Rainfall increases similarity in plant species composition between grazed and ungrazed sites in a tropical semi-arid grassland

In peninsular India, semi-arid grasslands support marginal farmers and rich endemic biodiversity, yet are commonly designated as wastelands, and fragmentation has caused heavy grazing by domestic and wild mammals. The impacts of unmanaged grazing and increasingly variable monsoon rainfall on the plant community are unknown. 2. We studied an ancient tropical semi-arid grassland in a drought year and a subsequent wet year. In replicated grazed and long-term ungrazed sites, we assessed plant community composition, richness, biomass, traits and functional groups, as well as invertebrate herbivory, fungal infection, soil and plant nutrient stoichiometry. 3. We found that grasses dominated over forbs and legumes. The species richness rose from 35 in the dry year to 84 in the wet year, reflecting the grassland's long evolutionary history of high variation of annual rainfall. However, community composition differed greatly between grazed and ungrazed sites, especially in the dry year. Grass Heteropogon contortus was abundant in the wet year, while Chrysopogon fulvus dominated ungrazed sites in the dry year. H. contortus may persist under heavy grazing owing to its low palatability, whereas fast-growing C. fulvus may outcompete it given sufficient rain and no grazing. Specific leaf area was unrelated to grazing, but increased in the wet year, probably reflecting more acquisitive species. 4. Most plants had insect damage, which was 10% lower in the wet year despite high community-level plant nitrogen, perhaps because high plant density reduced per capita herbivory. Fungal disease incidence was high in the wet year, possibly due to rapid pathogen growth in the humid microclimate. Plant and soil C:N ratios were low in the wet year, when greater legume cover may have increased N availability. 5. Synthesis. Both grazing and rainfall shaped the plant community. Grazing affected species composition, whereas rainfall affected species richness, herbivory and disease incidence. As climate change makes the Indian monsoon more extreme and land use confines grazers to smaller areas, plant species diversity is likely to decline, even among plants adapted to both aridity and grazing.