Geoscience Reference
In-Depth Information
body and antler size as well as starvation of their calves, for overall higher sur-
vival, which enabled this population to persist while herds that remained on
the mainland perished from predation. One could argue that the animals on
the mainland made poor choices in habitat selection.
Animals may not always correctly perceive risks, especially in a novel or
changing situation, so their choice of habitats may not necessarily be best in
terms of their fitness (Wiens et al. 1986, Holt and Martin 1997). Pollution is
one such risk that is rarely evaluated in habitat studies. Consider Mallory et
al.'s (1994) study of the effects of acid rain on habitat quality for common
goldeneyes (
Bucephala clangula
). Acid rain killed fish in lakes, which enabled
proliferation of invertebrates, thus providing more food and enhanced repro-
duction for goldeneyes. However, these ducks may inevitably suffer other
consequences of the acidified habitat. In some situations the choices made by
animals may maximize fitness over the long term (e.g., if the changes are
ephemeral); in other cases animals may require time to adapt.
Suppose that an animal uses alternate habitats more often when the nor-
mally preferred habitat becomes less profitable because of increased density
and hence competition for resources. If the alternative habitats are more avail-
able than the preferred habitat, then use of these alternative habitats can end
up higher than that of the otherwise preferred habitat. Situations like this
seemingly justify considering habitat use in terms of availability. However,
Hobbs and Hanley (1990) showed both intuitively and with computer mod-
eling that intraspecific competition may alter use:availability ratios such that
they may not reflect the true quality of a habitat in terms of its potential con-
tribution to population growth. Certainly interspecific competition and pre-
dation are integral components of habitat quality, and managers concerned
about a given species should not evaluate habitats independent of these factors.
However, perceptions of habitat quality that are affected by density of the tar-
geted species are an artifact of the method. Recall, as Peek (1986) noted, that
habitat preference should be innate, and intricately tied to fitness; if our per-
ception of preference declines as the preferred habitat becomes more densely
populated, then our perception is clearly mistaken. Hobbs and Hanley (1990:
520) concluded that “use /availability data inherently reflect differences in ani-
mal density among habitats” (independent of social behavior) and therefore
reveal little about habitat quality. Consequently, they recommended abandon-
ment of the use-availability design in favor of an approach that enables a direct
investigation of the link between habitats and fitness.
Site attribute studies often focus on sites of biological importance (as
reviewed earlier) and therefore may provide more direct insights into habitat
