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Joseph H. Connell within its fundamental niche need not, as defined originally by Hutchinson (1958) and more recently by Vandermeer (1972), be limited to competitive interactions. A major distinction was made earlier between species that reached a size much larger than their predators and those that did not. However, many species occur in both categories. A young member of a large species must spend some time in the latter category when predators are larger than it is. Even birds and mammals are vulnerable for a short time during infancy, and other groups may spend a much greater proportion of their lives in " vul- nerable state. In addition, small prey are usually unattractive to much larger pred- ators. Thus at anyone time there is a restricted range of sizes of predators will- ing and able to attack an individual. As an individual grows, it may be attacked by a series of ever larger predators, or by predators operating in larger social groups. Another important assumption under- lying the present model is that predators attack with less intensity as physical con- ditions become harsher, i.e., more severe, variable, or unpredictable. Evidence sup- porting this generalization comes from gradients across the edges of lakes or the seashore, or from comparisons such as those between deep and shallow alpine ponds or between tropical and temperate latitudes, e.g., that nest predation of juve- nile birds is greater in tropical than in temperate latitudes (Ricklefs, 1969; also data of G. Orians quoted in MacArthur, 1972, p. 2 18). Although further evidence is clearly desirable, this generalization seems to apply to a wide range of species. 480 Thus we have found two relationships 10 be of prime importance: I) the change in vulnerability of prey as its size varies in relation 10 that of the predator, and 2) the change in intensity of predation as physical conditions vary. The first of these applies if prey and predator are to live intermingled together. The prey must be either so much smaller than the predator as to be economically unattractive, or so much larger as to be impossible to attack successfully. The size of either predator or prey refers to the operational unit, whether it be an individ- ual or a pack. The second relationship is important for prey that do not grow large enough to escape their predators. Then prey must have a refuge from predation. One way this may occur is when predation dimin- ishes as physical harshness increases; the prey may then evolve tolerance to physi- cal regimes too harsh for the predator to attack effectively. These points are combined in a dia- grammatic way in Figures 2 to 5. Each of these represents a surface of varying probability of mortality per unit time, showing "contours" of equal probability. The shading indicates a region of very low probability of mortality. A gradient from left to right in all the ligures represents increasing harshness of the physical re- gime, such as would obtain for a marine species from low to high levels in the in- tertidal zone. A gradient from bottom to top on the ligures represents increasing body size of the prey. Figure 2 indicates the probability of being killed by a predator under different physical regimes and body sizes. Let us 16 Producing Structure in Natural Communities . . . ;;; Increasinc:l Ptl1Sical Honhneu ----+ Figure 2 Mortality due 10 predation under different physical regimes and body sizes. A point on the surface represents a particular probablhty P that ~ prey individual will be killed by a predalor per .un1l time. The curves are contours of equal probabIlity, high toward the left, the shaded region being one of very low probability of mortalllY from I'redatlOn. A gradient from lefl to ril?ht represents mcreasmg ph)'sical harshness. A gradIent from bottom to top represents increasing relative body sIze of prey com. pared with predalor. . ;;; ~ ." o III " < o . . < Increosin; Physical HQrlhnen_ Figure 3 Mortality due to the direct effects of harsh physical conditions on prey of different. body sIZes. The gradients and curves are ex plaIDed ID Figure 2. A poinl on the surface now re~res~nts the proba- bility P Ihat a prey individual w,l! dIe due to har;h physical conditions per unJt tJrne. 481 '0 . ;;; / / Inereoline) Physical Harshness ~ Figure 4 Mortality due 10 both predation and phys. leal conditions, for prey that are small in relati~n to the size of their predatots. The solid curves III region A represent probabilities of m~rtaJity from predalion, as in the lower pomon of FIgure 2. with the scale expanded. The dasbed curves m regJo~ B represent probabilities of mortality ~rom phYSIcal factors, as in Figure 3. The sbaded regIOn represents a region or very low prohability of mortality. See Figure 2 for more explanation. . Vi Figure 5 Mortality due to both predation and phys. ical conditions, for prey that grow to a large Slze m relation to the size of their predators. The curves for mortality from predation (solid lines, region A) are taken from Figure 2, those for physical effects (re- gion B) from Figure 3. See Figure 2 for more explanation.