Background/Question/Methods Experiments from the last 25 years indicate that plant species richness is a strong predictor of net primary productivity (NPP) when native species are considered. However, relationships may be altered by invasive (exotic) species, which are hypothesized to reduce richness but increase productivity (i.e., ‘paradox of invasive species’). If invasive species reduce diversity, but increase NPP, then the slope of richness-productivity relationships would shift from positive to neutral or negative. This shift in slopes is predicted to be larger when invasive species are C4 than when they are C3.
We compared relationships between richness and NPP using three data sets. The first is a comparison of 42 invaded or native-dominated sites from Minnesota to central Texas. The others were long term experiments, and compared native and exotic plant communities under common environments in the southern (MEND experiment, in C4 dominated grasslands, n = 64) and northern USA (Iowa, in C3 dominated grasslands, n = 64). ANPP was measured using peak biomass clipping in all three studies, and BNPP was measured in one study with root ingrowth cores using root-free soil. Richness dropped over time from planted levels in the experiments, producing a range of values in realized richness across plots.
Results/Conclusions In the comparative study, there was a significantly positive relationship between ANPP and species richness across native-species dominated sites (P < 0.05) from Minnesota to Texas. The slopes were significantly reduced across exotic dominated sites (native-exotic x richness interaction, P < 0.05), and the relationship between ANPP and richness was not significantly different from zero. In the MEND study, species diversity was positively related to ANPP and BNPP in native plots (P < 0.05), but again, the relationship was not significantly different from zero across exotic plots. The exotic plots in the MEND experiment had higher ANPP and BNPP than more diverse native plots. At the northern site, where the invasive species were C3, we again found that a positive relationship between richness and ANPP across native plots, and a relationship that was not significantly different from zero across exotic plots. Exotic plots had lower ANPP than native plots at the northern site. Taken together, our results indicate that exotic plant species alter biodiversity-ecosystem functioning relationships.