Grass leaf silicification: Natural selection for an inducible defense against herbivores.

Plants from four populations of three species of African grasses were collected from grasslands in Tanzania's Serengeti National Park that differ in the grazing intensity that they experience. Plants were grown in the laboratory in a factorial experiment in which variables were plant origin, species identification of plants, defoliation intensity, and supply of soluble silicate in the nutrient medium. All plants accumulated silica in leaf blades in the absence of soluble silicate from the nutrient medium. Plants native to the more heavily grazed grassland accumulated more silica in their leaf blades than did plants from the less heavily grazed site. Blade silica content was higher when plants were defoliated, indicating that silicification is an inducible defense against herbivores. The quantitative heterogeneity of this qualitatively homogeneous plant defense system may have contributed to the evolution of high species diversity in the grazing fauna.