Reference
Isbell, F., Reich, P.B., Tilman, D., Hobbie, S.E., Polasky, S. and Binder, S. 2013. Nutrient enrichment, biodiversity loss, and consequent declines in ecosystem productivity. Proceedings of the National Academy of Sciences USA 110: 11,911-11,916.
Background
The authors write that "anthropogenic drivers of environmental change often have multiple effects, including changes in biodiversity, species composition, and ecosystem functioning," but they say that "it remains unknown whether such shifts in biodiversity and species composition may, themselves, be major contributors to the total, long-term impacts of anthropogenic drivers on ecosystem functioning."
What was done
Desirous to discover how this phenomenon may operate, Isbell et al. analyzed temporal trends in the effects of nitrogen enrichment on the productivity, plant diversity, and species compositions of naturally assembled grasslands in a long-term nitrogen addition experiment they conducted at the Cedar Creek Ecosystem Science Reserve in central Minnesota (USA), where they measured aboveground peak biomass, as well as the number and abundances of plant species in each plot from 1982 to 2008, after which they similarly analyzed data from the BioCON (Biodiversity, CO2 and N) experiment located at the same reserve, in order "to quantify the extent to which N enrichment and elevated CO2 influence productivity by non-randomly changing grassland plant diversity," which they did in the latter instance from 1998 to 2011.
What was learned
The six scientists discovered that "although chronic nitrogen enrichment initially increased productivity, it also led to loss of plant species, including initially dominant species, which then caused substantial diminishing returns from nitrogen fertilization." In contrast, they report that "elevated CO2 did not decrease grassland plant diversity," but that it "consistently promoted productivity over time," both by its direct aerial fertilization effect and by its non-significant yet real tendency for species diversity to actually be somewhat enhanced by elevated CO2.
What it means
The good intentions of man (nitrogen fertilization) may sometimes have unintentional negative consequences for certain ecosystems, while other actions of humanity (the mining and burning of fossil fuels, such as coal, gas and oil) can sometimes prove to have unexpected positive effects.