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Morphological Variation and Width of Ecological Niche Author(S): Leigh Van Valen Reviewed Work(S): Source: the American Naturalist, Vol

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Morphological Variation and Width of Ecological Niche Author(S): Leigh Van Valen Reviewed Work(S): Source: the American Naturalist, Vol The University of Chicago Morphological Variation and Width of Ecological Niche Author(s): Leigh Van Valen Reviewed work(s): Source: The American Naturalist, Vol. 99, No. 908 (Sep. - Oct., 1965), pp. 377-390 Published by: The University of Chicago Press for The American Society of Naturalists Stable URL: http://www.jstor.org/stable/2459179 . Accessed: 05/10/2012 09:12 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. The University of Chicago Press, The American Society of Naturalists, The University of Chicago are collaborating with JSTOR to digitize, preserve and extend access to The American Naturalist. http://www.jstor.org Vol. XCIX, No. 908 The American Naturalist September-October, 1965 MORPHOLOGICAL VARIATION AND WIDTH OF ECOLOGICAL NICHE LEIGH VAN VALEN Department of Vertebrate Paleontology, American Museum of Natural History, New York How does a species maintainits genetic variabilityin a formsuitable for adaptive evolutionwithout an unbearablegenetic load? One of the answers to this question has been thatdifferent individuals in a populationmay be adapted to somewhatdifferent environments. This hypothesisnormally re- quires the existence of all the followingthree conditions, where the popu- lation may be divided into sets a, b, . n and the niche inhabitedby the populationinto moreor less separable units (althoughcomplete gradation is by no means excluded foreither phenotype or environment),A, B ... M, at least at the timeof year whenpopulation size is limited: 1) The individuals of some set a surviveor reproducebetter than those of some set b in some environmentA, while the reverse is truein some en- vironmentB. 2) The above difference between a and b is in part genetic. 3) Some appropriate mechanism of distributional or mating preference exists. The third condition can be met by individuals of set a choosing environmentA with a greater than random frequency, or by homogamic mat- ing (at the limit of which a and b are two reproductively isolated species) if A and B differ spatially and there is a positive correlation between the locations of birth and mating. It is importantto note that continuous variation as well as polymorphism can be maintained by this model, the advantage of one or the other depend- ing in part (not as an absolute condition in either direction) on the degree of regularity and continuity of the environment. Stabilizing selection is weaker with greater intra-niche diversity, and may even be absent or nega- tive (destabilizing selection) when strong discontinuities are present within the niche. Intrapopulation variation in niche occupied permits a greater population size than would otherwise be possible, and is advantageous even when a polymorphismis maintained by some other means. Genetic control need not be present in such cases. As an example of this advantage, if a and b are the sexes (however determined), equally abundant and of equal size, and eat differentfoods of the same abundance, and if population size is limited by the availability of food, then the population size will be twice what it would be if the sexes ate the same food. Coexistence of morphologically or physiologically differentphenotypes, as well as the behavioral or microgeographic diversity mentioned above, 377 378 THE AMERICAN NATURALIST would be useful on both the individual and the population levels for ex- ploitation of a diversified niche, and it is thereforeprobable that under the above conditions the population -would be more variable than if it occupied a narrowerniche. Various aspects of the above theory have been formalized by Ludwig (1950), Levene (1953), Li (1955a, b), Lewontin (1955), Maynard Smith (1962), and Levins (1962, 1963); see also Dempster (1955). It was perhaps first explicitly proposed by Mayr (1945), although others approached it earlier. Mayr (1963) has summarized some of the evidence for the sub- sidiary hypotheses of the model. The niche-variation model is compatible with a dependence of fitness on relative frequency of genotypes or phenotypes, but neither situation implies the other. For example, learning by predators can produce a frequency- dependent polymorphismin their prey, and if one part of the niche cannot be tolerated or reached by some individuals, reduction in the frequency of individuals occupying that part will not change the relative fitness of those excluded fromit. Even in the more usual case, where a reduction in fre- quency of one formleads to an increased relative fitness in its part of the niche, this occurs only when that part of the niche remains habitable and- would not apply when an environmentalchange reduced the frequency by re- ducing the parts of the niche