TREE vol. 1, no. I, July 7986 could be expected to rise periodically some other nuclear power stations 6 Bumazyan, A./. f1975)At. Energi. 39, (particularly during the spring snow are beinq built much nearer to these 167-172 melting) for many years. cities than the Chernobyl station to 7 Mednik, LG., Tikhomirov, F.A., It is very likely that the Soviet Kiev. The Chernobvl disaster will Prokhorov, V.M. and Karaban, P.T. (1981) Ekologiya, no. 1,4C-45 Union, after some initial reluctance, affect future plans, and will certainly 8 Molchanova, I.V., and Karavaeva, E.N. will eventually adopt the same atti- make a serious impact on the nuclear (1981) Ekologiya, no. 5,86-88 tude towards nuclear power stations generating strategy in many other 9 Molchanova, I.V., Karavaeva, E.N., as was adopted by the United States countries as well. Chebotina. M.Ya., and Kulikov, N.V. after the long public enquiry into the (1982) Ekologiya, no. 2.4-g accident at Three Mile Island in 1979. 10 Buyanov, N.I. (1981) Ekologiya, no. 3, As well as ending the propaganda of Acknowledgements 66-70 the almost absolute safety of nuclear The author is grateful to Geoffrey R. 11 Nifontova, M.G., and Kulikov, N.V. Banks for reading the manuscript and for power plants and labelling them as (1981) Ekologiya, no. 6,94-96 comments and editorial assistance. 12 Kulikov, N.V. 11981) Ekologiya, no. 4, ‘potentially dangerous’, the US gov- 5-11 ernment also recommended that 13 Vennikov, V.A. (1975) In new nuclear power plants be located References Methodological Aspects of Study of in areas remote from concentrations 1 Medvedev, Z.A. (1979) Nuclear Disaster Biosphere(in Russian), pp. 55-71, of population. The official Soviet in the Urals, W.W. Norton Moscow policy until the Chernobyl disaster 2 Trabalka, J.R.,Eyman, L.D.,and 14 Polushkin, K. (1980) Nauka iilhizn, no. did not take the Three Mile Island Auerbach, S.I. (1980) Science, 209,345 11.44-52 lesson into consideration. The long 353 15 Devel, L., Tovedal, H., Bergstrom, U., 3 Soran, D.M., and Stillman, D.B. (1982) Appelgren, A., Chyssler, J. and term planning continued to locate An Analysis of the Alleged Kyshtym Andersson, L. (1986) Nature, 321, nuclear energy plants of different Disaster, Los Alamos National Laboratory 192-193 types (including the fast breeders) Report LA-9217-MS 16 Beardslev. T. 119861 Nafure. 321. 187 near large cities in order to heat 4 Die Presse, March 18,1959. Vienna 17 Aleksakhin, RIM., and Naryshkin, M.A. towns centrally with hot water. 5 Gus’kova, A.K. (1967) Med. Rad. 7 7, (1977) Migration of Radionuclides in Voronezh, Gorky, Leningrad and 53-64 ForestBiocoenosis (in Russian), Nauka Since Darwin accepted the Malthu- ThePresent Status of the Competitive Sian population theory to solve the demographic problems he thought to be logically connected with the ExclusionPrinciple universal operation of natural selec- tion, the numerical processes in both Pieter J. den Boer populations and communities were generally supposed to be governed stocked with inhabitants; and it each takes possession of certain by competition. For interspecific re- follows from this, that as the peculiar kinds of food and modes of lations this found expression in the favoured forms increase in number, fife in which it has an advantage over ‘competitive exclusion principle‘. so, generally, will the less favoured its competitor”. The ‘niche’ of a After it was shown that coexistence decrease and become rare”. Darwin species is thought to consist of the rather than exclusion of closely re- thought that this generally occurred essential resources of the species lated species is the rule, this prin- by competition, not only between inclusive of conditions of time and ciple gradually changed into the varieties of the same species, but space as well as the strategy of life ‘competitive niche shift principle’. also between species: ‘We have which enable it sufficiently to make Recently the universality of competi- reason to believe that species in a use of these resources. Therefore, tion has been increasingly ques- state of nature are limited in their ‘competition’ is that kind of inter- tioned, so that other interspecific ranges by competition of other orga- action between individuals of two or relationships (especially predation) nic beings quite as much as, or more more species, by which at least one are revaluated as possibly governing than, by adaptation to particular of the species is kept from sufficient many natural population and inter- climates”. Especially closely related ly using its essential resources. population processes. species would compete severely: ‘As In the 1920s the principle of com- species of the same genus have petition was formalized in the Lotka- By considering natural selection as usually, though by no means invari- Volterra equations. Given the highly the driving force of evolution Darwin ably, some similarity in habits and restricting assumptions of these created a new problem: to be natur- constitution, and always