Geoscience Reference
In-Depth Information
(Elston et al. 1996). In the wild, however, preferences must be inferred from
patterns of observed use of environments with disparate, patchy, and often
varying resources.
Generally, the purpose for determining preferences is to evaluate habitat
quality
or
suitability,
which I define as the ability of the habitat to sustain life
and support population growth.
Importance
of a habitat is its quality relative to
other habitats—its contribution to the sustenance of the population. Assess-
ments of habitat quality and importance (i.e., habitat
evaluation
) are thus
based on the presumption that preference, and hence selection, are linked to
fitness (reproduction and survival) and that preference can be gleaned from
patterns of observed use.
Use of habitat is generally considered to be selective if the animal makes
choices rather than wandering haphazardly through its environment. Typically,
the disproportionate use of a habitat compared to its availability is taken as
prima facie evidence of selection. Although technically resource
availability
encompasses accessibility and procurability (Hall et al. 1997), these attributes
are difficult to measure, so it seems reasonable to equate habitat availability with
abundance (typically measured in terms of area), as is normally done in habitat
selection studies. A habitat that is used more than its availability is considered
to be selected for. Conversely, a habitat that is used less than its availability is
often referred to as being
selected against,
or even
avoided.
This is poor termi-
nology, however, in that it suggests that the animal preferred not to be in that
habitat at all, but occasionally just ended up there. Use that is proportional to
availability is generally taken to be indicative of lack of selection, which is also
unfortunate terminology, as illustrated by the following examples.
Consider an animal living in an area with only two habitats and using each
in proportion to its availability; from this we might assume that the animal was
not exhibiting habitat selection. However, unless the animal was a very low life
form, it certainly made choices as to when it visited each habitat and what it
did when it got there; anytime it made a choice, and either stayed or moved, it
selected one habitat over the other. Arguably, if one analyzed these movements
on a short enough time scale, habitat use would be disproportionate to avail-
ability, enabling detection of habitat selection. As the time scale is shortened,
though, the sheer physical constraint of moving between the two habitats (i.e.,
the distance between them) also affects their relative use.
On the flip side, imagine a dispersing animal attempting to traverse an area
with no regard for habitat. If its route was frequently diverted by the presence
of other, more dominant resident animals, living in their presumably preferred
habitats, the disperser's movements would appear to reflect habitat selection
