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Joseph H. Connell (1970), even with augmented numbers the invertebrate predators did not completely eliminate the larger herbivores as did the fish. The benthic flora and fauna of fresh- water lakes are also affected by predation. Field experiments excluding fish (Black, 1946; Threinen and Helm, 1954) and tad- poles (Dickman, 1968) resulted in rapid grov.'th oflarger plants. However, some of the fish were introduced asian carp, which may not have had such devastating effects in their native waters. Experimental re- moval and addition of fish (Lepomis sp.) in ponds or lakes have resulted in a greater standing crop of benthic inverte- brates without fish (BaIl and Hayne, 1952; Hayne and BaIl, 1956), or no change (HaIl et a/., 1970). This difference may have been the result of the much higher density of fish used in the two former studies (97 and 179 kg/ha, respectively) than the lat- ter (50 kg/ha). HaIl et al. (1970) found that the fish reduced the larger animals (insect larvae and amphipods), and the smaller species increased, maintaining the total biomass. The fish selected the larger pupae, thereby greatly reducing the num- bers of emerging insects. v,'hen invertebrate predators were re- duced, the biomass of benthic inverte- brates rose, but when these predators were increased, biomass rose in the first sum- mer and feIl in the second. Increased pre- dation had predictable effects on the two principal species, the midge Chironomus being reduced and the ephemeropteran Caenis increasing. Some experiments and observations have shown that fish may also be affected 470 by their predators. In lakes and rivers young salmon are heavily preyed upon by predatory fish and birds. Removal of these predators increased the survival to both the smolt and adult stages (White, 1939; Foerster and Ricker, 1941; Foerster, 1954; Elson, 1962). Jackson (1961) observed I~kes with and without the large predatory tiger fish (Hydrocyon villatus), which eats all fish small enough to swaIlow. Only adults of species larger than this occur outside the shelter of aquatic plants in lakes with the tiger fish, but in lakes with- out it, small fish swim in open water. In most of these studies, the predators selectively removed the larger inverte- brates, regardless of species, and the smaller herbivores usually increased. This is interpreted by most of the authors to indicate that without the larger competi- tors, the less efficient smaIler species could secure more of the limited resources and so increase. However, an alternative hy- pothesis is that after the predatory fish were added, they removed not only the larger competitors but also the predators of the smaller species. Fish removed the predatory insects in the experiments of Hall et al. (1970), and presumably also in some of the other lakes studied. However, when invertebrate predators were added by HaIl et at. (1970), it is less likely that they ate the predators of the smaIler her- bivores. Thus Coenis increased, almost certainly because its competitor Chirono- mus had been reduced by invertebrate predators. Likewise the larger cladocerans in the zooplankton decreased progres- sively through the summer, and the roti- fers increased. Invertebrate predators, by 16 Producing Structure in Natural Communities reducing the larger herbivores, which are probably more efficient coIlectors of sus- pended food (Brooks and Dodson, 1965), probably allowed the less efficient smaller species to increase. However, to elucidate the role of competition properly, field ex- periments are essential. Sprules (1972) has done a small-scale pilot experiment that indicated that the large Daphnia pulex has a deleterious effect on the smaller Daphnia rosea. Repetition of this interesting exper- iment on a larger scale would be welcome. The effects ~of grazing by zooplankton on phytoplankton has recently been esti- mated by Porter (1973). She suspended clear plastic bags holding about 0.5 m3 of water a short distance below the surface of a lake. The numbers of zooplankton grazers were either reduced, increased, or kept the same in the bags. Counts of the phytoplankton after four days in the bags and examination of gut contents of the grazers showed that some groups of plants were reduced, others increased, and some were unaffected. The growth of some gelat- inous green algae may have been in- creased by passage through the gut of zooplankters (as discussed by Patrick, Chapter IS, for grazing on algae by crus- tacea), whereas small edible species are reduced. Thus grazing may completely chane.e the relative abundance of fresh- wate; phytoplankton. Where physical conditions become very harsh in fresh water, predation is reduced. For example, in the alpine ponds studied by Dodson (1970) and Sprules (1972), shallower ones froze to the bottom each winter and sometimes dried up in late summer, whereas deeper ones never did. di 471 The predators in the deeper pools were the axolotl salamander and Chaoborus, neither of which apparently tolerates freezing or drying. Both were absent from all shallow pools, where the only predators were a large copepod, which apparently tolerates the harsh conditions, and/or a salamander, which could leave the pond and hibernate elsewhere; this salamander was never as abundant in these pools as the axolotl was in the deeper pools. Thus, predation was probably less intense in harsher conditions. A second example involves large lakes. The deeper hypolimnion of temperate lakes often becomes anoxic in summer, reducing the activity of fish. Therefore, fish feed less of the year in the deeper than in the shallower depths, which do not be. come anoxic. Jonasson (1971) found that the midge Chironomus anlhracinus reached much higher population densities in the hypolimnion than at shallower depths of Lake Esrom, Denmark. Since almost all of the mortality occurred during the short period when the predatory eel (Anguilla) could feed, between the autumn overturn and the onset of winter, Jonasson ascribed most of the mortality in deep water to predation. At shallower depths Chirollol11us never attain such high densi- ties, possibly because the predators are able to feed on them for the entire warm season. Because growth is also inhibited when oxygen is lacking in the hypolimnion, not all the Chironol11uS larvae complete devel- opment in one year. However, some do and emerge in May, leaving others be- hind. But in most years there are so many ;-J I : ~j : : : ~ f _i;t