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Joseph H. Connell have absolute refuges from their preda- tors; Where the prey species are free ofpred- ators, they may increase in numbers until they compete for resources with other species. This is the usual explanation for the low abundance of smaller zooplank- ters in the lakes without large predators: that the larger zooplankters are more effi- cient competitors. However, the small zooplankters may be more common when fish are present because the fish also re- move the predatory invertebrate species, which in lakes without fish reduce the smaller zooplankters. A cautionary note is relevant here. The number of species is usually lower on smaller or more isolated islands. The fact that species there often undergo "niche shifts" on islands is usually interpreted as being due to the absence of one or more competitors (Crowell, 1962; Diamond, 1973, and Chapter 14, Figures 39-43). But an alternative hypothesis is that a signifi- cant predator may also be absent. Clearly, controlled field experiments are necessary to decide between the two alternatives in such instances. 2. In a completely different category are those species that defend themselves against predation by evolving adaptations allowing them to coexist with predators without having to live in refuges. The defenses may be morphological (spines, bark, stinging cells), chemical (tannins, alkaloids), behavioral (aggressive nature, social groups, parental care), or simply growing too large to be attacked success- fully. The problem with most of these de- fenses, most obvious with the last, is that 476 they are less effective in younger than in older individuals. Parental care is an ex- cellent adaptation to bridge the vulnera- ble young stage. Parental care is particu- larly well developed in groups such as birds and mammals. Having also evolved . adaptations such as homeothermy and large size, these animals have escaped many of the hazards of the physical envi- ronment and of predation. Thus, it would be expected that their populations might be limited by competition for resources, as is assumed in many of the chapters in this book. A similar conclusion may be reached from consideration of survivor- ship curves: intraspecific competition among adults is more likely in species in which many young survive to adulthood (e.g., birds and mammals) than in species in which most young perish before adult- hood. In species with little parental care, the young survive until they reach a less vul- nerable larger size only when predation is occasionally reduced, as in the case of Balanus cariosus described earlier. Once an individual or group survives to a size at which attack by a predator is much less probable, it will continue to grow and hold more resources, suppressing its neighbors or smaller individuals beneath it. Thus, competition involving the larger individuals is to be expected, once they escape their predators. Another advantage of large size is that it renders the individ. ualless vulnerable to extremes of physical conditions. This is attested by dominant year-classes, representing escapes during occasional mild spells, in populations in normally harsh regimes. An interesting aspect of physical harsh- , f 16 Producing Structure in Natural Communities :~ :.(< ness is that if prey and predator have similar physiological requirements, the prey species can sometimes tolerate physi- cal extremes in which the predator cannot attack it. For example, sessile animals such as barnacles or mussels can survive on the seashore at such high levels that their predators cannot attack them, as described earlier. This relationship obvi- ously does not apply if the two species have different physiological makeups, e.g., a land predator such as a seabird attack- ing a marine invertebrate. If we consider only species of prey and predator from the same habitat, it follows that if physical conditions become more harsh, i.e., extreme, variable, unpredict- able, or any combination of these, preda- tion would be expected to be reduced. This category includes the larger plants and animals on land and in aquatic habi- tats. The large plants and sessile aquatic animals provide much of the physical structure of ecosystems, modifying the cli- mate or water movements and providing the vertical structure inhabited by many smaller species. Therefore a more detailed analysis of the mechanisms determining the relative abundance, distribution, and diversity of these species is in order here. A Model of the Communi!)' Dynamics of Large Sessile Species of Animals and Plants Communities dominated by large ses- sile species are the rule in most terrestrial habitats, and in shallow aquatic ones. Terrestrial plant communities. coral reefs, beds of turtle gr.ass or kelp. oyster reefs, rocky shores covered with large barnacles, I 477 mussels or algae, and the macrophjtic vegetations in the littoral of lakes are common examples. All are mosaics of patches of "dominant" species, and gaps are continually appearing as individuals or groups are killed by predators, storms, floods, fires, etc. The ecological events and evidence de- scribed above for marine communities can be summarized in a general scheme, which may serve as a testable model of use in predicting which species of "domi- nant" succeeds in occupying a gap. Benthic plants and animals are ar- ranged in a mosaic of patches, some held by long-lived dominants, others inhabited by a mixture of opportunists and young individuals of the dominants. The latter are usually killed within the first year after settlement. Many other species live only in the sheltered conditions created by the dominants. This is the situation shown in Figure I as step 1. Let us now suppose that a patch of dominants is removed, by unpredictable variations in weather, damage by floating objects such as logs, increases in preda- tion, or simply because the large older dominants die as they reach old age. Then the vacant patch is quickly colonized by opportunists, which characteristicaIly have long breeding seasons and numerous motile spores or larvae. In addition, young stages of dominant species that happen to be available may be among the colonists (steps 2 and 3, Figure I). The e,:ents t~at foIl ow are dilTerent in harsh and to bemgn conditions. In very harsh conditions, such as at the upper margin of the intertidal zone or in places with much abrasion, most newly