Exclusion Or Coexistence and the Taxonomic Or Ecological Relationship Between Species

EXCLUSION OR COEXISTENCE AND THE TAXONOMIC OR ECOLOGICAL RELATIONSHIP BETWEEN SPECIES

P. J. DEN BOER (Biological Station of the Agricultural Universityof Wageningen,Wijster, The Netherlands)

SUMMARY

The history of the "principle of competitive exclusion" is briefly reviewed. Next, it is shown that taxonomically closely related (carabid) species, i.e. species in the same genus, can indeed be considered to be also ecologically closely related. This opened the possibility to test whether or not exclusion plays a demonstrable role in the distribution of species belonging to the same or to different genera over different habitats. The method used was proposed by SIMPSON(1949) and applied for the first time by WILLIAMS(1951). With the help of the pitfall-catches of 149 carabid species during nine years and in 73 different habitats, and by using three, different "values" it could thus be shown that congeneric species coexist more frequently in the same habitats than could be expected from a random distribution of the available species over habitats. This "coexistence principle" was further illustrated by two examples from habitats studied during a number of years. It could also be shown that interaction groups of coexisting congeneric (carabid) species do not die out more frequently than those of species belonging to different genera, not even in the course of a century. These findings are thus in agreement with the conclusion of BIRCH(1979) from a review of the field evidence, that "competitive exclusion" must be considered an only exceptional outcome of the possible interactions between species. The most parsimonious hypothesis for understanding the findings discussed in this paper therefore is: Taxonomically closely related (carabid) species are also ecologically closely related, and will thus more often than not be found coexisting in the same habitats.

1. INTRODUCTION

During many years already I have been puzzled by the thoughts of ecologists who have apparently access to knowledge about "laws of nature" which seems not to be obtained from a comparative study of a large number of populations. To give an example, NICHOLSON ( 1954) deduced, that populations must be "regulated by density-governing factors", which will keep population density "in a state of tending towards a position of equilibrium" (i.e. "a state of balance"). This seems to be more than a hypothesis that can be tested and thus possibly

(communication no. 214 of the Biological Station, Wijster) 279

rejected (see discussion following REDDINGIUS, 1971b), for NICHOLSON (1933, p. 133), also stated: "populations must exist in a state of balance for they are otherwise inexplicable" (see also: REDDINGIUS, 1971a, p. 12). Of course, with "for they are otherwise inexplicable", NICHOL- SON wants to say: "for otherwise they would not be in keeping with my presuppositions". A predominant one among these presuppositions seems to be the expectation that population density will persist between finite and positive bounds, or in the words of NICHOLSON ( 1958) : in the centre of distribution "density-governing reaction permits a species to persist indefinitely in all favourable places". REDDINGIUS ( 1971 a) convincingly showed that under not too unrealistic conditions (i.e. in a not completely constant and homogeneous environment) "density governing reactions" cannot even theoretically be expected to accomplish this job. In addition to this DEN BOER (1979) showed that also in nature populations-taken as "interaction-groups" (see also BAKKER, 1971)- do not persist, but die out and are (re)founded more or less frequently. See also: DIAMOND ( 1979) . A "law of nature", arisen in a comparable way by deduction from a presupposition, is "the principle of competitive exclusion". Although for a considerable time already I have data at my disposal that indicate that under more or less natural conditions related carabid species as a rule do not exclude each other, either because they do not compete or because other "factors" are apparently more important than a possible interference, up to now I was not highly motivated to devote a paper to this subject, because "the principle" is gradually disappearing now from the ecological literature. After reading the review of the present field evidence on exclusion and coexistence by BIRCH (1979), however, I thought it useful to support his conclusions and considerations by presenting another type of data, at the same time recalling attention to some papers of WILLIAMS (1947, 1951) which appear to have been generally overlooked by field ecologists.

2. THE EXCLUSION PRINCIPLE

2.1. History In fact, the exclusion principle was already given the status of a "law of nature" by DARWIN (1859, 6th ed. 1888), who clearly was of the opinion, that species with similar needs cannot coexist; the competi- tively inferior species will be eliminated by the superior one (s) . GRINNELL (1904) for the first time connected this principle with the concept of "niche". He wrote: "Two species of approximately the same food habits are not likely to remain long evenly balanced in numbers in 280 the same region", and drew the conclusion: "It is, of course, axiomatic that no two species regularly established in a single fauna have precisely the same niche relationships". VOLTERRA and LOTKA gave the principle a mathematical base (see: ScuDO & ZIEGLER, 1978) by modelling exclusion as the outcome of competition for a resource in short supply under fairly severe, limiting conditions. GAUSE ( 1934) showed that exclusion may indeed occur in model experiments, in which the conditions underlying the Lotka- Volterra-equations are approximated as closely as possible. A symposium of the British Ecological Society (ANONYMOUS, 1944), where the question was considered: "Can two species with similar ecology live together in the same place?", was the start of a considerable number of publications on this subject. Most of these papers dealt with competition experiments carried out in the laboratory under constant conditions (for a review see e.g.: AYALA, 1970). By historically interpreting special cases of the geographical distribution of taxonomically related species (mainly birds) some other authors tried either to support the "principle of competitive exclusion" (e.g. LACK, 1946, 1971), or to question its generality (e.g. MAYR, 1942, 1963). Field experiments and field observations with the aim to actually test this principle were mainly started more recently (for a review see: BIRCH, 1979). Only a few ecologists severely critized the principle (e.g. ANDREWAR- THA & BIRCH, 1954; COLE, 1960). They think it to be either trivial or invalid, depending on how it is formulated. If it is formulated in the sense OfSLOBODKIN (1961, p. 122): "If two species persist in a particular region it can be taken as axiomatic that some ecological distinction must exist between them", the principle is true, but trivial. If it is re- versed in the sense, that two species with identical ecological requirements cannot coexist, it is invalid, since two species, whether coexisting or not, will not have exactly the same requirements. The latter will not even be true for two individuals from a bisexual population (see further: AYALA, 1970).

