Do the effects of species richness on ecosystem function rival other forms of environmental change?

Background/Question/Methods

Many experiments have shown that species richness of primary producers have significant effects on resource uptake and biomass production.  While the effects of biodiversity are unequivocally significant, a key question remains: how do these effects compare in magnitude to other forms of environmental change.  We used meta-analysis to compare the influence of altered species richness with that of realistic variation in several types of environmental change, including eutrophication, elevated CO₂, and warming.  We assessed these effects on two key ecosystem processes – primary production and decomposition – using two different approaches.  First, we compiled data from existing meta-analyses that have summarized how the different forms of environmental change influence NPP and decomposition in terrestrial and aquatic ecosystems and compared these to species richness effects from a database of experiments testing diversity effects on ecosystem functioning.  Second, we compared effects of environmental variation and variation in species richness from experiments that factorially manipulated both.  In this second analysis, we only had sufficient data to assess effects on primary production.   We used two metrics of the effects of changes in species richness: 1) maximum richness compared to the mean monoculture, and 2) loss of 50% of local autotroph diversity.

Results/Conclusions

Altered species richness had effects on biomass production that were intermediate to other forms of environmental change.  Nutrient addition, particularly N and P together, had the largest effects, increasing production 1.8 to 3.3-fold.  Maximum effects of species richness changed production ~1.6-fold, on par with effects of single nutrient additions and of elevated CO₂ in some studies.  Warming and elevated CO₂ in mixed species communities had the smallest effects (1.2-fold), as did 50% loss of species, because of typically saturating relationships between richness and productivity.  For decomposition, nutrients and CO₂ had minimal effects in terrestrial systems; species richness effects did not differ significantly from 0, but varied widely among studies.  In the meta-analysis of factorial diversity x environment experiments, effects of nutrient addition, temperature, and water availability were ~3x stronger than maximal effects of species richness.  Effects of 50% loss of species were small (1.1-fold), and on par with effects of elevated CO₂.  These results suggest that random reductions of species richness to monocultures can have effects on ecosystem processes that are similar in magnitude to those of other prominent forms of global change.  However, for even a 50% loss of species, changes in ecosystem processes were comparatively small.