Geoscience Reference
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more often as fact than as assumption. However, there are many reasons why
this relationship may not hold. Habitats that are used infrequently may be
more important than suggested by the time spent there. Conversely, habitats
used preferentially for activities that require a lot of time (e.g., resting) may be
less important than indicated by their use; a resting habitat may be substi-
tutable, with little effect on fitness, whereas a specific foraging habitat (or a
water hole in an arid environment) may be more essential, even if not used
very much. Differentiating activity-specific habitat use may help alleviate this
difficulty (Forsman et al. 1984; Cavallini and Lovari 1991; Ternent 1995;
Salas 1996; Tew et al. 2000), although this, too, is seldom done, and moreover,
does not necessarily circumvent the problem. For example, Powell (1994)
studied winter foraging of fishers, which preyed heavily on porcupines (
Erethi-
zon dorsatum
) but spent a disproportionately small amount of time hunting
in upland hardwood habitats where porcupines were common. The reason,
he found, was that fishers rapidly located porcupines at known den sites, thus
minimizing their search and chase times. In contrast, fishers had a harder time
hunting snowshoe hares, so they spent a larger amount of time in low-
land conifer habitats where hares were more common. Clearly, the relative
importance of the two habitats to fishers was not reflected by their time spent
hunting in each. For animals that feed on a variety of different foods, a mix-
ture of different habitats may be more beneficial in many respects than a sin-
gle, highly preferred type, and the time spent in each may be a poor indicator
of importance of either the specific type or the overall mix. Omnivorous
Egyptian mongooses (
Herpestes ichneumon
), for example, favor one specific
habitat for resting, but use a mix of habitats when feeding (Palomares and
Delibes 1992).
Fitness also may be affected by predation and interspecific or intraspecific
competition. These factors can change the cost:benefit ratio of a habitat and
hence alter an animal's habitat use (Douglass 1976; Holbrook and Schmitt
1988; Hughes et al. 1994; also see review and other citations in Lima and Dill
1990). Cowlishaw (1997) found that baboons (
Papio cynocephalus
) spent more
time feeding in a habitat with poor food but a low risk of predation than in a
food-rich habitat with a high risk of predation. Fitness was probably enhanced
by this choice of habitats.
Fitness certainly was enhanced for a herd of caribou in Ontario, Canada,
that spent most of the time on an island. The nearby mainland had higher-
quality forage but also a high density of wolves. Ferguson et al. (1988) found
that the island occupants sacrificed nutrition, which was reflected in smaller