2.2. Laboratory tests The explanatory value of a theory or hypothesis will directly depend, of course, on the number and admissibility of the underlying assumptions, To study these assumptions we start from one of the more strin- gent formulations of the principle (see e.g. ELTON, 1927) in which it is said that two species cannot coexist if they share one or more resources essential for the survival of the species. Frequently, but not always ex- plicitly, the shared resource (s) is (are) also required to be in short supply for the greater part of time. In the mathematical formalizations of LOTKA & VOLTERRA (see ScuDO & ZIEGLER, 1978) this is indeed sup- 281 posed, whereas the biologically most unrealistic assumptions concern the environment common to the competing species: it should be homogeneous and constant in all respects, and it must be "closed" (i.e. no immi- and emigration). In many relevant laboratory experiments in which these assumptions were satisfied as good as possible these indeed appeared to be sufficient conditions to bring about the extermination of one of the species (e.g. GAUSE, 1934; CROMBIE, 1945; PARK, 1948, 1954; MER- RELL, 1951; BIRCH, 1953), though some of these exclusions took a remarkably high number of generations (e.g. PARK, BIRCH). However, many of these experiments also showed the above conditions to be necessary ones: a small change in pH (GAUSE), in temperature and/or moisture conditions (PARK, BIRCH) may be sufficient to transfer the advantage from one species to the other. A periodical fluctuation in the quality of the food may give two Drosophila-species the opportunity to coexist permanently (MERRELL). Coexistence can also result from a small difference in "life tactics" between the species concerned, i.e. Rhizopertha-Oryzaephilus (CROMBIE) ; Paramecium caudatum (or aurelia)- P. bursaria (GAUSE), and/or from the introduction of small heterogeneities in the environment, e.g. small glass tubes in the culture medium of Tribolium-Oryzaephilus (CROMBIE). For more details, see: ANDREWARTHA & BIRCH (1954, p. 421-433). A more recent example of this kind was even presented as "experimental invalidation of the principle of competitive exclusion" (AYALA, 1969). A discussion in Nature led to the statement, that the (small) fluctuations in temperature permitted to occur in the incubators "will have exerted large selective pressures, by which a state of disequilibrium was maintained" (Bo- ROWSKY, 1971). This conclusion was refuted again by AYALA ( 1971 ), but the fact remains, that his Drosophila-species coexisted under experimental conditions that only slightly deviated from those necessarily dictated by the generally favoured (simple) competition model (or by a non-linear version of it: GILPIN & JUSTICE, 1972).

2.3. Exclusion in the field Hence, even under biologically fairly unrealistic conditions (in a closed and nearly constant and/or nearly homogeneous environment) the exclusion principle seems not to be a "law of nature". For the field ecologist, who is more inclined to trust the reality of natural phenomena than the assumptions underlying mathematical deductions, thus the question arises: "What will be more general in the everchanging and heterogeneous natural environment, exclusion or coexistence of ecologically related species? Such questions can only be answered, of course, when we leave our writing desks, incubators and computer 282 terminals, and go out into the field. Fortunately, many biologists did so already, so that gradually relevant field evidence is accumulating, which could thus be reviewed by BIRCH (1979). This review clearly shows two points: (1). It is very difficult to actually demonstrate the occurrence of exclusion, whereas inferring exclusion usually leaves open the possibility to give other reasonable explanations. (2). As far as data are available about the numerical effects of interactions between species that share resources, these clearly show that complete exclusion can only be considered to be a not very common, extreme outcome, and not a "law of nature". The majority of reliable cases of exclusion under field conditions appear to belong to a few categories: (1) direct aggression or even killing of one species by the other, as e.g. among certain ants; (2) interspecies territoriality, as in some groups of Pomacentrid fishes and among hummingbirds; (3) animals living in a closed environment, some good examples of which are found among worms inhabiting intestines and among insect larvae living in other insects, carrion or in fruits; (4) sedentarily living animals, such as barnacles, limpets, anemones, and the like. See further: BIRCH (1979); see also ZWBLFER (1979). The latter category closely links up with plants, where reliable cases of direct exclusion will be more common than among animals. In this connection must be pointed to a promising approach (also for zoolo- gists) to study competitive phenomena, that was introduced by DE WIT (e.g. 1960, 1971; DE WIT & GOUDRIAAN, 1974): the processes running in monocultures in the field, which again result in a certain yield, are simulated with the computer, and thus used to predict the yields in cultures of ecologically related species mixed in different ratios. In this way not only many cases of exclusion (replacement) in mixed field cultures could be predicted, but also cases in which replacement could not be expected. In fact, BAKKER (1964) already worked along the same lines when he analysed the competition between the larvae of two mutants of Drosophila melanogaster; these processes were modelled by DE JONG (1976). In all instances where exclusion could be shown to occur under field conditions the excluding species somehow made essential resources unavailable for the excluded species (BIRCH, 1979). Although this can thus be considered a necessary condition for exclusion to occur, it is not always a sufficient condition. Before the exclusion is completed many other factors may increase again the availability of essential resources, especially in the field (but also in model experiments, cf. section 2.2), such as changing and/or fluctuating physical conditions, 283 possibilities to escape, heterogeneities in the environment by which in different places the relevant processes do not run parallel, the actions of predators and parasites, etc. Especially the latter "factors", by restricting numbers, may be very powerful in preventing some resource from becoming irreversibly exhausted, not only for other animals (see e.g. CONNELL, 1971, 1975), but also for plants by the actions of phyto- phages (see e.g. HARPER, 1970).