in structure, models, which state that populations ally selected implies a better chance the struggle will generally be more of identical individuals of two spe- of leaving descendants, and if con- severe between species of the same cies are growing together in a closed ditions remain favourable numbers genus, when they come into com- and homogeneous environment are expected to increase. In his own petition with each other, than be- with constant physico-chemical words: ‘Owing to the high geometri- tween species of distinct genera”. properties, the principle is tautolo- cal rate of increase of all organic With this conclusion of Darwin the gical: the conclusions are implicit in beings, each area is already fully ‘competitive exclusion principle’ was the assumptions3. Laboratory ex- born, though it is generally referred oeriments in the 1930s and 194Os, to as Gause’s principle: ‘It is admit- usually with genetically homo- Pieter den Boer is at the Biological Station, Wijster (Agricultural University Wageningenb ted that as a result of competition geneous stocks of Tribolium or Dro- Kampsweg 27 9418 PD Wijster. The Nether- two similar species scarcely ever sophila species, aimed at satisfying lands. This article is Communication no. 309 of occupy similar niches, but displace as closely as possible the premises the Biological Station at Wijster. each other in such a manner that of the competition equations. They 25 TREE vol. 7, no. 1, July 1986 Fig. 1. Two species of Crateroscelis individual records relative abundance warblers on Mt Karimui in New summit Guinea segregate in their niches by 4oOG altitude, replacing each other . abruptlv at 1543 m. Each circle rep- . resents one observation of C. rob&- ta ffilled circles) or C. murina Looen ing the species evolved separately, . circlesl. (Paucity of svmbols~ be- . and thus became adapted to dif- tween 650 and 1050 m is because ferent sets of conditions, or because little time wes spent there.) On the they coevolved under pressure of right, these observations are re- \ expressed es the percentage each competition, and thus diverged in warbler species contributes to the resource utilization’2,‘3. The latter total number of bird individuals of phenomenon, which was called all species at each altitude. (Based ‘character divergence’ by Darwin, on Ref. 9, with permission of the American Association for the but is better known as ‘character Advancement of Science.) displacement”4, and is also indi- cated as ‘niche shifVg or ‘niche seg- regation’, is at present one of the most controversial aspects of com- munity ecology15. After the war, ecologists had to 1 find their way between axiomatic theorizing and multi-interpretable field observations. As the gap be- tween these poles is wide, and hard- _L III II I ly bridgeable, explorations could be 0 2 4 6 8 10 12 undertaken in many different direc- 91;of all bird individuals tions. Several authors tried to adapt their conviction of the universality of showed that the logic of the models generic species replace each other competition to the growing collec- is indeed irrefutable, but also that the abruptly over a short distance, e.g. tion of apparently conflicting field assumptions are so severe that even Fig. 1 (Ref. 9). But even observations observations, especially those con- small deviations from constant or of this kind (which cannot often be cerning coexistence of supposed homogeneous conditions might give made) are open for other explana- competitors: fundamental niche”j, unpredicted results4. Doubts grew tions, such as food preferences, competition theory17, and niche about the power of the principle to requirements for nesting, or highly theory’8-21. Others tried to tackle explain distributional patterns of specific physico-chemical toler- these questions more directly by closely related species living under ances. The same can be said when performing elegant field experi- natural conditions, ending in 1971- the invasion of some species into a ments22f23. Although the possibilities 72 in a discussion in Nature (cited in new area coincides with, or is fol- for such experiments are restricted, Ref. 5). The principle mainly survived lowed by, retraction of distribution it has at least been shown that the as the widespread conviction that or at least a striking decrease in num- coexistence of sessile organisms natural selection should necessarily bers of related species’,‘. Such may be significantly favoured by pre- be ‘competitive selection’6,7. observations indeed suggest com- dation, including the feeding of herb- During most of this century it has petitive interactions, but other ex- ivores on plants22,24. It was con- been common to consider that the planations cannot be excluded’O,“. cluded that predation may prevent occupation of separate habitats by Where congeneric species are found potential competitors from exceed- closely related species is due to com- to coexist it is generally supposed ing the carrying capacity of the en- petitive exclusions.