Herbivores and the dynamics of communities and ecosystems

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 ( 1 1 , 27, 73, 1 15, 162, 1 63). Insects and mammals are the most conspicuous terrestrial herbivores (2, 46-49, 125) , but nematodes (20), crustaceans ( 152), molluscs (78) , birds, and reptiles (69) can also be signifi­ cant. Marine ecosystems are grazed primarily by crustaceans (57, 79, 1 12 , 143), molluscs (15 , 126), fish (83 , 84, 92), echinoderms (24, 25 , 65), and a few insects ( 170) , reptiles and mammals ( 123, 185). These herbivores affect plant communities in many ways. Feeding selectiv­ ity and feeding modes are highly varied; the terrestrial insect herbivores alone include phloem and xylem feeders, root grazers, gall formers, and folivores that mine, chew, roll, rasp, or pit leaves ( 1 94). Herbivores may select among plant parts, individuals, species, patches, and portions of landscapes (7 , 55 , 83 , 95 , 120, 126 , 137, 138 , 140). Many herbivores also clip or tear loose much plant biomass that is not consumed (5 , 70, 104, 105 , 122, 169). Herbivores change the environment by their trails, burrows, wallows, den building, foraging, social behavior, and other activities ( 1 , 4, 15, 40 , 50, 53 , 94, 96, 98 , 138, 148 , 165 , 167, 177). They convert plants into dung, frass,