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Joseph H. Connell move the allelopathic effects also destroy or drive out the grazing animals (Halligan, 1972; Christiansen, 1973). Either experi- mental removal of the tops of allelopathic plants or exclusion of grazing vertebrates by cages or fences, or both, has been done in several instances, When vertebrate grazers alone were excluded, herbs have sometimes become established in abun- dance near allelopathic shrubs (Bartholo- mew, 1970; Halligan, 1972), sometimes much less abundantly or not at all (Mc- Pherson and Muller, 1969; Muller and del Moral, 1971; Chou and Muller, 1972; Christiansen, 1973). The biggest change has been reported when both grazing and aIlelopathy have been experimentally eliminated together in these studies. The conclusion from these studies is that both allelopathic interference between the plants and grazing by herbivores influence the community structure of chaparral vegetation. Some field experiments testing the inter- action between grazing and competition between grasses and forbs have been done on sheep pastures (Sagar and Harper, 1961; Putwain and Harper, 1970). They indicate that grazing may reduce compe- tition, but without knowledge of whether wild herbivores graze natural vegetation at the same intensity as these sheep did, it is difficult to generalize from these re- sults. The effect of predators on terrestrial animals has seldom been estimated from field experiments under natural condi- tions. The only controlled field experiment r know of concerns the effect of wood- peckers on Englemann spruce beetles 468 (Knight, 1958). The birds concentrate their attacks both in the groves and on the individual trees where the beetles are commonest. By using exclusion cages on 250 trees in the Rocky Mountains, Knight was able to show that the predators caused proportionately greater mortality where the prey were denser. Murdoch (] 966) has shown that carabid beetles will survive well through the summer only when pro- tected from predation. Predators of terrestrial vertebrates do not seem to be so effective in reducing their prey, for reasons which will be dis- cussed later. Predators of ruffed grouse were removed almost completely in two different places; nesting losses were re- duced but the adult populations did not increase (Edminster, 1939; Crissey and Darrow, 1949). This supports the sugges- tion of Levins (Chapter I) that a change in production of one life stage may not affect the abundance of another life stage limited by some other factor. When about half of the predators of the vole (Microtus calif amicus) were removed during a peak in vole numbers, the population declined to the same degree as it had from a previ- ous peak (Pearson, 1966). The often-cited instance of an irruption of the deer popu- ]ation following the reduction in their predators on the Kaibab plateau of Ari- zona has been shown to be incorrect by Caughley (1970). There is one instance in which predators have been effective in reducing mammalian populations, i.e., wolves and moose on Isle Royale in Lake Superior. Although no controlled experi- ments have been done, the evidence strongly suggests that before the wolves 16 Producing Structure in Natural Communities arrived the moose were so dense that their feeding was injuring the vegetation. This is not happening now that wolves are feeding on the moose (Mech, 1966; Jordan Shelton, and Allen, 1967). In freshwater. The zooplankton of open water in lakes without planktivorous fish usually consists of several species of relatively large crustaceans, together with smaller species of various groups. After planktivorous fish have been introduced and increased substantially, the larger species of zooplankton are absent from the open water and the smaller species are much commoner. Several studies have compared the same pond or lake before and after fish were introduced, and all have found that the large species dis- appeared and the smaller species in- creased, suggesting that the fish selectively ate the larger zooplankters (Hrabacek et al., 1961; Brooks and Dodson, 1965; Macan, 1965; Reif and Tappa, 1966; Galbraith, 1967; Wells, 1970; Hall, Cooper and Werner, 1970; Warshaw, 1972). Brooks (1968) has confirmed that planktivorous fish selectively eat the larger zooplankton in laboratory experiments, as have Galbraith (1967) and Green (]967) by comparing stomach contents with plankton samples. Where large aquatic vegetation is dense, the herbivorous zooplankton is much more abundant, even though phyto- plankton productivity is less. A likely rea- son for this is that fish predation is less effective in dense vegetation. Hall et 01. (1970), found that in ponds with dense vegetation (caused by addition of large amounts of mineral nutrients) the biomass 469 of zooplankton was not reduced signifi- cantly by fish. But in replicate ponds with less vegetation, fish reduced the biomass of zooplankton significantly below that in control ponds. Other predators besides fish may change the biomass and/or relative abun- dance of zooplankton. In a series of alpine ponds in Colorado where salamanders (Ambysloma) were common, the herbivo- rous zooplankton species were small. \Vhere salamanders were rarer or absent, larger herbivores occurred (Dodson, ] 970; Sprules, 1972). Like fish, salamanders also tend to select the larger individuals as prey, judging by stomach contents and ]a bora tory experiments. Field experiments on invertebrate pred- ators have been conducted in only one study, by Hall et 01. (]970). The biomass of zooplankton was consistently lower in treatments with increased invertebrate predation. The predators removed the larger herbivores first, thereby changing the species composition of the community in the same fashion as the vertebrate predators did. It is difficult to decide whether fish or invertebrate predators have a greater effect on the relative abundance of species in the zooplankton. The field experiments of Hall et 0/. (1970) underestimated the effects of invertebrate predators since a small predator, Chaoborus, was left be- hind in the "reduced" treatment. Dodson (1970) calculated that the predation rate on Daphnia by the population of preda- tory midge larvae was about 10 times that of the salamander population. However, in the experimental ponds of Hall et a/. II 4 :If