2.4. How to test the significance of competitive exclusion Although the interests are gradually shifting from "competitive exclusion" towards the conditions for coexistence (see: CODY & DIAMOND (ed.), 1975) many ecologists will not conclude from the preceding sections that at least among non-territorial, free-moving animals, that do not inhabit a small, closed environment in some stage of the life- cycle, competitive exclusion must be a phenomenon of only minor importance. That is because assumptions about exclusion as well as thoughts about coexistence are generally deduced from the same presupposition: competition is the major or even the principal process that determines the distribution of species (compare CONNELL, 1975, who protests against this view). At present this conviction is stimulated again by recent theoretical developments on the evolution of communities (CODY & DIAMOND (ed.), 1975), which are largely based again upon the equilibrium models of LOTKA & VOLTERRA (cf. 2.1). Instead of continuing the construction of more and more sophisticated models on this same base and thus piling up assumptions, we should try to investigate the explanatory value of this base itself. Just as REDDINGIUS ( 1971 a) tried to replace the dogma of "the regulation of population density by density governing factors" (cf. 1) by statistical methods for testing hypotheses on the occurrence of density-dependence in field populations, we should try to replace the dogma of competition as the major process determining the distribution of species by statistical methods for testing hypotheses on the occurrence of possible effects of competition on the distribution of species. In this paper we will try to give a modest start in this direction. For the moment our approach will only be a qualitative one, a more quantitative approach being possible as soon as we have solved the problem of how to compare and evaluate reliably the numbers in populations of different (carabid) species: how many individuals of species X are equivalent to a single specimen of species Y in relation to the relevant process (e.g. consumption of different kinds of preys) ? Because of the above competition-dogma most ecologists will-just as NICHOLSON (1960, p. 501)-still consider competitive exclusion 284 associated with the normal course of evolution, i.e. being the ultimate cause of the extinction of enormous quantities of-supposedly com- petitively inferior-species. Therefore, most of them will also be inclined to assume that the often striking differences between the distributional patterns of closely related species have been highly influenced-if not determined-by competitive relationships, if not in the present than undoubtedly in the past (compare e.g. LACK, 1947; DIAMOND, 1979). It is the merit Of WILLIAMS ( 1947, 1951 ) to have realized that the above view is open to statistical testing i.e. the actual distribution of related species can be compared with those that would be expected under the hypothesis of a major influence of competitive exclusion. If we assume that species in the same genus will usually be more closely related ecologically than species in different genera, and also that competition between closely related species will generally be more severe than between unrelated species, a relevant null hypothesis will be: among closely related species (species in the same genus) distributional patterns will not differ more than among comparable less related species (species in different genera), either because competition does not influence distribution significantly, or because competition does equally affect the distributional patterns of both closely related and unrelated species. More specifically, under the null hypothesis we will not expect species belonging to different genera to occur more frequently together in the same habitat than congeneric species. Such a null hypothesis can be tested against the alternative hypothesis: Closely related species (congeneric species) will occur more frequently in different habitats than comparable less related species (species in different genera), in which case it seems plausible to suppose a major influence of competitive interactions. WILLIAMS ( 1951 ) tested these hypotheses for 172 species of Passerine birds belonging to 92 genera and 9 families, and inhabiting 32 different types of habitat in the Usambara Hills of north-east Tanganyika (data of MOREAU, 1948), and could thus show that congeneric species are more frequently inhabiting the same habitats than could be expected under the null hypothesis. Hence, this result is just the reverse of the expectation under the "competitive exclusion principle", and may thus point to the possible validity of a "coexistence principle". Although this analysis by WILLIAMS (1951) already significantly supports the general impression from the review by BIRCH ( 1979), it seems useful to check the generality of this statistical result independently on a quite different and less diversified group of animals. Our data on carabid beetles (DEN BOER, 1977) seem to be very suitable for such a test, because it concerns data that are obtained by an objective and standardized technique giving for many species the 285

chances to coincide within many different habitats in a small area, and covering many years. These data will moreover allow us to go somewhat further into questions concerning exclusion and coexistence under field conditions. To make such an analysis meaningful, however, we will first have to test the (reasonable) assumption, that taxonomically closely related species are also ecologically closely related.

3. THE TAXONOMIC AND ECOLOGICAL RELATIONSHIPS BETWEEN CARABID SPECIES

3.1. Available information At present carabid beetles possibly will be one of the biologically best known groups of invertebrates (see LINDROTH, 1945, 1949; THIELE, 1977; DEN BOER, 1977; ERWIN et al. (Ed.), 1979; DEN BOER et al. (ed.), 1979). As in many respects our knowledge is no longer restricted to a few species only we can tentatively start comparative ecological studies on carabid species, to get a first insight (at least within this group) into the degree of generality of ecological phenomena that up to now often were considered "laws of nature". One of these "laws" is: "the ecological relationship between species is sufficiently reflected by the taxonomic relationship".

