Geoscience Reference
In-Depth Information
els, and data used to estimate a distribution are sequential locations of an indi-
vidual animal and may not be independent observations of the true distribu-
tion (Gautestad and Mysterud 1993, 1995; Gautestad et al. 1998; Seaman and
Powell 1996; Swihart and Slade 1985a, 1985b). However, lack of indepen-
dence of data may not be a great problem for some analyses (Andersen and
Rongstad 1989; Gese et al. 1990; Lair 1987; Powell 1987; Reynolds and Laun-
drè 1990). After all, data that are not statistically autocorrelated are nonethe-
less biologically autocorrelated because animals use knowledge of their home
ranges to determine future movements. Boulanger and White (1990), Harris
et al. (1990), Powell et al. (1997), Seaman and Powell (1996), and White and
Garrott (1990) reviewed many home range estimators and Larkin and Halkin
(1994) summarized computer software packages for home range estimators.
GRIDS
To avoid assuming that data fit some underlying distribution (for example,
that an animal's use of space is bivariate normal in nature), many researchers
superimpose a grid on their study areas and represent a home range as the cells
in the grid having an animal's locations (Horner and Powell 1990; Zoellick
and Smith 1992). Each cell can have a spike as high as the number or propor-
tion of times the animal was known or estimated to have been within that cell
(figure 3.3) and the resultant surface is an estimate of the animal's utility dis-
tribution. For small sample sizes of animal locations, or for finely scaled grids,
a home range can be estimated to have several disjunct sections (see especially
figure 3.3b). The resident animal traversed the areas between the disjunct sec-
tions too rapidly, or the interval between locations was too long, for the animal
to be found in intervening cells. These areas were not used for occasional sal-
lies and therefore should probably be included within the animal's home
range. One can include in the home range all cells between sequential loca-
tions, but no objective method exists to incorporate these cells into the esti-
mated utility distribution. If possible, one should collect data until the animal
has been found at least once in each cell connecting formerly disjunct loca-
tions. Using this approach to estimate home ranges, a researcher risks not
including significant areas in an animal's home range.
Doncaster and Macdonald (1991) estimated the home ranges of foxes
(
Vulpes vulpes
) as a retrospective count of the grid cells known to be visited at
any one time. This approach is equivalent to treating the cells as marked indi-
viduals for a mark-recapture study and estimating home range size (popula-
tion size of the cells) from a minimum number known alive approach (Krebs
