Meta-analysis of ecosystem function response to variation in native-exotic dominance

Background/Question/Methods

Exotic species composition is expected to increase in many ecological communities in the decades ahead.  However, it is unclear to what extent, and in what circumstances, exotic species affect ecosystem function.   We assembled from the literature studies that measure ecosystem functioning (e.g. plant productivity, decomposition rates) and also provide the relative prevalence (percentage, richness, or abundance) of non-native species.  We focused on systems that were grass-dominated at the time of measurement.  We analyzed these data to examine evidence for any significant patterns between the dominance of exotic species and ecosystem function. 

Results/Conclusions

We identified >30 grassland, prairie, and savanna studies that quantified ecosystem productivity (ANPP or peak above ground biomass) and provided some measure of the relative abundance of native and exotic species.  Productivity increased significantly with increasing rainfall.  At sites with more than 500 mm of rainfall per year, increasing numbers and abundances of exotic species were correlated with increasing above ground biomass.  However, this trend was driven by a few productive tropical sites dominated by exotic monocultures.  At lower rainfall levels there was no correlation between the amount of exotics and grassland biomass.  These data suggest that the presence of exotic species in grasslands do not have predictable effects on ecosystem functioning.  Effects on ecosystem function may be more linked with functional identity of exotic relative to native species and the ability of some exotic species to form dense monocultures.