Abstract:

In the past two decades a considerable debate was generated among ecologists due to differences in the results of experiments which evaluated the relationship between diversity and ecosystem functioning. Recent experimental and theoretical work in this area has also led to animated debates and controversies. A large number of hypotheses linking species richness and ecosystem functioning have been tested. This relationship has been studied primarily through manipulative experiments but little is known about descriptive studies. Grasslands and their grazers provide one of the strongest and widespread cases for studying this relationship. Evidences of both, positive and negative relationships between species richness and Aboveground Net Primary Productivity (ANPP) were founded in the Río de la Plata grasslands. The aim of this paper is to examine the relationship between species richness and ANPP in natural grasslands under continuous grazing of the Southern-Central region of Uruguay. We used two estimators of the ANPP, the Normalized Difference Vegetation Index (NDVI) and the accumulated green biomass. In a heterogeneous environment in terms of soil fertility, species richness and NDVI were measured in 44 plots of 30 cm of diameter. The plots were harvested to estimate the accumulated green biomass and the species were grouped in three Plant Functional Types. The results indicated that both descriptors were positively related to species richness (only the NDVI was signiºcant). Erect grasses and herbs showed higher values for species richness. On the other hand, the prostrate grasses despite being a group with only four species (8% of the total species), contribute with abundant biomass (about 35% of the total biomass) and explain much of the productivity of these grasslands. This paper provides evidence for a linear and positive relationship between species richness and ecosystem functioning descriptors