Geoscience Reference
In-Depth Information
HOME RANGE CORE
Particular parts of an animal's home range must be more important than other
parts. In general, foods and other resources are patchily distributed (Curio
1976; Frafjord and Prestrud 1992; Goss-Custard 1977; Mitchell 1997; Powell
et al. 1997), so the parts of a home range with greater density of critical
resources ought, logically, to be more important than areas with few resources.
For years, biologists have conceived the core as the part of an animal's home
range that is most important to it (Burt 1943; Ewer 1968; Kaufmann 1962;
Samuel et al. 1985; Samuel and Green 1988). To understand home ranges
well, identifying cores is important, if cores do indeed exist.
My understanding of a home range core has two major parts (Powell et al.
1997; Seaman and Powell 1990). First, a core must be used more heavily than
the apparent clumps of heavy use that occur from uniform random use of
space within a home range. Random use of space leads to some areas being
used more than others, even though no place is more important to the animal
than any other. Therefore, random use of space leads to apparent clumps of use
in some places and little use of other places. Consequently, the core of a home
range must be used more than expected by random use, which means that for
a home range to have a core, use of space within that home range must be sta-
tistically clumped and not random or even. Testing for clumped versus ran-
dom (or for clumped versus even) use of space is usually straightforward
(Horner and Powell 1990; Mitchell 1997). In a uniform random distribution,
the mean equals the variance. If an animal uses space at random, then the
mean number of locations in each cell in its home range equals the variance. If
the variance is significantly greater than the mean, then use is clumped; if the
variance is less than the mean, then use is even. Note that many nonrandom
distributions have means equal to their variances. Therefore, equal mean and
variance does not prove uniform random use of space, but unequal mean and
variance does disprove random use of space.
Second, a core must not be strictly determined by home range area. Ani-
mals with home ranges of equal size but with different patterns of home range
use (e.g., central place foragers, strongly territorial animals, extensive wander-
ers) should have differently sized cores. Any technique developed to identify
the core of a home range must reflect this biological understanding of what a
core is.
Most definitions of cores have been ad hoc or subjective. Many define the
core as the smallest area with an arbitrary probability of use (e.g., the smallest
area enclosing 25 percent of total use). A crucial problem with such defini-
