Geoscience Reference
In-Depth Information
tions, beyond their subjectivity, is that the cores so defined are not strictly tied
to intensity of use of space. In fact, animals that use their home ranges ran-
domly or in an even fashion have cores by these definitions. Samuel et al.
(1985) developed an objective method for identifying the maximum possible
core: all parts of the home range used more heavily than would result from
evenly distributed use. Although this definition is objective, it is still arbitrary,
it allows cores to incorporate space used little more than adjacent space, and it
defines cores for animals that use space randomly. Seaman and I (1990; Pow-
ell et al. 1997) introduced a technique for identifying cores that is objective,
not arbitrary, and that allows the animals themselves to define their cores. Our
technique is based on the logic used to identify behavior bouts (Fagen and
Young 1978; Slater and Lester 1982). Bingham and Noon (1997) used the
same method and Harris et al. (1990) may have, but their explanation is not
clear.
If an animal's use of space is random within its home range, a plot of home
range area at a certain percentage use versus probability of use should yield a
straight line going from the 100 percent home range at probability of use equal
to 0 (100 percent of the home range has probability of use 0 or above) to the
0 percent home range at probability of use greater than the maximum proba-
bility of use (0 percent of the home range has probability of use greater than
the greatest probability of use; figure 3.6a). If probability of use is transformed
to percentage of largest probability of use, then both
x
and
y
axes on the graph
range from 0 to 100 (figure 3.6a) and the descending line representing random
use has a slope of -1. If use of space by an animal is clumped, however, its
curve will sag below the line of random use (figure 3.6b) and if use of space is
even (all areas used with equal intensity), the graph will remain as a high
plateau from
x
equals 0 probability of use up to
x
equal some large probability
of use and then plummet (figure 3.6c).
When use of space by animals is clumped (figure 3.6b), Seaman and I
defined as an animal's core the areas in its home range that are used most inten-
sively. The parts of the home range mapped onto the steeply descending slope
of the area-probability curve along the
y
-axis are used least and constitute the
periphery of the home range. The parts of the home range mapped onto the
nearly horizontal slope along the
x
-axis are used most intensively and consti-
tute the core. The curve can be divided into two pieces at the point whose tan-
gent has a slope of -1, that is, whose tangent is parallel to the line for random
use. This is also the point that is furthest from the line with a slope of -1 (fig-
ure 3.6b). Plots of actual data may not yield smooth curves; these plots can be
fit with reasonable curves.