differentially. In the present paper I will compare the variation within populations of some birds that differ regionally as to width of niche. In one region, usually on the zoogeographic mainland, the niches of the species are rela- tively tightly packed together by the action of stabilizing selection im- posed by ecologically adjacent species. In the other region, usually on is- lands, the environment available for the birds is partitioned into wider niches, which implies weaker stabilizing selection by adjacent species. The wider niches would permitgreater phenotypic variation if this variation is controlled to a significant extent by the adaptive diversity of the niche. The results suggest that this is in fact the case. By "niche width" I mean the proportion of the total multidimensional space of limiting resources used by a species or segment of a community. "Niche" is used here in an observational sense, not a theoretical one. The relative sizes of the niches in the regions compared have been studied for the species used and found to be differentin the directions stated (see next section). The relative number of species in the total fauna is a re- lated question, but one that I will not consider because it seems relevant only to the average relative width of the niches in each fauna, and the ac- tual relative width for each species is known fromdirect observation. Let us consider a segment of a communitycalled a 'ttrophic-A level" de- pendent upon some set, usually a relatively diverse set, of limiting re- sources. There are three extreme ways in which such a segment of a com- munitycan exploit the available supply of the limiting resources: 1) There can be relatively few species, each individual of each of which is adapted to occupy a relatively broad segment of the resource space. MORPHOLOGICAL VARIATION AND NICHE WIDTH 379 Each individual may occupy this entire segment, or there may be a be- havioral partitioning of the segment. 2) There can be relatively few species, but these are relatively variable and differentindividuals within a species are fitted to, and do in fact, oc- cupy on the average different narrow niches (or, equivalently, different parts of a broad niche). 3) There can be a relatively large number of species, each of which is restricted to a relatively narrow segment of the resource space, more or less uniformfor each individual. These alternatives are not mutually exclusive on the communitylevel, and most trophic-A levels of most communities probably contain more than one kind of species. Species intermediate between the extremes given are presumably common. Genetically controlled interindividual behavioral and morphological variation related to the occupation of the niche should on the average be large in the second kind of species and small in the first and third, although environmentally controlled behavioral (and in a few taxa morphological) variation may sometimes also be large in the first kind. In small and unstable communities (for example, on islets), and in stable trophic-A levels where the limiting resource changes in quality from time to time (for example, changes in composition of prey), I would expect the first kind of species to predominate. In large and exceptionally stable communities I would expect a preponderance of the thirdkind, which should also predominate among the rare species of most communities, excluding species maintained by immigration. The set of moderately to very stable communities should contain the greatest proportion of the second kind of species, which perhaps may never comprise a majority of the species in a community. This list is obviously suggestive rather than exhaustive, and may be compared with an analogous list, with supporting theory and in- volving mainly other criteria, given by Levins (1963). I regard the fact that the present comparisons are between island and mainland populations as logically (not necessarily causally) irrelevant in- sofar as the relative number of species in the avifaunas is concerned. It is unknown to what extent niches are vacant or incompletely occupied on the islands, and to what extent the relative impoverishmentof the island biotas is cybernetic by, in itself, restricting the numberof potential niches for any trophic-A level. Because of these considerations, no a priori prediction of greater average niche width on the islands can be given with assurance. Island and mainland populations of most of the species on which comparisons have been made of niche width are, at the levels of resolution used, indistinguishable in niche width. I have ignored these species, although they are relevant to any consideration of relative niche width on the islands. Some species clearly have broader niches on the islands, and these species have been used here. In one case the island niche is clearly narrower than that on the mainland, and it is interesting that in this case the mainland populations, rather than the island ones, seem to be more variable.
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