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Joseph H. Connell probably the first demonstration of this: "If turf which has long been mown, and the case would be the same with turf closely browsed by quadrupeds, be let to grow, the more vigorous plants gradually kill the less vigorous, though fully grown plants; thus out of twenty species growing on a little plot of mown turf (three feet by four) nine species perished, from the other species being allowed to grow up freely." Hutchinson (1961), Paine (1966), and Connell (1971) have suggested the same thing, and MacArthur (1972) states: "If abundant predators prevent any spe- cies from becoming common, the entire picture changes. Resources are no longer of any concern and our Eqs. (I) and (2) are irrelevant. More correctly, resources are still a concern, but their manner of subdivision is irrelevant." The question I would like to try to answer is, when does predation, or any factor other than com- petition, limit the distribution or abun- dance of a species and so affect its role in cOmmunity structure? Under which conditions does competition determine the shape and size of the realized niche and which not? A Note on Methods I will first consider the problem of how to detect and measure the extent of bio- logical interactions such as competition or predation. In my opinion this has been the main stumbling block in testing the valid- ity of models of community structure. There seem to have been at least three general methods used to detect or measure biological interactions under natural con- 462 ditions. The first is to describe the pattern that exists at a point in time to see whether it does or does not conform to the predictions of the model. Some examples of this are gradient analysis (Whittaker, 1967, 1970; Terborgh, 1971), fitting re]a- tive abundances to various mathematical models (MacArthur, 1960; Whittaker, ]965; Levins, 1968), and examining the distribution patterns, food habits, etc., of closely related species to see how much their niches overlap. This method is very useful in detecting patterns and suggesting hypotheses for testing. The second approach is an attempt to apply the experimental method but with- out using controls. It consists of searching for a "natural experiment." For example, if two similar species occupy the upper and lower halves of a mountain, rt"spec- tively, with little or no overlap, one hy- pothesis is that they exclude each pther by competition for resources. To test this hypothesis, one looks for a nearby moun- tain where one or the other is missing (e.g., Diamond, Chapter 14, Figure 39). If competition determines the boundary, the species present should extend beyond the boundary on this second mountain. The trouble with such "natural experiments" is that an essential part of all true experi- ments is missing: a control. There is no certainty that the only difference between the two mountains is the absence ,)f one of the species. A predator may have been absent, or an essential food organ;sm, soil nutrient, etc., may have been present be- yond the boundary on the second moun- tain. 16 Producing Structure in Natural Communities The third and, in my opinion the best, method for revealing the extent of biolog- ical interactions is the controlled field ex- periment, whose essential aspect is that everything varies in the same way be- tween treatment and control except for the factor being tested (see review by Connell, 1974). In the example above, instead of looking for a mountain with one species missing, one finds another with both spe- cies in the same arrangement and then removes one species. The most efficient thing would, be to remove it near the boundary; if the other species extends its range into the vacant territory, competi- tion is the most likely mechanism pro- ducing the original boundary. If not, the hypothesis can be rejected forthwith. Such field experiments need careful desion. The trick is to make sure that all envi~onmental factors, except the one being tested, vary in the same way and to the same degree on both experimental and control sites. Since there are bound to be some differences, replication is es- sential. Often it is more difficult to arrange adequate control sites than to perform the treatments. The effect of the experimental treat- ment itself must be taken into account. The simplest treatment, removal of one or more species, is probably the best. Alter- natively, animals are sometimes intro- duced into pens or cages (Jaeger, 197 I; Wilbur, 1972; Grant, 1972). This may cause problems. First, if emigration is the usual result of aggressive encounters, this will be prevented and unusual outcomes may ensue (Krebs, Keller, and Tamarin, 463 1969). Secondly, other species of competi- tors or predators, which would normally have influenced the outcome, may be ex- cluded. If enclosures must be used, these and other such effects need to be taken into account in the final interpretation of the experimental results. The superiority of such controlled ex- periments to "natural" ones seems obvi- ous, yet field experiments are rare in all ecological literature. One reason is that it is very difficult to change the abundance of large or highly mobile animals such as birds or large mammals. Also, tradition dictates that experiments are done in lab- oratories, whereas ecology is done out- doors. Field trials are relegated to agricul- ture or forestry, and ecology students seldom take courses in these areas. But applied scientists need answers, and ne- cessity stimulated the development both of statistical techniques and of field ex- periments. Most ecologists have adopted the former but not the latter, to their cost. In the following section I present a re- view of the evidence of where competition occurs in natural communities and where it is prevented by predation or physical conditions. The aim of this review is to see whether there is any pattern in the occur- rence of competition. I have used evidence from controlled field experiments when- ever possible, both to illustrate their value and to suggest where they could be used in the future. Although the number of instances where field experiments have been used are few, they differentiate be- tween the alternatives clearly in a way that correlations between abundances,