HABITAT SELECTION THEORY (concluded)

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 disadvantages outweigh the advantages. Examining Fig. 30?, you can see that the first habitat should fill until realized suitability has been reduced to the basic suitability of Habitat 2. At that point it becomes highly advantageous to settle in Habitat 2 because suitability increases with density as long as densities there are low. Once individuals start to settle in habitat 2, others may shift from habitat 1 to habitat 2. This type of effect may be important in the evolution of coloniality, a topic to which weíll 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. Each sheep uses several home ranges in the course of a year. 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.

TERRITORIALITY

Territoriality is thought to evolve when resources are clumped, rather than evenly or randomly distributed (see handout, Fig. 6.6), predictable at least on short time scales, and individuals (or groups) can economically defend areas sufficient for their needs(see handout, Fig. 7-2).. Territorial defense seems to be related primarily to food resources: among species with similar food habits larger species defend larger territories, and predatory species defend larger territories than herbivorous species of the same body size (see handout, Fig. 7-8). However, territoriality may also function to reduce the probability of predation of prey species that rely on crypticity to avoid predators, particularly if predation rates are density dependent.

Whether a territory can be defended depends on the mobility of the animal (in relation to energetics of defense), distribution of the resource, resource abundance, and resource predictability. If resources are scattered, vary little in quality, or are unpredictable, costs of territoriality will usually outweigh benefits and nondefended home ranges will occur. If resources are abundant and are spatiotemporally predictable, territoriality will be favored. Generally, territories are defended for long periods of time (e.g., months, years). However, territories may be ephemeral; for example, during the nonbreeding season, hummingbirds defend feeding territories during the day but move from one area to another every several days.

Definitions

The area in which an individual or a group lives is called its home range. Territories are home ranges that are used exclusively by an individual or a group. (Territoriality also has been defined as "site-specific" dominance -- an individual or a group is dominant to all others at a particular site or in a specific area.) Exclusive use may be maintained by advertisement (e.g., song, scent marking) and/or aggression. Many mammals are not territorial in the true sense but may defend a core area (the portion of the home range that is used most extensively) within the home range. In contrast, most birds are territorial, defending their entire home range. The regular occurrence of territoriality in breeding birds but its relative rarity in small mammals with similar food habits probably relates to differences in mobility; a bird can economically defend a much larger area than a mammal with similar energy requirements can.

Cost-benefit approaches

Alcock discusses costs and benefits of territoriality. The benefit of exclusive access to resources in a particular area must be weighed against the cost of defense, as well as other costs, such as predation. The general ideas can be understood by examining models of optimal territory size. If costs outweigh benefits at all possible territory sizes, then there will be no territoriality. If benefits outweigh costs at some sizes, then territories of those sizes should be defended, and the optimal size is defined by the maximal difference between benefits and costs (see handout, Fig. 7-11). Generally, it seems reasonable that benefits would increase with size of the territory but less than linearly, plateauing at the point where the territory has more resources than the individual (or group) can effectively utilize. On the other hand, costs likely increase more than linearly because the area that must be defended increases exponentially and competitors will be attracted more to large than small territories (see handout, Fig. 6.1, left panel). What if resource density increases? This would shift the benefit curve towards the origin and upward and smaller territories should be defended (see handout, Fig. 6.1, middle panel). What if population size and thus competition for areas increases? This will shift the cost curve up and to the left (for any particular size of the territory costs are higher than before)--see handout, Fig. 6.1, right panel.

Alcock discusses territoriality during the nonbreeding season in the nectar-feeding Golden-winged Sunbird. Gill and Wolf (1975. Ecology 56:333-345) found that the cost of defense was more than offset by higher nectar levels of flowers within territories than in flowers in undefended areas (see handout, Fig. 7-12). This discrepancy is accentuated by the birds themselves, because exclusive use permits regulation of return time to particular areas and regeneration of nectar by flowers after feeding. Territory size varies tremendously, but each territory contains just enough nectar to supply the sunbird's daily energy requirements (see handout, Fig. 11.3). When flower densities are high, territoriality breaks down in association with high intruder density.

Carpenter and MacMillan (1976. Science 194:639-642) studied territoriality in Hawaiian Honeycreepers (Vestaria coccinea). Shifts to nonterritorial behavior occur at low (why defend an area with no food?) and high flower densities (see handout, Fig. 11.4). They suggested that there is simply no reason to defend territories when resources are superabundant.

Peter Myers (e.g., Myers, JP. et al. 1979. Auk 96:551-561) studied winter territoriality in Sanderlings (Calidris alba) along the California coastline. In his study area wintering Sanderlings defend linear stretches of beach from 10 m to 120 m wide in the wave-washed zone at high tides. There is generally only 1 prey species encountered along these beaches, an isopod that reaches peak densities in the upper intertidal zone. Territory size is inversely related to prey density, and areas of low prey density aren't defended (see handout, Fig. 2), as in honeycreepers and sunbirds. In areas of the highest prey density, the territory holders attempt to exclude other Sanderlings but are unsuccessful when flocks move in. At such times the territory holder gives up its defense and joins the foraging flock as long as the flock remains. This study shows that individuals did attempt to maintain territories in areas of highest prey densities but were unable to do so when the costs of defense were too high.

Sanderling winter territoriality broke down completely in Dyers' study area when a Merlin appeared on the scene. Merlins are efficient predators of shorebirds, but flocking is an effective means of defense. For example, Kenward (see Alcock) found that success rates of goshawks pursuing pigeons are inversely related to flock size due to increased vigilance in larger flocks.

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