Nearly all carabid species caught regularly in the surroundings of the Biological Station, Wijster could also be classified according to a few ecological "characters": ( 1 )Annual reproductive rhythm, i.e. reproduction either mainly in spring, or in some other time of the year, usually in autumn; (2) favoured type of habitat, i.e. either "unstable", divided again in unpredictably wet sites, and "other" (usually waste sites, felled forest, and other sites disturbed by man), or "more stable", divided in forest or other places with trees, and "other" (mainly old heath and peat moor, or blown sand fixed by heather; (3) dispersal power, i.e. either capable to bridge rather large distances (flying has been established directly, or individuals are big enough to be able to walk about one kilometer), or the dispersal power is poor (smaller individuals without wings, or wings that are too small to fly, or winged individuals are very rare and flight observations are lacking up to now). Although neither being an ecological nor a taxonomic "character", we added to this "size of the individuals", because for polyphagous predators "size" will highly determine the kind of preys that can be attacked. Moreover, "size" seems to be a most important "character" with the help of which "niche width" and/or "niche breadth" can be estimated (see e.g. MACARTHUR& LEVINS,1967; HESPENHEIDE,1975).

3.2. Results In Table 1 for the 16 genera with 3 or more species occurring in the surroundings of Wijster the distributions of numbers of species over different classes of "ecological characters" are presented. First of all we can test these distributions within genera against the null hypothesis 286 TABLE 1 Some "ecological characters" of carabid species belonging to the same genus. On the 16 genera with 3 or more species occurring at present around Wijster (Drenthe, Tl Netherlands) are considered here (116 species).

* Size groups: I, <5 mm; II, 5-8.5 mm; III, 9-12.5 mm; IV, >12.5 mm. 287 288 that overall they will not deviate from the general distribution for all species together (at the bottom of Table 1 ), i.e. particular genera are generally not characterized by particular "ecological proporties". The results of these tests are shown in Table 2. The conclusions from these tables will be evident: (1) In many genera most species do show the same type of annual reproduction rhythm, and as a whole this phenomenon is highly significant (at the bottom of Table 2).* It can easily be computed that on the average 84% of the species belonging to the same genus show the same kind of annual rhythm, whereas the highest possible expectation is 61 % (bottom of Table 1: 71/116 for spring breeding). Ten out of 16 genera exclusively show reproduction in only one season, either in spring or in autumn (winter), together 50 species; three more genera show reproduction nearly exclusively in one season (respectively: 11 of 12, 10 of 12, or 9 of 10 species). Hence, 13 of our 16 genera (covering 80 of the 116 species) are highly characterized by a particular annual rhythm. More about annual reproduction rhythms in carabid beetles can be found in THIELE (1977) and PAARMANN (1979). (2) In most genera the majority of species show the same "size of the individuals", and this phenomenon deviates very significantly from the expectation under the null hypothesis (Table 2). On the average 77% of the species in the same genus have individuals in the same size-class, whereas the highest expectation for some class is only 39% (45/116 for size-class II). On the average even 96% of the species in the same genus show individuals in the same or in two adjacent size classes (highest possible expectation is 65% for size-classes II + III). Because we have 4 size-classes this is especially informative, of course, for genera with more than 4 species. For these 8 genera on the average 94% of the species have individuals in the same or in two adjacent size-classes (highest expectation 75% for size-classes II + III). (3) On the average 53% of the species in the same genus occur in the same type of habitat (highest expectation is 31% for open, stable habitats); only for the 8 genera with more than 4 species this is 47% against a highest expectation of 33%. On the average 82% of the species in the same genus is restricted to only two types of habitat (highest expectation is 55 % for the two classes of stable habitats) ; only for the 8 genera with more than 4 species this is 78% against a highest expectation of 58% (for man-made sites + open, stable habitats). Although less evident than annual reproductive rhythm and "size of the individuals", also the habitat preference of species belonging to

* The separate Chi-square values given in the body of Table 2 are, of course, the more reliable, the higher the number of species in the genus concerned. 289 TABLE 2 Chi-square values of the deviations from expectation (totals in Table 1) of the distributions of species within genera over the different classes of "ecological characters" (body of Table 1).

* Chi-square value higher than the value for p = 0.05. d.f. = degrees of freedom.

the same genus is highly significantly clustered within one or two classes of habitat (Table 2). (4) We cannot expect much reliable information from the distributions of species within genera over classes of annual reproductive rhythm or over size-classes which are combined again with habitat classes (8 or 16 classes respectively). As far as we did not run too evidently into unallowed statistics we could establish that the above conclusions were confirmed also for combinations of "ecological characters", e.g. highly significant "clustering" of "ecological characters" within large genera such as Amara and Harpalus, and no important "clustering" within the also taxonomically very heterogeneous genus Pterostichus. 290

(5) The distributions of species within genera over the two classes of dispersal power do not significantly deviate from the expectation that about 75% of the species in a genus will possess reasonable powers of dispersal (Table 2). This unexpected "behaviour" of dispersal power results from the established fact that under the present conditions in our cultivated countryside both wing size and the ability to develop flight muscles are subject to an intensive, negative selective pressure in many species (DEN BOER et al., 1980). This means that in "modern" genera such as Agonum, Amara, Harpalus, Pterostichus most species with a poor dispersal power will have obtained this property only recently. (6) Other "ecological characters" are less completely known for all species to allow a similar analysis, but our knowledge is already sufficient to conclude that also many other "ecological characters" are highly "clustered" within certain genera: last column of Table 1. 3.3. Conclusion The general conclusion from section 3.2 can be: Among carabid beefles the relationship between taxonomic and ecological features appears to be sufficiently close to confidently handle the rule: taxonomically closely related carabid species are also ecologically closely related. This conclusion is evidently not restricted to the 16 genera with 116 species tested in 3.2, since in many carabid genera all species are characterized by particular ecological features. To give some examples: Species of Abax and Molops show particular forms of brood care behaviour that are unknown from other carabids (BRAND- MAYR et al., 1979) ; Cychrus-species are specialized predators of snails; Calosoma-species are specialized predators of tree-dwelling caterpillars; Lebia-species are parasites of certain Chrysomelid-larvae; Brachynus- species have a very effective defense-mechanism (explosive secretion); species of Elaphrus and Stenolophus are typical inhabitants of the borders of pools and rivers; Laemostenus-species are cave dwellers (LAMPRECHT & WEBER, 1979); Pogonus-species inhabit salt-marshes; Aphaenops- species lay only one large egg; Clivina-species are burrowing into the soil; etc. See further: THIELE (1977), who emphasizes the adaptive significance of the high degree of behavioural variation among carabids.

