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1990), their home range selection is negatively influenced by conspecific den-
sity. Other competing species have a further interacting effect on habitat avail-
ability and hence selection (Ovadia and Abramsky 1995). Because competi-
tion changes each animal's perception of habitat availability, human
measurement of availability, based on the assumption of free and equal access,
is inevitably inaccurate. As a result, animals tend to be more uniformly dis-
tributed across patchy landscapes than predicted from studies of habitat selec-
tion (Kennedy and Gray 1993).
Another major problem in measuring habitat availability is the recognition
and treatment of areas of nonhabitat that may exist within home ranges. Part
of the difficulty arises simply because our concept of home range is too nebu-
lous. Home range is typically defined as the area used by an animal for its nor-
mal activities (generally attributed to Burt 1943), but home range area is a
human perception, not a biological entity. Humans may perceive the land-
scape as a mosaic of habitats that fit together like a jigsaw puzzle, on which are
superimposed home ranges of animals. In contrast, animals may perceive the
landscape as series of corridors or islands sprinkled in an ocean of nonhabitat.
If we unwittingly define available habitat from our human perspective, and
include large patches of nonhabitat that the animal does not really perceive as
among its choices of places to live, a comparison of use to availability might
demonstrate nothing more than avoidance of the nonhabitat. This would be
grossly accurate, but not particularly insightful. An example was presented by
Johnson (1980), where mallards (
Anas platyrhynchos
) rarely used open water
areas far from shore, but the area of open water was large. Standard means of
comparing use to availability, such as the chi-square test, might show open
water to be avoided and all other habitats selected; however, a knowledgeable
duck biologist would recognize this as a trivial result, and might elect to
exclude this obvious nonhabitat from the analysis. Other cases may not be so
clear-cut (figure 4.1). Manly et al. (1993:45-46) presented an example with
California quail (
Callipepla californica
), taken from a study by Stinnett and
Klebenow (1986). Bonferroni confidence limits, and hence perceptions of
selection, depended on whether habitats that were not used as escape cover
when the birds paired for mating were included or excluded from the analysis.
In this case the habitats that were not used as escape cover during mating were
not obvious nonhabitats because the birds used them in other circumstances
and for other activities.
An advantage of Johnson's (1980) technique is that the results are rather
robust to inclusion or exclusion of habitat types that are rarely used. A prob-
lem with Johnson's (1980) technique is that because it is based on rankings of
