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Joseph H. Connell About every 5 years they escape from smaller predators; they may then survive for 15 years or so before being eaten by larger predators. Where is competition prevented by physical conditions? On land. In certain very harsh deserts, the germination of seeds and the estab- lishment of seedlings of perennial plants may be prevented by the absence of rain- fall for many years. For example, in cen- tral Australia, Acacia aneura produces viable seeds in most years, but no seed- lings get established. In the occasional years when rainfall is higher, seedlings may survive longer, but are usually de- stroyed by insects. Only if at least three successive years of higher than normal rainfall occurs will a crop of seedlings become established; this happens about every 40 to 50 years, judged by the age structure of the trees. Then competition for water between the seedlings ensues, and some may die (Slatyer, 1975). The population of Acacia aneura thus consists of widely spaced year-classes, produced by occasional escapes during rare periods of mild weather. In aquatic habitats. Near the upper margin of their distribution in the inter- tidal zone, marine organisms are exposed for long periods to extreme and variable weather; young colonists are usually killed by the harsh physical conditions. Evidence for this comes from various field experi- ments in which the conditions were im- proved at high levels. For example, Hatton (1938), Frank (1965), and Dayton (1971) arranged streams of sea water above the intertidal zone; algae and bar- 474 nacles survived much higher than usual in these streams. Barnacles survived better under shades set up by Hatton (1938) on the upper shore. Conversely, barnacles transplanted to higher levels quickly died (Hatton, 1938; Foster, 1971); the smaller barnacles died before the larger ones. In general, younger or smaller individuals are more vulnerable to harsh physical conditions. This is probably a conse- quence of the greater surface-to-volume ratio of smaller individuals; they are rela- tively more exposed than larger ones to such hazards of the ex.ternal environment as desiccation, increased radiation, ex- treme temperatures, fresh water, etc. (Lewis, 1964). Weather being variable, there will occur periods when the harsh conditions are temporarily ameliorated. If favorable conditions last long enough, the young colonists may reach a size where they can survive the usual harsh weather. Many marine species at higher latitudes have short seasons of breeding and settlement each year, so that such "escapes" may happen only once every few years. For example, out of four year-classes of the barnacle Balanus balanoides that I ob- served at high shore levels, survival was high in only one, so that the population was composed mainly of the survivors of that year-class (Connell, 1961a, Figure 12). Other examples of populations domi- nated by older year-classes at high shore levels are three species of barnacles stud- ied by Foster (1971) and a limpet, Acmaea sea bra, studied by Sutherland (1970). An interesting example for marine algae is given by Kain (1963). The physi- cal conditions in the sublittoral zone de- 16 Producing Structure in Natural Communities teriorate as the light intensity diminishes with depth. Atthe lower limits of distribu- tion of two populations of the large sea- weed Laminaria hyperborea, the growth rate and population density diminished, and the age distribution consisted of dominant year-groups. In contrast, the shallower populations showed little evi- dence of such dominant age groups. Re- cruitment evidently happens inter- mittently in the populations near the lower edge of the range. Kain (1963) felt that recruitment occurred only occa- sionally when the conditions become tem- porarily favorable, but there is no direct evidence in support of this suggestion. What Determines When Competition Will Occur? The evidence from controlled field ex- periments on the occurrence of competi- tion and on the instances when it is pre- vented by predation or harsh weather has now been reviewed. It seems clear that competition is often prevented by preda- tion, less often by harsh physical condi- tions. In fact, so many instances have been demonstrated by controlled field experi- ments, in contrast to being simply sug- gested by correlations, that I suggest the following priority be followed in adopting simplifying assumptions to use in models of community structure. Predation should be regarded as being of primary importance, either directly determining the species composition or in preventing competitive exclusion, except where the effect of predation is reduced for some reason. There seem to be two , 1 475 principal situations in which predation is reduced, both the result of evolution of defensive adaptations by the prey. 1. Some prey species have evolved the ability to live in refuges that the predator cannot invade, either because the condi- tions are too harsh for the predator or the habitat structure too difficult to search. Outside the refuges the prey are eaten, e.g., Balanus glandula on the middle and lower seashore or larger zooplankton in open waters. The highest levels on the shore provide a refuge where the preda- tors cannot drill and consume a barnacle during the short period at high tide. In the dense vegetation of ponds the fish cannot effectively search for zooplankton. There is a great difference in the rela- tive abundance and species composition of the zooplankton or benthic inverte- brates in lakes with or without large pred- ators such as fish or amphibians. Large species of zooplankters or benthic inverte- brates are eliminated where these large predators are present. These larger inver- tebrates must have evolved in lakes with- out fish, and depend for their existence on the fact that these larger vertebrate pred- ators have low rates of movement between lakes and so have not reached many small lakes. Only in recent years have they done so, thanks to the stocking by government fishery departments. This category of species protected by refuges includes many prey species that are smaller than their predators. They may exist as "fugitive" species, invading isolated patches of habitat such as lakes or islands before their preda tors, which have lower powers of dispersal. Alterna- tively, they live in the same habitat but \! -----~