Reversal of Grazing Impact on Plant Species Richness in Nutrient-Poor vs. Nutrient-Rich Ecosystems

To test the hypothesis that the impacts of grazers on plant species richness reverse under contrasting nutrient richness, we analyzed unpublished and published data from lake, stream, marine, grassland, and forest ecosystems. We analyzed data from 30 studies providing 44 comparisons of plant species richness under low vs. high grazing pressure in enriched or nutrient-rich and non-enriched or nutrient-poor ecosystems. All 19 comparisons from non-enriched or nutrient-poor ecosystems exhibited significantly lower species richness under high grazing than under low grazing. In contrast, 14 of 25 comparisons from enriched or nutrient-rich ecosystems showed significantly higher species richness under high grazing than under low grazing. However, nine of these 25 comparisons showed no significant impact of grazers on species richness, while two comparisons showed declines in species richness under high grazing. Based on all the comparisons, plant species richness decreases with high grazing in nutrient-poor ecosystems, while it increases with high grazing in nutrient-rich ecosystems. Although nutrient-rich ecosystems seemed to produce more variable responses to grazers than did nutrient-poor ecosystems, in rare cases high grazing produced a decline in species richness in nutrient-rich environments. We suggest that species richness declines with high grazing in nutrient-poor ecosystems because a limitation of available resources prevents regrowth of species after grazing, which may not be the case in nutrient-rich ecosystems. It is also possible that an increase in species richness under high grazing in nutrient-rich ecosystems may be due to an increase in the dominance of inedible species. Our observation of a grazer reversal of plant species richness under contrasting nutrient richness may have important implications for management of species diversity.