Does livestock grazing prevent species diversity loss from fertilizer addition in high-productivity grasslands?
Native grasslands are increasingly subjected to anthropogenic nutrient inputs, especially from fertilizer addition aimed at increasing biomass production for domestic animals. This practice may have, at least, two unwanted consequences from a biodiversity standpoint. First, it has been shown that fertilization often decreases plant species richness in productive herbaceous systems through a reduction in light availability. Second, elevated nutrient loads may facilitate community dominance by invasive exotic species. Here we asked whether seasonal grazing by domestic livestock ameliorates the negative impact of fertilization on grassland species diversity. We also tested whether nutrient additions differentially altered diversity of dominant and subordinate species, and of native vs exotic species. The study was conducted in a highly-productive grassland in the Flooding Pampa of eastern Argentina. The design was a 2 x 2 factorial with grazing exclusion and fertilization as main effects. Fertilizer (NPK) was applied three times a year to 5 x 5 m plots, paired with unfertilized control plots, and established within 6 exclosures and in the adjacent grazed grassland (total = 24 plots). Diversity was estimated by the number of species (richness) and the reciprocal of Simpson’s dominance index, both calculated from 1-m2 species cover data in spring and summer.
Grazing exclusion had a strongly negative effect on plant diversity (40%) and promoted dominance by exotic perennial grasses. Fertilizer addition (first year) similarly reduced plant richness in grazed and ungrazed plots by nearly 5 species/m2, which represented a 20% loss in richness, relative to pre-treatment levels. Fertilization further increased community dominance (decreased Simpson’s diversity) in the absence of domestic livestock, but had only a small effect on dominant species diversity in grazed grassland. Our results indicate that seasonal grazing ameliorated the loss of diversity induced by fertilizer addition among dominant plant species, presumably by reducing biomass production and increasing light availability to least competitive species. However, grazing by domestic herbivores could not prevent the significant loss of total species richness that occurred after just one year of fertilization. Many such species were subordinate and rare native grasses and forbs that might contribute to ecosystem functions in the long term.