Geoscience Reference
In-Depth Information
and unused sites may be quite subtle; these subtleties would not be discernible
in site attribute studies if the investigator chose unused or random sites that
were very different from the used sites. The scale of comparison in this case
would be too coarse. In an attempt to circumvent this difficulty, Capen et al.
(1986) eliminated available sites in habitat types that were “radically different”
from those that were used (analogous to eliminating nonhabitats in use-avail-
ability studies). Conversely, if in attempting to use a finer scale of comparison
one picked random sites from too narrow a universe, such that they were all
very similar to the used sites, habitat differences might not be detected if a
large portion of the random sites were used. This points out the advantage of
distinguishing unused sites instead of just random sites and of selecting
unused sites that are similar in many respects to the used sites.
Use-availability studies do not distinguish unused areas and so may be
especially prone to problems of too fine or too coarse a scale of comparison.
The coarse-scale problem (used and available areas are too dissimilar to detect
the true basis for selection) may occur when composition of home ranges or
habitat use within home ranges is compared to some broader study area. The
fine-scale problem (available area is too similar to the used area to detect dif-
ferences) may occur when habitat use is compared to availability within home
ranges. Thus these two scales of comparison may yield different results (Kil-
bride et al. 1992; Aebischer et al. 1993b; Boitani et al. 1994; Carroll et al.
1995; Paragi et al. 1996; MacCracken et al. 1997). McClean et al. (1998)
examined the effects of varying the definition of available habitat, from the
entire study area to progressively smaller-sized circles around individual radio-
locations. They found that selection became increasingly difficult to detect as
availability was defined by a smaller and smaller area. This result is not sur-
prising because the radiolocation represents use, so habitat composition
within smaller areas around each location more closely matches areas of actual
use.
ASSESSING HABITAT SELECTION: FATAL FLAW #1
Perceived habitat selection may vary with the technique chosen to compare use
and availability or to compare attributes of used and unused (or available)
sites. Some of this variation in perceived selection stems from the fact that dif-
ferent methods actually test different biological hypotheses (Alldredge and
Ratti 1986, 1992; McClean 1998) and some is from the different assumptions
inherent in these techniques and their sensitivity to violation of these assump-
tions (Thomas and Taylor 1990; Aebischer et al. 1993b; Manly et al. 1993).
