HABITAT SELECTION

How choice is involved in the restricted distributions of animal populations is the subject of habitat selection. Are preferences learned or innate? Wecker's studies of deermice (Peromyscus maniculatus) provide an example of innate preferences. Individuals from prairie and forest populations strongly prefer the type of vegetation in which they are normally found. When they are reared in captivity with a choice of both habitat types, they still prefer the habitat in which their parents were raised. Preference of many small mammals seems to have a strong innate component.

Studies of passerine birds indicate that learning (habitat imprinting at the postfledging or juvenile stage) influences habitat preferences to a greater extent, but there is still an important innate component. Gibb found that Blue Tits (Parus caeruleus) prefer broad-leaved trees and Coal Tits (P. ater) prefer conifers, but the preferences are less pronounced in hand-reared birds than in wild-caught birds (see handout).

Linda Partridge further studied the adaptive significance of habitat choice in Blue Tits and Coal Tits in laboratory experiments. She tested the two species on their feeding skills and found that each species is better than the other at exploiting the type of food supply which it would encounter in its own habitat. The differences have a hereditary basis (see handout), i.e., there are differences in the foraging abilities of hand-reared birds. However, the differences are more pronounced in wild birds, showing that learning (more positive reinforcement in the preferred habitat)also plays a key role.

Field studies have documented that animals certainly show preferences for particular habitats. Rick Douglass studied the habitat preferences of two vole species, Microtus pennsylvanicus and M. montanus, in enclosures of natural vegetation and found that the types of vegetation used by each species were similar to those in natural conditions. However, the experimental animals also utilized a greater variety of habitats in the absence of congeners than they do in natural conditions (in the presence of congeners), showing that preferences and interspecific competition are both likely to be involved in determining different habitat distributions of these species in nature. These types of results probably often occur in nature.

Douglas Morse emphasized another important point about habitat selection. He studied black-throated green warblers in the northeastern US, where the warblers nest in red spruce and white spruce. Numbers of warblers were higher in red spruce in all four years of his study, but food abundance was lower in red spruce when the warblers established territories (see handout, Fig. 4.2). This study showed that animals don't simply pick the habitat that is most suitable at the present time; the warblers settle preferentially in the habitat that will have the most food soon after young have left the nest.

HABITAT SELECTION THEORY

Ideal-free distribution

Steve Fretwell has provided a very useful theoretical framework for considering where an individual should live. He begins by stating that an individual's options will be determined both by the suitability of the habitat and densities of conspecifics in that habitat. Since conspecifics will be the strongest competitors for resources, the suitability of the habitat should decrease as density increases (handout - Fig. 30).

If several habitat types are available but differ in their basic suitability and each individual is free to settle wherever his or her expected fitness is highest, the ideal-free distribution will apply. If a population is distributed among habitats in accordance with this distribution, densities will be highest in the most suitable habitat and lowest in the least suitable. If a population colonizes (or regularly migrates to) an area, only the most suitable habitat should be filled until its suitability is reduced to a value that is equal to that of the second-most suitable habitat. Subsequent settling would occur in both, and so on (see handout - Fig. 31). This idea was tested for mallards on a pond in England by Harper, who showed that relative numbers in 2 artificial food patches were proportional to quality of the patches (see handout).

The ideal-free distribution would apply to territorial as well as nonterritorial systems if territorial behavior simply results in uniform spacing of individuals or groups within habitats (case (i) on Fig. 11.10a on handout). If two habitats (one (A) of higher basic suitability) are available and individuals settle in habitat A until the suitability is reduced to the basic suitability of B, the ideal-free distribution would apply.

Great tits are territorial and breed in a variety of habitats. In the Netherlands, great tits nest in mixed woods and conifers. Mixed woods have higher basic suitability. In years of low densities the birds settle only in mixed woods. In years when densities are higher they settle in pine woods once densities in mixed woods are about 0.5-0.6 males per hectare (see handout, Fig. 36). Because average reproductive success is equal in pine and mixed woods, the ideal-free distribution appears valid even though the males are territorial--pairs in pine woods have equal fitness to pairs in mixed woods. This is because as densities get high in mixed woods per capita resource abundance decreases and predation by weasels increases.

In England, great tits use mixed woods and hedge rows; hedge rows are clearly inferior habitats. Reproductive success in hedge rows is lower than in mixed woods. If pairs in deciduous woods are removed, they are replaced quickly by pairs from the hedge rows whose reproductive success is then similar to other pairs in the mixed woods. Thus, it appears that the ideal-free distribution does not explain selection of hedge row habitat by some Great Tits.

