Geoscience Reference
In-Depth Information
gated. For example, Coker and Capen (1995) examined cowbird (
Molothrus
ater
) selection for habitat patches of various size, shape, and location relative to
other habitats by entering these variables in a logistic regression with use (used
or not used) as the dependent variable. Similarly, Chapin et al. (1998) com-
pared habitat variables (including an index of the extent of habitat edge) in
grid cells of different sizes that were used (i.e., had at least one telemetry loca-
tion) by American martens with those in cells not used by martens, and also
compared characteristics of forest patches that were used and not used.
McLellan (1986) argued that observed use is a better indicator of habitat
selection than use relative to availability. He reasoned that an animal familiar
with its home range knows the availability and location of resources, so an ani-
mal's location at any given moment represents selection. He gave an example
of a person at a buffet selecting a slice of beef from a 500-kg steer and an equal-
sized slice of pork from a 100-kg pig; based on use alone, pork and beef were
selected equally, but compared to availability, pork appears to be selected over
beef, which is obviously absurd. However, had the steer and pig been cut up in
equal-sized chunks and distributed over a large area, and after considerable
searching the person still returned with an equal quantity of the two foods,
active selection for pork would indeed seem apparent. The key difference is that
in the latter case the person had to search for the food; selection was evidenced
by the extra effort expended in finding the pork (and apparently bypassing
chunks of beef ). This searching for resources is really the basis for the develop-
ment of use-availability comparisons and explains why it originated with stud-
ies of diet. In most cases animals do not know the location of all foods in their
home range, so dietary selection based on availability may be appropriate.
However, habitats are not spread around like chunks of pork and beef, but occur
in large patches, the locations of which are known by the animals; thus habitats
are probably more like McLellan's (1986) whole steer and whole pig than the
cut up chunks of meat spread randomly around (figure 4.3).
Consider some actual examples of how observed use and use versus avail-
ability can lead to disparate interpretations of selection. Prayurasiddhi (1997)
investigated use and selection among two large ungulates, gaur (
Bos gaurus
)
and banteng (
B. javanicus
), in Thailand. He differentiated two general study
area boundaries, one of which more closely matched the area that his radio-
collared animals used most intensively. He also used actual home range bound-
aries as a third representation of the study area and hence the available habitat.
He found that this variation in the area considered to be available habitat
resulted in drastic differences in perceived habitat selection (table 4.3). One
habitat that received 46 percent of use by gaur was deemed to be selected for,
