RELATIONSHIP BETWEEN PRODUCTIVITY, AND SPECIES AND FUNCTIONAL-GROUP DIVERSITY IN GRAZED AND NONGRAZED PAMPAS GRASSLAND

Most hypotheses addressing the effect of diversity on ecosystem functi on indicate the occurrence of higher process rates with increasing div ersity, and only diverge in the shape of the function depending on the ir assumptions about the role of individual species and functional gro ups. Contrarily to these predictions, we show that grazing of the Floo ding Pampas grasslands increased species richness, but drastically red uced above ground net primary production, even when communities with s imilar initial biomass were compared. Grazing increased species richne ss through the addition of a number of exotic forbs, without reducing the richness and cover of the native flora. Since these forbs were ess entially cool-season species, and also because their introduction has led to the displacement of warm-season grasses from dominant to subord inate positions in the community, grazing not only decreased productiv ity, but also shifted its seasonality towards the cool season. These r esults suggest that species diversity and/or richness alone are poor p redictors of above-ground primary production. Therefore, models that r elate productivity to diversity should take into account the relative abundance and identity of species that are added or deleted by the spe cific disturbances that modify diversity.