4. COEXISTENCE AND EXCLUSION AMONG CARABID BEETLES

Now we have shown that congeneric species can indeed be considered ecologically closely related species we are ready to test the hypothesis, "Species belonging to different genera will occur more frequently 291 together in the same habitats than congeneric species", against the null hypothesis, "Species belonging to different genera will not occur more frequently together in the same habitats than congeneric species". 4.1. Material Before being able to execute this test we will have to define "occurring in the same habitats". As our carabid beetles were sampled with the help of pitfalls we can define "occurring in the same habitats" as "being caught in the same pitfalls". In other words, individuals of the species concerned generally will have a positive chance to meet, i.e. to interact, and the possibility to interact must be considered a necessary but not sufficient condition for competition-and thus exclusion-to occur. To make our test not intractable we will use the year as a unit of time, i.e. species caught within the same year in the same pitfall will be considered to occur in the same habitat. This large time-unit is suitable for our purpose: as shown in section 3.2 (1) more than 80% of the species belonging to the same genus have the same annual reproduction rhythm. This argument is fortified again by the fact that there appears to exist a clear relationship between the moisture conditions of a habitat and the annual rhythm of the carabid species occupying it: wet sites are nearly exclusively inhabited by spring breeders, whereas in very dry habitats (higher parts of blown sand areas) the majority of carabid species consists of autumn and winter breeders: Table 3; see also 3.2 (4). Hence, most congeneric species that were sampled with the same pitfall within the same year may indeed have been potential competitors. In the course of nine years (1959 up to and including 1967) many remnants of natural habitat as well as a few less natural sites around Wijster (most of which within a radius of 10 km) have each been sampled continuously during at least one year, and in many cases during a number (up to 8) succeeding years, with a standard set of pitfalls (3 pitfalls at mutual distances of 10 m): 175 year-samples from 73 different sites in total. More about this technique of sampling can be found in DEN BOER (1977: Ic), whereas the sampling sites are described in DEN BOER (1977: Appendix A, Part II). In this way the occurrence in these sites of 149 carabid species belonging to 41 genera could be established. These species are divided over the genera as follows: - - - - number of species/genus 1 2 3 4 5 6 7 10 12 15 21 - - - number of genera 17 8 4 4 1 1 1 - 1 2 1 1 - - - - total numbers of species 17 16 12 16 5 6 7 10 24 15 21 292 TABLE 3 Distribution of the annual reproduction rhythms of the carabid species captured in the environments of Wijster (Drenthe, The Netherlands) over eight categories of habitat, arranged from very wet to very dry (in brackets: the number of these species that also about equally occupy other types of habitat).

* One or more of these species are reproducing in spring as well as in (summer) autumn (i.e. larvae both in summer and in winter). Interestingly enough, these species also occur about equally abundantly in quite different types of habitat with respect to moisture conditions: Calathuspiceus Marsh. (see: DENBOER, 1979) and Notiophilusbiguttatus F. in moist as well as in dry forest, Abax parallelepipedusP & M. in moist and in wet forest, Notiophilusaquaticus L. and Carabus nitens L. in moist heath, peat bog and in blown sand areas (DENBOER, 1977: Appendix A, part III, pp. 136, 149, 151, 161 and 162).

4.2. Methods of testing To compare the actual distribution over genera of the species, present in some year- sample, with the distribution expected under the null hypothesis, we will use the method proposed by SIMPSON(1949) and described in WILLIAMS(1951), because it is independent of any assumption about some mathematically expressed frequency distribution possibly underlying the data. The total number of different ways in which two species can be taken at random from any collection of species (N), classified into genera-each with ni species respectively-is N (N-1 )/2 (possible pairs), whereas the total number of ways in which two species can be taken within a genus is ni (ni - )/2 (congeneric pairs). Hence, the chance that two species taken 293 at random from N will be congeneric, is by be N(N (N-- I ) 1 ) , which N¿n¡(n¡-l) (N-1 ) will a measure of the "Generic diversity" (G.d.) of N, i.e. a number that will be the higher the greater the number of genera over which the N species are distributed. Hence, the total number of ways two species can be taken from our 149 carabid species, is 149 x 148/2 = 11026, whereas the number of ways in which two species can be taken so as to belong to the same genus, is (cf. 4.1 ) : (8 x 2 x 1 )/2 + (4 X 3 X 2)/2 + (4 X 4 X 3)/2 + (1 X 5 X 4)/2 + (1 X 6 X 5)/2 + (1 X 7 X 6)/2 + (1 x 10 x 9)/2 + (2 x 12 x ll)/2 + (1 x 15 x 14)/2 + (1 x 21 x 20)/2 = 582; G.d. = 18.9, i.e. once out of 18.9 times we take a pair of species at random from our 149 carabid species these will be congeneric. We can now compare this value with those to obtain from the pitfall catches in separate habitats. To find a congeneric pair we expect that under the alternative hypothesis on the average we will have to take more than 18.9 pairs of species at random from those living together within some habitat, whereas under the null hypothesis we will have to take 18.9 or a smaller number of pairs for that. In other words, species living together in the same habitat will belong to more separate genera than "normal" under the alternative hypothesis (G.d.-values higher than 18.9), and to about the same or a lower number of genera than "normal" under the null hypothesis (G.d.-values equal to or lower than 18.9).

