Geoscience Reference
In-Depth Information
Pooling individuals is common because sample sizes are typically too small
to test for selection by individual. However, the statistical tests usually used
assume independence among sample units, which is often not the case in stud-
ies that consider each location a sample. Some techniques ( Johnson 1980;
Aebischer et al. 1993a, 1993b; Manly et al. 1993) consider animals as sample
units, so lack of independence among locations within individuals is not prob-
lematic. However, these methods are still subject to difficulties with lack of
independence if animals are gregarious (attracted to the same habitats because
they are attracted to each other; e.g., bed sites of deer; Gilbert and Bateman
1983) or territorial (social exclusion precludes use of certain habitats), or if the
study subjects are related (habitat preferences possibly affected by a common
learning experience) or are from the same social group (group leaders dictate
habitat use for all).
In an effort to alleviate the problem of a lack of independence among indi-
viduals, Neu et al. (1974) used groups of moose (
Alces alces
) and Schaefer and
Messier (1995) used herds of muskoxen as their sample units, rather than indi-
vidual animals. Similarly, although Gionfriddo and Krausman (1986) moni-
tored habitat use of individual radiocollared mountain sheep (
Ovis canadensis
),
they considered groups of sheep their sample unit. However, Millspaugh et al.
(1998) contend that animals in a herd should be considered independent indi-
viduals if they congregate because of a resource rather than because of a bio-
logical dependence on each other. They provide a hypothetical example with
elk, where 99 of 100 radiotagged animals congregated at a winter feeding area
in one habitat and the remaining individual used a second habitat; at other
times of the year the elk did not associate with each other. In this case, they
argue that each radiotagged individual should be considered an independent
sample. In contrast, predators that hunt together in a pack and are thus
dependent on one another cannot be considered to use habitats indepen-
dently. Millspaugh et al. (1998) recommend tests to evaluate independence of
habitat use by seemingly associated individuals.
MEASURING HABITAT AVAILABILITY
Measuring habitat availability is often more problematic than measuring use.
Use-availability studies inherently assume that study animals have free and
equal access to all habitats considered to be available. That is, at any given
moment each study animal should be able to use any available habitat. This
assumption may hold if use and availability are measured for each animal indi-
vidually. However, the assumption may be violated when animals are pooled