Ideal-despotic distribution

In the ideal-free distribution each individual is free to choose its territory regardless of repulsion attempts by other individuals. In the ideal-despotic system an individual's options for settling are constrained by the territorial behavior of already established individuals. This is an extension of Huxley's elastic disk concept - territory sizes may not shrink indefinitely as population size increases. Consequently, individuals may be forced to settle in secondary habitats before the realized suitability in the preferred habitat drops to the level of the basic suitability of the secondary habitat. Fretwell has suggested that lower success of breeding birds in secondary habitat would indicate that territoriality is acting to limit density because resources in the preferred habitat would be underutilized; he has called this the ideal-despotic distribution.

On Fig. 33 on the handout, Fretwell depicts this concept from the point of view of established individuals and prospecting individuals. First, if individuals settled according to the ideal-free distribution, the S1 and S2 functions would apply for both settling patterns and "realized suitability". However, if residents repel prospectors and make prospectors settle in inferior habitat, the T1 and T2 functions apply to the settlement patterns of the prospectors. Once settlement in the two habitats is complete, the S1 and S2 functions show that the realized suitabilities are very different in the two habitats (see starred points on Fig. 33). Thus, if the ideal despotic distribution is operative, fitness (e.g., breeding success) should be higher in the primary habitat and lower in the secondary habitat. Great tits in deciduous woods and hedgerows in England fit this model.

Both the ideal-free and ideal-despotic distributions seem to be implicated in patterns of spacing among female red-winged blackbirds. Red-winged blackbirds are polygynous. Males defend territories and several females may settle on the best territories. Females behave territorially only briefly prior to egg-laying. In a late spring all females tend to arrive at breeding areas synchronously, and territorial behavior of the first females to settle may force others into suboptimal habitats. The largest harems will be in the best habitats, but these will be smaller than if each female were not constrained in her choice of where to breed. In such years there is thus a positive correlation between harem size and reproductive success; females in the best areas experience higher suitability than those in poorer areas. In early springs arrivals of females are less synchronous, and there is less pressure to initiate breeding quickly; consequently, late-arriving (or late-prospecting) females would be attempting to settle once the early settlers are already laying eggs or incubating and would be able to successfully settle nearby. In such years (1) densities will be higher and harem sizes larger in the best habitats than in years when arrival is synchronous and (2) there is no relationship between harem size and reproductive success; i.e., the ideal-free distribution applies.

Allee-type distribution

Fretwell also discusses one additional type of habitat-distribution. W.C. Allee, in a series of classic experiments, showed many advantages of group living to individuals and only as densities become high would the decreases be detrimental. This is the basis for Fretwell's "Allee-type ideal-free distribution" (Fig. 32). In this case there is a dynamic, changing relationship of density and suitability such that once individuals start to settle in habitat 2, others would be expected to shift from habitat 1 to habitat 2. This type of effect may be important in the evolution of coloniality, a topic to which we'll return after a discussion of territoriality.

TRADITION

Tradition plays an important role in habitat selection in at least some animals but is difficult to document. One of the best examples comes from Val Geist's (1971. Mountain Sheep, Chicago) studies of bighorn sheep (Ovis canadensis). Geist studied populations in Banff where suitable seasonal ranges of alpine tundra are small habitat islands separated by boreal forests at lower elevations. Young individuals learn appropriate patterns of home range use from associating with older individuals. Older ewes lead the movements of ewe bands which are composed of ewes, lambs, yearlings and young rams. When rams are about 4 years old they begin to associate with ram bands and learn a somewhat different pattern of home range use by following the pattern established by older rams in the ram band. Tradition plays such an important role in habitat use in such habitats that reintroduction of bighorns to former alpine tundra ranges in the mountain states is sometimes a difficult proposition.

Mating sites in the blue-headed wrasse, a coral reef fish, are traditional (Warner, R. R. 1990. Amer. Natur. 135:205-217). The same sites are used repeatedly at least over several generations. If local populations are removed experimentally and naive fish introduced; however, the introduced fish typically choose different sites. If these colonizers are removed, naive replacement fish tend to choose the same sites as the previous colonizers. These results show that colonists base their choice of mating sites on resource assessment but established fish base their choice on tradition. As the habitat gradually changes, traditional sites may become suboptimal. The reason that sites are traditional seems to be due to predation. Juveniles settle out of the plankton after an extended larval period and remain near the coral where groups occur and never have the opportunity to explore the reef because predation rates are so high. The fitness difference between using a traditional versus a best-quality (unoccupied) sites may be minor compared to the risk of assessing unused sites.

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