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Joseph H. Connell examine the situation in which the prey is relatively much smaller than the preda- tor, i.e., as in the lower portion of the figure. Along any horizontal line, with a constant ratio of body sizes of prey and predator, the probability of mortality from predation decreases toward the right as the physical conditions become more harsh, because of a progressive reduction in the period of activity of the predator. Along a vertical line, representing a par- ticular degree of physical severity, the probability of mortality increases up- wards, as the prey become relatively larger. The reason is that very small prey are usuaIly not eaten by a large predator; they are ignored, or not caught by the meshes of its filter, etc. Presumably there is a size of prey below which there is too Iowan energy return for the effort ex- pended. These two tendencies cause the contour lines of equal probability of death to slant upwards to the right in the lower portion of Figure 2. In the upper part of Figure 2, the prey are relatively larger. As before, along any horizontal line the probability decreases as the physical regime becomes steeper. However, along a vertical line at a partic- ular physical regime, the probability of death decreases upward. As a prey indi- vidual grows, it becomes too large for a particular predator to attack. This also tends to reduce the number of species of predators that can attack it. These ten- dencies cause the contour lines to slant downwards to the right. The shape of the contour lines would vary with different combinations of prey, predator, and physical regime. I have drawn them as they probably apply to the 482 species on rocky seashores. The situation in Figure 2 applies to predators and prey that have similar physiological charac- teristics, for example, marine organisms. In other instances, they may have quite different physiologies. For example, along shores of lakes or oceans, land animals feed on aquatic species. In this case, a mirror image of Figure 2 would be appro- priate, since as the physical regime be- comes less severe for the aquatic prey (toward the left), it would become more severe for the terrestrial predator. The effect of the direct action of physi- cal conditions on organisms of different sizes is shown in Figure 3. Along a hori- zontal line the probability of death per unit time increases toward the right. Up- ward along a vertical line the probability decreases since, as an individual grows, it becomes less vulnerable to the physical environment. These trends cause the con- tour lines of equal probability to slant upwards to the left. Again, the shape of the curves will depend upon the charac- teristics of the species and physical re- gime. In Figures 4 and 5 I have combined the first two diagrams to illustrate the effect both of predators and of different physical regimes on the mortality of the prey. Fig- ure 4 represents small prey, so uses only the lower portion of Figure 2. When prey are very small, they tend to be ignored by larger predators so that mortality is due mainly to physical harshness. As prey grow, they escape this hazard except where conditions are very severe. In very benign conditions, larger prey soon attract the notice of predators that tend to be active much of the time. In conditions of 16 Producing Structure in Natural Communities intermediate severity, they escape the at- tentions of the less active predators for a longer time, and having passed the very young stages at which they are vulnerable to variations in physical conditions, they may be safe for a while. If local popula- tion densities are high enough, competi- tion may take place during this interval before predation begins to reduce the populations. Fil1.ure 5, which combines most of Fig- ure 2' with Figure 3, represents the situa- tion in which the prey grow to a relatively large body size. Once they have grown large enough to be less vulnerable to physical factors, they mayor may nOI be safe from predators for a short while, but soon are attacked. However, if they sur- vive long enough, they may reach an in- vulnerable size. (These diagrams do not take into account the fact that prey even- tually become vulnerable again in old age.) The situation in Figure 4 is exemplified by the two species of barnacles thaI com- peted only at intermediate intertidal levels in Scotland (Connell, 1961b). After elimi- nating Chthamalus in competition, Bala- /lUS was eaten by predatory snails, except in a refuge high on the shore. Balanus cariosus fits both Figures 4 and 5, depend- ing upon the relative size of its predators. With small predators it is able to grow too large to be attacked (Figure 5), but with the large starfish it fits Figure 4. Many prey species probably pass through a sim- ilar series of ever larger preda tors. Species of prey that never grow very large compared with their'predators, such as the aquatic plankton or small land 483 plants or animals, are represented by Fig- ure 4. Large plants and animals would be more closely represented by Figure 5. Summary The distribution and abundance of a species are ultimately determined by tol- erances to extremes of physical conditions, but a species is usually limited to a smaIler range of habitats and population size by interactions with other organisms. The evidence reviewed in this paper, taken mainly from controlled field experiments on invertebrates and plants, suggests that many species seldom reach population densities great enough to compete for re- sources, because either physical extremes or predation eliminates or suppresses them in their young stages. Species may sometimes escape these hazards to reach high densities in various ways. One way is through large size, which reduces vulnerability to harsh weather and predation, but the problem for a young individual is to reach a large enough size before it is killed. Strong parental care helps as in many vertebrates, but offspring without care will escape only during occa- sional reductions in predation or harsh weather. Such escapes may produce widely spaced "dominant year-classes." Once they reach large size, they may compete for resources and suppress smaller individ- uals. Examples are forest trees, large ses- sile aquatic animals, etc. A model of this situation is presented. Second, some species can never grow so large as to escape predation. They may escape for a while if they are too small