4.3. Different values As at the moment we have no reliable method to test the significance of deviations of the actual G.d-values from the expectation, we can only make some qualitative comparisons. It seems therefore the more necessary to consider first the "factors" that may possibly mask the real relationships between carabid species living together in the same habitat. In this connection the following points should be brought out: ( 1 ) Populations, taken as "interaction groups", i.e. as they are sampled with the help of pitfalls (DEN BOER, 1977), die out and are (re)founded more or less frequently (DEN BOER, 1977, 1979). Therefore, the relevant information hidden in the distribution of species over genera from separate year-samples will be mixed with a certain amount of "noise" originating from incidental and short-lived settlements of species that only marginally occupy the habitats concerned. Especially species with high powers of dispersal will contribute to this "noise", of course. In 3.2 (5) (Tables land 2) we saw already that "dispersal power" is not distributed in some special way over genera, so that we need not expect some systematic error from these irrelevant species. However, in separate year-samples this phenomenon will sometimes result in un- reliable G.d.-values, so that it seems not desirable to argue from single year-samples. Therefore, we will consider summed (or averaged) results only, i.e. we will examine values of EN (N-1 ) /2 and of summed over definite groups of year-samples. (2) Individuals of such incidental and short-lived settlements will either not be caught at all in the definite pitfalls, or there will only be 294 captured single individuals (after screening many year-samples for marginal species it appeared that only rarely more than a single individual of such a species is caught). Hence, we can get rid of an important part of the above "noise" (but also of a few relevant species) by rejecting all species of which only a single specimen was caught during that year in that set of pitfalls (year-sample). Values "cleaned" in that way will be marked with "S". (3) Genera with only a single species in our area cannot contribute to the numbers of congeneric pairs, and are thus irrelevant to our problem. A few or even many of such species will be incorporated, however, in the respective values of N (N-1), and may thus increase the "noise-level" of our computations. As throughout all parts of the Netherlands (and often throughout Western + Central Europe too), 12 out of these 17 genera (cf. 4.1) are represented by the same single species only, it seems better to leave these 17 genera out of our comparisons. Values which are thus "cleaned", and from which also are removed the 12 species in other genera of which only a single specimen was caught in 175 year-samples althogether (i.e. only marginal in our whole area of study), will be marked with "R". In the following we will thus present three kinds of values of EN (N-1 ) /2, of and of mean G.d. (computed from these summed values): (1) values in which all genera and apecies are incorporated (which will be marked with "T"), (2) "R"-values and (3) "R + S"-values. The mean G.d.-values will be compared with the respective G.d.-values for the "collection as a whole", consisting either of the carabid species captured in all habitats together within the same year (or in all nine years), or of the species caught in the same habitat during a number of succeeding years. We will also compare the numbers of congeneric pairs observed (summed over habitats) with the numbers that can be expected by random selection, which also are the lowest numbers under the null hypothesis; these expected numbers are obtained by dividing EN(N-1)/2 by the pertinent G.d.-value for the "collection as a whole".

4.4. Results The results of our test are presented in Table 4, which shows that in each of the nine years as well as for each kind of values the numbers of congeneric pairs of species observed in separate habitats were higher than those expected. This not only means that the carabid species occurring within the same habitats cannot be considered random samples from the "collection" of available species, but also that we have to question the hypothesis that the distribution of carabid species over habitats should be affected demonstrably by "competitive ex- 295 296 clusion" among related species. On the contrary, congeneric species show a distinct tendency to live together within the same habitats. This is most convincingly shown by the "R+S-values", which are "cleaned" as good as possible from all incidental catches and irrelevant species (cf. 4.3). This result is not the outcome of a "play" with some statistical formalization : if one looks through the lists of species, representing the catches in separate habitats, especially those derived from sampling during a number of years in the same sites, one is struck by the fact that particular habitats are indeed characterized by the occurrence of species belonging to the same genera. Wet sites with a closed vegetation are often occupied by a number of Agonum- and/or Ptero- stichus-species; in many sandy sites with a sparse and/or disturbed vegetation often a number of Amara- and/or Harpalus-species occur, small and light birch forests on dry sand are often inhabited by a number of Notiophilus-, Calathus- and/or Amara-species, etc. In Table 5 two examples of such a coexistence of congeneric species are presented.

TABLE 5 Numbers of individuals of some congeneric species caught during a year in the same standard set of pitfalls (3 pitfalls at mutual distances of 10 m).

* Pterostichus-speciesin A G (mosaiclystructured heath vegetation)

* For more detailed descriptions of these habitats, see DENBOER (1977, p. 119)

