Geoscience Reference
In-Depth Information
use and availability, habitats will not appear to be selected if their proportion-
ate use is ranked the same as their availability. Thus even if the animal spends
an inordinate amount of time in the habitat that is most available, selection for
this habitat will not be detected using this technique because both use and
availability are ranked the same.
The Manly-Chesson index of habitat selection also does not fluctuate with
inclusion or exclusion of seldom-used habitats, and Manly et al. (1993)
showed that this index is much more versatile than Johnson's in many other
respects. Recently, it was adapted by Arthur et al. (1996) to handle situations
in which habitat availability changes. These authors recognized that habitats
available to polar bears (
Ursus maritimus
) varied with changes in ice conditions
and with movements of bears across their enormous home ranges. Thus they
defined availability separately for each radiolocation, using the habitat compo-
sition of a circle with a radius (from the radiolocation) equal to the expected
distance a bear would travel during the time between radiolocations; habitat
availability within these circles was then compared with the type of habitat the
bear actually used the next time it was located.
Another attribute of Manly et al.'s (1993) procedure is that it can be used
to analyze data from site attribute studies as well as use-availability studies,
although site attribute studies also face problems in assessing availability. If
used sites are compared to random sites, the universe from which the random
sites are drawn must be defined. As discussed earlier, that universe can be some
arbitrarily defined study area, a composite home range of study animals, or
each individual home range. Additional difficulties may arise if the compari-
son is between used and unused sites because errors may arise in distinguish-
ing unused sites (i.e., nonobservation of use may not mean nonuse). Further-
more, unused sites may be vacant for a variety of reasons, some of which are
unrelated to the physical habitat (e.g., human disturbance, exploitation, pre-
dation, parasites, interspecific competition). Some predictive models have
fared poorly when they did not consider such variables (Diehl 1986; Laymon
and Barrett 1986; O'Neil and Carey 1986). Geffen et al. (1992) found, unex-
pectedly, that Blanford's foxes (
Vulpes cana
) in desert environments were rarely
observed near springs, where water and food were most abundant, probably
because this habitat was favored by and provided cover for potential predators.
In order to assess the criteria used by a species in selecting sites, investigators
ideally should choose for comparison sites with both available resources and
predators (or other confounding agents) present, as well as sites with only one
or the other; however, such comparisons are unavailable in most field studies.
If a species is very selective in its choice of sites, differences between used
