Some organisms have smaller niches than others. Niche breadth, also called “niche width” and “niche size,” can be thought of as the extent of the hyper volume representing the realized niche of an organismic unit.

Thus a koala bear, Phascolarctos cinerus, which eats only leaves of certain species of Eucalyptus, has a more specialized food niche than the Virginia opossum, Didelphis virginianus, which is a true omnivore that eats nearly anything. Statements about niche breadths must invariably be comparative; we can only say that a given organismic unit has a niche that is narrower or ‘broader than that of some other organism unit.

Highly specialized organisms like the koala usually, though not always have narrow tolerance limits along one or more of their niche dimensions. Often such specialists have very specific habitat requirements, and as a result they may not be very abundant.

In contrast, organisms with broad tolerances are typically more generalized, with more flexible habitat requirements, and are usually much more common. Thus specialists are often relatively rare while generalists are more abundant. Rare organisms may, however, frequently occur in clumps so that their local density need not necessarily be low.

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The only currency of natural selection is differential reproductive success. This fact raises a question: If specialization involves becoming less abundant, why have organisms become specialized at all? Since generalized organisms can usually exploit more food types, occupy more habitats, and build up larger populations, they might be expected by their very numbers to overproduce slightly more specialized competing members of their own population and thereby swamp the population gene pool; The answer to this apparent dilemma lies in the old adage that a jack-of-all-trades is a master of none. More specialized individuals are more efficient on their own ground than are generalists.

Under what conditions will a jack-of-all-trades win in competition with more specialized species? MacArthur and Levins (1964, 1967) considered this question and developed the following model. First imagine an ant-eating lizard in an environment that contains only a single food resource type, colonies of ants 3 mm in length, and a variable population of ant-eating lizards that exploit the ant food resource.

Assume that ants are eaten whole and that lizards differ only in the size of their jaws, forming a fairly continuous phenotypic spectrum. Some phenotypes will be well adapted to use 3 mm ants and very effective at harvesting them; others will be less efficient, either because their jaws are too large or too small.

Next consider the same phenotypic spectrum of lizards in another “pure” environment, this one composed solely of colonies of 5 mm ants. Almost certainly the best-adapted phenotype will differ from that in the 3 mm ant environment, and the phenotype most efficient at using 5 mm ants will be one with a larger mouth.

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Now consider a mixed ant colony with equal numbers of 3mm and 5 mm ants (says, two castes) in a homogeneous mixture. Which phenotype will be optimal in this new mixed environment? Assuming that the lizards encounter and use the two ant sizes in exactly equal proportions, the relationship between phenotype and harvesting effectiveness must be exactly intermediate between the two similar relationships in pure environments.

The dashed line can thus be drawn in Fig. midway between the first two (if the two resources were not in exactly equal proportions, this new line would simple be closer to one or the other of the original lines); depending on the shapes of the curves and the distance between them, this new line can take either unimodal or bimodal shape.

In the formers case, the phenotype of highest harvesting efficiency is intermediate between the best “pure 3 mm ant eater” and the best “pure 5 mm ant eater,” and this “jack-of-both-trades” (probably the phenotype that could best exploit 4 mm ants) is competitively superior.

In the latter case, because the two types of ants are very different in size, the two phenotypes with high harvesting effectiveness are separated from one another by intermediate phenotypes with lower efficiencies at exploiting a mixture of 2 mm and 8 mm ants, and the two specialists will eliminate the jack- of-both-trades.

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Effects of interspecific competition on niche breadth are complex and under different conditions may actually favour either niche contraction or niche expansion.

Thus a competitor may reduce food availability in some microhabitates but leave prey densities in other microhabitats unaltered, effectively reducing expectation of yield in some patches but not others.

A competitor that is an optimal forager should restrict its patch utilization to those with higher expectation of yield, thereby decreasing the breadth of its place niche. Conversely, a more generalized competitor that reduces food availability more or less equally in all microhabitats by reducing the overall level of prey availability can force its competitor to expand the range of resources it uses, thereby increasing the breadth of its food niche.

In a food-sparse environment, an optimal forager simply cannot afford to bypass as many potential prey items as it can in a food-dense environment; therefore more suboptimal prey must be eaten in the former type of habitat. Reduced interspecific competition is often accompanied by an increase in the range of habitats a species uses, but marked changes in the variety of food eaten with changes in interspecific competition seem to be much less common (MacArthur, 1972).

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A growing body of theory on optimal foraging predicts that niche breadth should generally increase as resource availability decreases. In an environment with a scant food supply, a consumer cannot afford to bypass many inferior prey items because mean search time per item encountered is long and expectation of prey encounter is low.

In such an environment, a broad niche maximizes returns per unit expenditure, promoting generalization. In a food-rich environment, however, search time per item is low since a foraging animal encounters numerous potential prey items; under such circumstances, substandard prey items can be bypassed because exceptation of finding a superior items in the near future is high. Hence rich food supplies are expected to lead to selective foraging and narrow foodniche breadths.

Two fundamental components of niche breadth have only relatively recently begun to be distinguished: the “between-phenotype” versus “within phenotype” components.

A population with a niche breadth determined entirely by the between-phenotpye component would be composed of specialized individuals with no overlap among them in resources used; a population composed of pure generalists with each member exploiting the entire range of resources used by the total population would have a between-phenotype component of niche breadth of zero and a maximal within-phenotype component. Clearly real populations will lie somewhere between these two extremes.