Effects of grazing,competition, disturbance and fire on species composition and diversity in grassland communities

Abstract. The effects of grazing, physical disturbance, interspecific competition, and fire on populations of individual species and on the structure and diversity of grassland communities were investigated in short-, mid-, and tall-grasslands in the Serengeti National Park, Tanzania. The treatments included deep and shallow artificial disturbances, early dry-season burning, and removal of dominant grass species from 2m² plots located inside and outside of large-mammal-proof exclosures. Species-cover data were collected for five years and analyzed by analysis of variance and diversity indices. In all three communities, protection from grazing and physical disturbance had greater impacts on species cover and diversity than removal of dominant species or fire. More species were significantly affected (positively and negatively) by disturbance and protection from grazing (50 - 100%) than by species removals and fire (< 41%). In most cases, the cover of tall, perennial species increased following protection from grazing while the cover of annual and short, perennial species decreased. Over the five years of the study, vegetatively reproducing species benefited more than sexually reproducing species from protection from grazing, and tall rhizomatous species benefited more than tall stoloniferous species. Disturbance caused annual and short perennial species to increase and tall perennials to decrease in cover. Following species removals, tall species were either unaffected or they increased in cover while some short species increased and others decreased in cover. Species diversity in two of the three communities decreased when the communities were protected from grazing but increased when disturbed, when dominant species were removed, and when burned.