These examples are also interesting in some other respect. Con- cerning the Pterostichus-species one may remark that severe competition between species could hardly be expected, because these species are very different ecologically: P. niger is an autumn breeder, whereas the 297 other species are spring (or early-summer) breeders; the individuals of P. diligens are rather small (5.5-6 mm), whereas the individuals of the other species are much bigger (P. nigrita and P. versicolor 9-12, P. lepidus 11-13, and P. niger even 16-21 mm) ; P. versicolor and P. lepidus are day-active, whereas the other species are night-active; see also Table 2, 3.2. However, similar remarks cannot be made concerning the Notiophilus-species, which nevertheless show the same pattern of catches over years: the individuals of all Notiophilus-species are about equally sized (4.5-6 mm), are day-active and specialized hunters of springtails (also the larvae are active at the surface, and preferably hunt springtails; see e.g. BAUER, 1979). Although two species are mentioned in literature to be spring breeders (LINDROTH, 1945; see also Table 1), we obtain more and more indications that all species are reproducing both in spring and in autumn (and possibly even in winter). Hence, the five Notiophilus-species presented in Table 5 are very similar ecologically, and are nevertheless coexisting in the same habitat. The most plausible general conclusion from this test thus is: As species of the same genus are also ecologically closely related (section 3), the habitats preferred by congeneric species will show more similarities than those occupied by species of different genera, by which the chance to find congeneric species together in the field will be higher than it would have been in the case of a random distribution of species over habitats. The advantages for congeneric species to coexist are apparently greater than the drawbacks of a possible interspecific interference. I suppose that in the case of carabid beetles (and in many-if not most-other cases concerning invertebrates) for congeneric species interspecific interferences will not be more important than-because they (copulations excepted) are not much different from-possible intraspecific interferences. Hence, instead of defending a "principle of competitive exclusion", both the data presented by WILLIAMS (1947, 1951) and the present data on carabid beetles give sufficient arguments to put forwards a "coexistence principle": "Species that are ecologically closely related will more often than not be found coexisting in the same habitats".

4.5. Exclusion as a process in time In 4.4 we studied the possible occurrence of "competitive exclusion" from a rather static point of view, i.e. we compared existing distributions of species with expectations. However, in our case we can also look more dynamically at distributions, i.e. we can try to trace the existence of exclusion as a process in time. We could establish that in the habitats sampled during a number of years extinctions and 298

(re)foundations of interaction groups occur frequently (DEN BOER, 1977: 6.8.3, Table 9), and we may thus wonder whether or not such a "turnover" of interaction groups is independent of the distribution of species over genera. Hence, we can put forward the hypothesis: the extinction of interaction groups will occur more frequently-and/or refoundation more difficultly-among congeneric species than among species belonging to different genera. This means that one or more year-samples from such a habitat would show a higher "Generic diversity" (viz.: after disappearance of the species concerned) than the "collection" of all species observed in that habitat (which is not influenced by these particular losses). Hence, we can test this hypothesis by comparing the general G.d.-value of some habitat with the G.d. averaged over year-samples (one or more of the composing values ofN(N-1) and Y-ni(ni - 1) are expected to increase mean G.d.). In Table 6 we brought together the relevant data for all habitats that were sampled uninterruptedly during at least 4 years. This table shows a kind of data one would expect if the "turnover" of interaction groups would occur independently of the identity of coexisting species, i.e. in some cases-by chance-the disappeared species will have had congeneric partners (mean G.d. higher than general G.d., as in Z, 0 and AV), in other cases they will have had not (mean G.d. lower than general G.d., as in AE, AM, M, P and AT). Moreover, the differences between general and mean G.d.'s are usually small (probably not significant), and often not consistent over the three kinds of values (T, R or Ras). Hence, this test does not give any indication that among coexisting congeneric species exclusion would occur to a significant extent. Against this test it may be objected that the periods over which exclusion is supposed to result in significant effects are very short. We can try to find better indications, however. Small, homogeneous, and stable habitats that have already been highly isolated for at least a century (say), in the course of time will have irreversibly lost a number of species by extinction (DEN BOER, 1977, 1979). If exclusion of closely related species would be an important process we may expect that at present in such habitats we will find less congeneric species than in comparable not-isolated habitats, where refoundation of such excluded congeneric species will still occur repeatedly. In other words: in the isolated, stable habitats "Generic diversity" is expected to be higher than in the not-isolated habitats. In Table 6 the habitats B, C, AL and AM are small, old, rather homogeneous, and highly isolated remnants of deciduous forest (a few ha at most), whereas N and Z are rather homogeneous parts of an old, stable and extensive heath area (1200 ha). All G.d.-values of 299 300 the remnants of old forest are lower than those of N and Z, by which is shown that also after a considerable number of years obviously no significant exclusion of closely related species does occur. On the contrary, even over a period of a century (say) the advantages for congeneric species to coexist still remain greater than the drawbacks of possible interferences. 4.6. Coexistence The advantages of congeneric species to coexist in preferred habitats is shown in an even more pure form (i.e. undisturbed by possibly supposed exclusions) in unstable habitats, which are also extreme. Unstable habitats usually can only be occupied by species with high powers of dispersal (cf. DEN BOER; see also: SouTHWooD, 1962). In 3.2 (5) (Tables 1 and 2) we showed that species with high powers of dispersal can be found all over the genera, so that unstable habitats that are favourable for many species (albeit only for short periods, usually in spring and summer) can be expected to show comparatively high G.d.-values. After the conclusions in 4.4 and 4.5 we may now expect, that the more extreme such an unstable habitat the more the (carabid) species occupying it will be restricted to certain genera, i.e. the lower the G.d.-value. In Table 6 the habitats P and 0 are not very extreme, unstable habitats, whereas M is a very extreme one (a wet and very dense stand of high tussocks of Molinia). Still more extreme are floating moor-vegetations. Because of technical difficulties we only sampled some of such habitats during one or a few years. The T-, R-, and R+S-values for mean G.d. of these habitats (in brackets the number of sampling years) are respectively: Q (3), 8.0, 6.7, 6.3; AJ (2), 5.2, 4.5, 3.5; RST (1), 10.1, 8.9, 9.0; AB (1), 5.1, 5.1, 5.1; TB (1), 9.3, 9.3, 7.0. For descriptions of these habitats, see: DEN BOER (1977: Appendix A, Part II).

