Grazing Lawns: Animals in Herds, Plant Form, and Coevolution

Many grazing animals in both terrestrial and aquatic ecosystems form dense herds that maintain the vegetation in their concentration areas at very low statures. Studies of the effects of large ungulates on the structure of grasslands in the Serengeti region of Tanzania and Kenya indicated that some vegetation was arrested in a short form throughout the wet season while other vegetation was only lightly grazed during that season and reached similar heights in fenced and unfenced areas. Biomass concentration, the mass of foliage per unit of canopy volume, was consistently and substantially higher in unfenced plots but the covariation of height and biomass concentration across plots along the Serengeti's habitat gradient indicated that the tendency toward production of a dense, prostrate grazing lawn in ecological time was accentuated by, but was not solely a consequence of, proximal grazing intensity. Transplant garden studies documented intrinsic differences within and between species in traits related to dwarfing. Comparisons of the Serengeti data with information on the foraging of domestic ungulates indicated that individual grazers obtain a foraging advantage by membership in a herd because of the greater forage yield per bite from grazing lawns compared with lightly grazed vegetation. Thus, natural selection at the individual level, acting on both animals and plants to produce coevolution among members of the same trophic web, can regulate such ecosystem processes as energy flow and nutrient cycling, and contribute to species coexistence and the resultant species diversity of communities.