on ecosystem function

High species diversity often enhances ecosystem function; however, the effects of diversity have not been commonly explored in the context of natural ecosystem variation. In intermountain grassland, it has been reported that Pinus ponderosa canopies and litter reduces the diversity of native species and increases the abundance of Bromus tectorum, an invasive species. Thus this is a realistic system for exploring context-dependency in the effects of diversity on ecosystem function. We established native plant assemblages that varied in species richness and crossed each species richness level with Pinus litter addition. All plots were experimentally invaded with B. tectorum, allowing us to explore context-dependency in the effects of native diversity on community productivity and resistance to exotic invasion. Native polycultures had higher native plant cover, a surrogate for productivity, and stronger resistance to Bromus invasion relative to monocultures. We found that species-independent complementarity effect was the main contributor to the diversity effect on native plant cover, which increased as species richness in polycultures increased. Species-dependent complementarity/selection effects on native cover were negligible, and neither complementarity nor selection effects on native resistance to invasion varied with native richness. Pinus litter had no direct effect on native cover or Bromus performance, but decreased the effects of high native diversity on native cover and increased the effects of high native diversity on native resistance to Bromus. Thus we found some evidence for context-dependency and that low species richness under Pinus canopies might facilitate Bromus invasion. Importantly, the diversity-dependent increase in native plant cover was not correlated with diversity-dependent increases in resistance to invasion among plots, suggesting different mechanisms for how diversity enhances productivity versus resistance.