4.7. Coexistence principle Summarizing, the distribution of (carabid) species over habitats with respect to their belonging to the same or to different genera can most parsimoniously be explained by the hypothesis: "Taxonomically closely related species are also ecologically closely related, and will thus be found coexisting in the same sites more frequently than could be expected by a random distribution of species (coexistence principle)". Although we do not exclude the occurrence of incidental exclusions, the general picture is apparently not noticeably influenced by it. Also JACOBS (1979) points to the fact that there must be many processes and situations that will allow related species to coexist. 301

5. DISCUSSION

In this paper we tried to show that the "competitive exclusion" of ecologically closely related species cannot be a "law of nature", and not even a more or less general "rule", at least as far as carabid beetles are concerned. In this way we could support the review of the present field evidence on exclusion by BIRCH (1979), from which it becomes evident that in the wide range of intensities of possible interactions between species complete exclusion must be considered a fairly exceptional outcome. In concordance with this the experiments discussed in 2.2 showed that exclusion of closely related species can only be brought about under severely restricting conditions. It can hardly be expected that anywhere in nature such conditions will be satisfied with the necessary precision: Nowhere are physical conditions really constant, not even in the tropics (WOLDA, 1978) or in caves (JUSTER- THIE, 1979); every natural habitat is somehow heterogeneous in space, which usually will give possibilities to "escape" (with the probable exception of small and closed habitats, such as fruits and flower-heads; see e.g. ZWBLFER, 1979). It may be objected, that when habitats are experienced as heterogeneous in space we are not observing at the right scale, and we should subdivide our habitat further to study the events occurring on this much smaller scale, i.e. it is expected that congeneric species will not live in exactly the same "niche". If we adhere to a sufficiently narrow definition of "niche" this will always be true, of course, since not only different species but also different individuals of a bisexual population will be different by definition. Ecological differences between individuals of the same population, e.g. differences in egg production (VAN DIJK, 1979a, b), are often as great (at least among carabid species) as to largely overlap with related species, which makes the whole endeavour to define and compare exact "niches" for separate species a fairly unpractical-if not im- possible-job (apart from some highly interesting exceptions, which- I am sure-can always be found). However, if we intend to keep as close as possible to the processes that run in Nature in general, we should not go much further in sub- dividing habitats than stating: "Species, the individuals of which are able to meet (e.g. are caught in the same pitfall) may be considered potential competitors", as we did in this paper. This means, we are interested in the possible frequencies of the occurrence of exclusion and coexistence to be able to understand why species are distributed in space and time (at the scale of interaction groups, DEN BOER, 1979) as they are, and this will only exceptionally be significantly influenced by events on a very small scale between single individuals of different 302 species, as long as there are also plenty of possibilities to "escape" (i.e. heterogeneities in the habitat). This was already nicely illustrated by the experiments of CROMBIE (1945) when he introduced small glass tubes in the culture medium of Tribolium-Orzyaephilus. Hence, exclusion at the scale of populations under field conditions can only be expected to be a process that overrules relevant ecological differences between individuals as well as the heterogeneity in space and time of the habitat of the populations concerned. This means that it can hardly be a very subtle process that depends on small-but consistent in spite of variability between individuals-ecological differences between closely related species. This is in accordance with the conclusions by BIRCH (1979), who could show that the best documented cases of exclusion indeed concern fairly crude processes, such as direct aggression or even killing, pushing each other off a rock, and the like (see further 2.3), in general, the making unavailable of essential resources for the excluded species by the activities of the excluding species, as it is defined by BIRCH. It also means that there is hardly any reason to expect exclusion merely to occur between closely related species. The only necessary-but not sufficient (cf. 2.3)-condition is that both species depend on the same essential resource. It may even be expected that mainly if unrelated species depend on the same essential resource the possibility of exclusion comes into the picture (at least under field conditions), because especially then there will be a positive chance that the differences in the manner of ex- ploitation of that resource will be sufficiently important to overrule differences between individuals as well as heterogeneities in the habitat. A convincing example of this was found by A. DAANJE (pers. comm.) : Both the chrysomelid beetle Agelastica alni L. and the leaf-rolling weevil Deporaus betulae L. use the leaves of Alnus-species as a resource; larvae and adults of A. alni directly feed upon the leaves, whereas the females of D. betulae in a special way roll the leaves to serve both as a protection and as a future food resource for the larvae (DAANJE, 1964). Leaves that are damaged by A. alni cannot be used by D. betulae for rolling, by which D. betulae is readily excluded from trees where A. alni reaches relatively high densities (the more so because the weevils generally fly away when a leaf has appeared to be unsuitable). In such cases-as was observed repeatedly-A. alni thus literally makes unavailable an essential resource for D. betulae. It is interesting to note, that in such cases (at least in the present one) there is only exceptionally question of "resources in short supply" (Alnus-leaves). Finally, we have to question whether it is justified to consider "competitive exclusion" to be "associated with the normal course of evolution" (NICHOLSON, 1960). If we support the "principle of topi- 303

cality" we should admit, that the experimental evidence (cf. 2.2 and ANDREWARTHA & BIRCH, 1954) as well as the review by BIRCH (1979) together with the statistical analyses by WILLIAMS ( 1947, 1951) and in this paper, show that this claim is not justified: exclusion occurs, but apparently only incidentally, so that other processes can be expected to be more generally "associated with the normal course of evolution", i.e. can be expected to be responsible for the extermination of enormous quantities of species in the course of time.

ACKNOWLEDGEMENTS S

Manuscript and tables were typed by Mrs. G. H. Weijenberg-Boer.

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P. J. den Boer, Biological Station of the Agricultural University of Wageningen, Kampsweg 27, 9418 PD Wijster (Dr.), The Netherlands.