Geoscience Reference
In-Depth Information
standing
why
an animal has chosen to live where it has, estimators are needed
that provide more complex pictures. An animal's cognitive map will have
incorporated into it the importance to the animal of different areas. The most
commonly used index of that importance is the amount of time the animal
spends in the different areas in its home range. For some animals, however,
small areas within their home ranges may be critically important but not used
for long periods of time, such as water holes. No standard approach exists to
weight use of space by a researcher's understanding of importance. Therefore,
to estimate importance to an animal of different areas of its home range, using
any home range estimator currently available, one must assume that impor-
tance is positively associated with length or frequency of use, which are mea-
sures of time.
UTILITY DISTRIBUTIONS
From location data such as those shown in figure 3.2, most home range esti-
mators produce a utility distribution describing the intensity of use of differ-
ent areas by an animal. The utility distribution is a concept borrowed from
economics. A function, the utility function, assigns a value (the utility, which
can be some measure of importance) to each possible outcome (the outcome
of a decision, such as the inclusion of a place within an animal's home range;
Ellner and Real 1989). If the utility distribution maps intensity of use, then it
can be transformed to a probability density function that describes the proba-
bility of an animal being in any part of its home range (Calhoun and Casby
1958; Hayne 1949; Jennrich and Turner 1969; White and Garrott 1990; van
Winkle 1975), as shown in figure 3.2. Utility distributions need not be prob-
ability density functions, although they usually are. A utility distribution
could map the fitness an animal gains from each place in its home range, or it
could map something else of importance to a researcher.
The approach using a utility distribution as a probability density function
provides one objective way to define an animal's normal activities. A probabil-
ity level criterion can be used to eliminate Burt's (1943) occasional sallies.
Including in an animal's home range the area in which it is estimated to have a
100 percent probability of having spent time would include occasional sallies.
Including only, say, the smallest area in which the animal spent 95 percent of
its time could exclude occasional sallies or areas the animal will never visit
again. Using a utility distribution, one can arbitrarily but operationally define
the home range as the smallest area that accounts for a specified proportion of
the total use. Most biologists use 0.95 (i.e., 95 percent) as their arbitrary but
