Herbivores are taxonomically and ecologically diverse, ranging in size from microscopic zooplankton to the largest of land vertebrates. Aquatic grazers include zooplankton (28, 182), larger invertebrates such as snails, insects, and crayfish, and vertebrates such as waterfowl, tadpoles, fish, muskrats, and moose (11, 27, 73, 115, 162, 163). Insects and mammals are the most conspicuous terrestrial herbivores (2, 49-49, 125), but nematodes (20), crustaceans (152), molluscs (78), birds, and reptiles (69) can also be significant. Marine ecosystems are grazed primarily by crustaceans (57, 79, 112, 143), molluscs (15, 126), fish (83, 84, 92), echinoderms (24, 25, 65), and a few insects (170), reptiles and mammals (123, 185).