Geoscience Reference
In-Depth Information
1993; Loeb 1993; Nadeau et al. 1995), wintering areas (Nixon et al. 1988), or
areas recolonized by an expanding population (Hacker and Coblentz 1993).
Other studies have compared habitat characteristics of randomly located sites
to sites where birds or mammals were observed, radiolocated, or known to have
been from remaining sign (Dunn and Braun 1986; Krausman and Leopold
1986; Beier and Barrett 1987; Edge et al. 1987; Lehmkuhl and Raphael 1993;
Flores and Eddleman 1995).
The statistical procedures used in such studies vary. Most have used multi-
variate analyses to differentiate combinations of variables that tend to be asso-
ciated with the used sites. Discriminant function analysis (
DFA
) is the most
popular of these. Logistic regression is an alternative, and is especially useful
when the data consist of both discrete and continuous variables (Capen et al.
1986) or are related to site occupancy in a nonlinear fashion (Brennan et al.
1986; Nadeau et al. 1995).
DEMOGRAPHIC RESPONSE DESIGN
Ideally, studies should identify relationships between habitat characteristics and
the animal's fitness. Studies employing use-availability and site attribute
designs assume that certain habitat features are selected because they improve
fitness. Demographic response designs attempt to test this more directly. How-
ever, although I refer to the measured demographic parameters in these studies
as response variables, they really only represent correlates with given habitats.
I identified 39 studies among those that I reviewed (25 percent) that mea-
sured an association between a demographic parameter and habitat (note that
percentages for the three designs total more than 100 percent because some
studies used more than one design). Most of these investigated differences in
animal density among habitats. Fourteen studies, all on birds, related repro-
duction (i.e., nesting success) to habitat of nest sites. Three studies, two on
birds and one on mammals, attempted to find an association between habitat
and survival (Hines 1987; Klinger et al. 1989; Loegering and Fraser 1995), but
only one (Loegering and Fraser 1995) detected such a relationship.
Problems with Use-Availability and Site Attribute Designs
j
DEFINING HABITATS
The first prerequisite for assessing habitat selection is that habitats be defined
as discrete entities. For use-availability studies in particular, the defined num-
