Laserfiche WebLink
<br />Joseph H. Connell <br /> <br />left that they still search the bottom com- <br />pletely with their feeding activities and eat <br />all the eggs of their own species that are <br />laid in early summer, thus completely <br />suppressing the next generation. Only <br />after the two-year-olds emerge can a new <br />generation of eggs start to develop in the <br />hypolimnion. In shallow water, growth <br />and development are faster, all emerge <br />at the end of one year, and space is left <br />for a new generation each year. Thus <br />predation thins out the population in <br />shallow depths, but in deeper waters harsh <br />physical conditions (anoxia) exclude pred- <br />ators for a greater proportion of the year. <br />The prey apparently survive these harsh <br />periods so that, with less predation, popu- <br />lation density is greater, as is competition <br />for feeding space. <br />The studies described above have dem- <br />onstrated the dominant role of predation <br />in determining the relative abundance <br />and distribution of species in fresh-water <br />lakes. The community structure is com- <br />pletely different, depending upon whether <br />fish happen to be present or not. If not, <br />the presence or absence of amphibian or <br />invertebrate predators produces a differ- <br />ent community structure. Competition <br />may occur if the intensity of predation is <br />so low that high population densities of <br />herbivores develop. <br />In the sea. On rocky shores in temper- <br />ate zones, grazers and predators often <br />keep their prey populations so low that <br />competition is prevented. The evidence <br />for this comes from experiments in which <br />grazers or predators were removed. The <br />classic experiment of Jones (1948), who <br />removed 15,000 limpets from a strip of <br />rocky shore, demonstrated the great effect <br /> <br />472 <br /> <br />of these herbivores on abundance of <br />algae. The algae quickly colonized and <br />covered the surface for several years. <br />Others have repeated the experiment <br />elsewhere with similar results (Southward, <br />1953, 1964; CastenhoIz, 1961; Haven, <br />1966, 1973; Dayton, 1970). The same <br />effect of grazing by sea urchins on algae <br />in tide pools or rocks below the intertidal <br />region has been demonstrated by experi- <br />mental removal (Kitching and Ebling, <br />1967; Jones and Kain, 1967; Paine and <br />Vadas, 1969; Dayton, 1970). Lastly, by <br />excluding herbivorous fish from intertidal <br />and subtidal areas on coral reefs, Stephen. <br />son and Searles (1960) and Randall (196 I) <br />have found that they too keep algae and <br />sea grasses grazed down. Unplanned ex- <br />periments on a larger scale have shown <br />the same effect. Grazers were killed by a <br />spill of fuel oil from a tanker wreck <br />(North, Neushal, and Clendenning, 1964), <br />and by detergents used to clean shores of <br />crude oil (Smith, 1968). Algae colonized <br />and grew profusely after the grazers were <br />gone. <br />On the middle and lower shore levels <br />experimental removal of predators has <br />shown that they also often eliminate ses- <br />sile animals before these reach maturity. <br />All barnacles were eaten by predatory <br />snails within 18 months in Scotland <br />(Connell, 1961a) or 12 to 15 months in <br />Washington (Connell, 1970; Dayton, <br />1971) and New Zealand (Luckens, 1970), <br />except where predators were excluded by <br />cages. Mussels survived only in cages <br />(Connell, unpublished; Dayton, 1971) or <br />when the predatory starfish were picked <br />off by hand (Paine, 1966, 1971). <br />Yet despite this very heavy grazing and <br /> <br /> <br />16 Producing Structure in Natural <br />Communities <br /> <br />predation, some plants and sessile or sed- <br />entary animals survive to maturity and <br />live a long time at low shore levels. The <br />clue to how this happens is in the age <br />structure of these populations: they often <br />consist of widely spaced older year-classes. <br />I have studied in detail the population <br />dynamics of one such species, the barnacle <br />Balanus cariosus on San Juan Island, <br />Washington (Connell, in preparation). At <br />the start the population consisted of <br />classes aged 2, 4, and at least 10 years. <br />Over the next I3 years, snail predators ate <br />. all the young that arrived every year at <br />two different study sites, with three excep- <br />tions: once at one site and once at each <br />of tWO different shore levels in different <br />years at the other site. In these instances <br />some individuals survived to the age of 2 <br />years, at which time they were invulner- <br />able to all the common predators except <br />the very large starfish Pisaster ochracells. <br />I tested the invulnerability to smaller <br />predators by protecting the barnacles in <br />cages for varying lengths of time and then <br />removing the cages or allowing predators <br />to enter. All B. cariosus younger than 2 <br />years were quickly eaten, whereas only <br />seldom were older ones eaten. Dayton <br />(1971) has since confirmed these findings. <br />As described earlier I have found that, <br />under natural conditions, B. cariosus sur- <br />vives to this invulnerable age only occa- <br />sionally, so that the populations consist of <br />"dominant year classes." This age struc- <br />ture is produced by occasional escapes <br />from the intense predation to which they <br />are usually subjected. <br />Other organisms may also reach a size <br />at which they are invulnerable to preda- <br />tors. Kitching, Sloane, and Ebling (1959) <br /> <br />473 <br /> <br /> <br />found that the mussel Mytilus edulis be- <br />came invulnerable to attack by crabs. <br />Only one species of very large crab, Can- <br />cer pagurus, could break open the larger <br />mussels available. This case is similar to <br />that of B. cariosus; the prey can reach a <br />size invulnerable to the smaller species of <br />predators, but there exists a very large <br />species of predator to which it never is <br />invulnerable. Dayton (1971) suggests that <br />Mylilus californian us may reach a size at <br />which it becomes invulnerable to Thais; <br />although this is probable, no evidence <br />exists, since the predators ate all sizes <br />offered in the experiments. <br />Some species of intertidal algae may be- <br />come invulnerable to grazers. Southward <br />(1964, Table 2) found that the alga Fucus <br />vesiculosus, which colonized a shore after <br />limpets, Patella vulgata, were removed, <br />maintained a complete cover for the next <br />2 years. During this period many small <br />limpets colonized the rock beneath the <br />canopy of large algae. After 4% years the <br />limpets were much larger and the algal <br />cover had been reduced to 22%. The <br />smaller limpets evidently did not attack <br />the large algae, feeding instead on the <br />smaller plants that colonized beneath the <br />canopy. When they grew larger, they ate <br />the large algae. It seems reasonable to <br />draw the conclusion that large algae are <br />invulnerable to small grazers but not to <br />large ones. <br />Grazing and predation are often ex- <br />tremely intense under the more "benign" <br />conditions of the lower shore, and "es- <br />capes" to invulnerable size occur only <br />rarely. The only data as to the frequency <br />of such escapes in natural conditions are <br />those I have given for Balanus cariosus. <br /> <br /> <br /> <br />