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Joseph H. Connell 478 A. c. !.!! ~ PHYSICAL CONDITlONS: 4b YOUNG DOMINANTS SVRVIvt llEL.L SI~C[ TH! Ptn'SlCAL CXlstlITIOlilS ARE KARSH ENOUGH TO JUDUCE THE EFFECTIVENESS OF !V.nrML 'ENEMI IS. BIl'T NOT SO Stn:RL AS TO DtSnOY THE PREY. But occasionally. certain species of natural enemies are reduced .j. !Ie THEN THE YOl'NG DOHIN,u,'T SPECIES WHICH IS THE PREY or THAT PAR.TlCULAJl NATURAL ENDrY SURVIVES TO A.\' 'INVULNERABLE' SIZE 1. PATC~ IS REEStABLISHED, ~'ITH THAT DOHI~M"T SPECIES A)."D ITS A.SSOClATtS Fi~re t A model of the mechanisms determining wh!ch "do~inants" (large plants or sessile aquatic ammals) WIll fill a gap. See lext for further discus- sion. settled or young individuals are killed by ~arsh ,,:eather each year so that the patch IS conlInually being colonized and va- cated; this cycle is shown between steps 3 and 4a. They survive only in particularly f~vorable years, and then may live a long lIme. Populations consist of dominant year-groups, the survivors from these oc- casional favorable years. Beca use they oc- c.upy more and more space as they con- lI~ue to grow, they exclude and may dIsplace other organisms (steps Sa to 6). In other words, once two or more individ- uals are past the vulnerable young stage, they may compete with one another. However, in many cases these populations never get dense enough to compete for resources. In benign environments, natural ene- mies tend to be much more effective (Connell, 1971) and most colonists are eaten. This tends to reduce competition and also to prevent the growth to an in- ,'ulnerable size of the young dominants. 16 Producing Structure in Natural Communities If the populations of grazers or predators are not reduced by some external event, they are capable of keeping the patch in this state of continual recolonization (the cycle between steps 3 and 4c). However, if some unpredictable event, such as a short spell of bad weather or an attack by their own predators or path- ogens, reduces the population of these natural enemies, the prey will have a pe- riod of good survival and growth. This then allows whichever dominant species happens to be present to grow to an in- vulnerable size (step 5c). Which, if any, do so depends upon which natural"enemy is reduced and whether the reduction lasts for a long enough period. Once the prey reaches invulnerable size. it excludes or displaces other organisms by further growth (steps Sc to 6). The situation then returns to the original state (steps 6 to 1) as "sheltering" species colonize the area beneath the dominant. In intermediate environments there is less mortality from harsh weather and natural enemies are less effective, so that the young of dominant species more read- ily escape being eaten. Thus these young dominants may reach high population densities and begin to compete with one another. The eventual winner, i.e., the dominant that eventually fills the space, is likely to be the one able to displace the others during a period of competition (steps 4b and 5b). Which species of dominant will succeed in filling the vacated patch depends upon several things. For example, in benign regions such as the lower seashore there are more species of predators, and some, 479 such as starfish, are quite large and able to eat grazers and smaller predators as well as the dominant sedentary animals (Paine, 1966; Menge, 1972). Thus starfish tend to reduce both animal dominants and grazers. Plants should therefore be favored, since both their competitors and their natural enemies are being reduced. This may be the reason why, in temperate latitudes over the world, plants rather than animals are the dominant organisms covering the lower seashore. They form the "su blitloral fringe" in Stephenson and Stephenson's (1949) universal scheme of zonation. In contrast, at middle shore lev- els, large predators such as starfish are less common; the main predators are muricid snails. They feed much more heavily on sedentary animals such as mussels and barnacles than on grazing molluscs, so that grazers are less likely to be reduced than they were at lower shore levels. This means that both plant and animal domi- nants are under attack, so that no a priori prediction can be made as to which will succeed in fil1ing the vacated patches of rock surface. Barnacles, mussels, tube worms, oysters, and various algal spe- cies-all may occur as dominants in mid- dle shore zones. What Determines Realized Niches and Community Structure? The distribution and abundance in which one finds a species, as opposed to its potential area and population size, is obviously not determined solely by com- petition. Therefore the variables that. con- strain the realized niche of a species ~-~~~~