Geoscience Reference
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habitat area. Similarly, Mysterud and Ostbye (1995) found that although roe
deer (
Capreolus capreolus
) in winter chose open canopy habitat for feeding and
dense canopy for resting, they had to balance the advantages of being in each
type of habitat against the energetic disadvantages of traveling between them,
so patch size (distance between patches) affected habitat selection. Mysterud et
al. (1999) suggested that for animals such as roe deer, which face tradeoffs in
using different habitats, selection is not directly related to resource availability,
so habitat rankings based simply on ratios of use to availability often are mis-
leading. Bowyer et al. (1998) used a site attribute analysis to examine habitat
selection related to various tradeoffs faced by black-tailed deer (
Odocoileus
hemionus
).
Assessing selection can be extraordinarily complex because each habitat is
not a single patch, but a series of patches of different sizes and shapes, each
bordering other patches of different sizes, shapes, and habitat types. Otis
(1997) offered a model that tests for the disproportionate use of habitat types
as well as habitat patches, thereby providing a means of assessing things such
as minimum patch size requirements. Data for this model (patch size distribu-
tions for each habitat type and locations of animals in specific patches) are
available with modern geographic information system (
GIS
) coverages. This
model still does not take into account habitat interspersion and juxtaposition,
which probably have significant effects on selection for many species. For
example, Porter and Church (1987) found that a standard use-availability
analysis of habitat selection by wild turkeys indicated an avoidance of agricul-
Figure 4.2
(opposite page) Hypothetical relationships between area and use of habitat. Use-
availability studies assume that habitat use increases linearly with area of available habitat. This is
unlikely to be the case in many situations. (A) Relationship between use and size of a patch used
mainly for foraging. A relationship like the one depicted might occur if different habitats offer differ-
ent foods; the animal increases foraging time with increased availability of one habitat type, but this
relationship asymptotes when the animal obtains enough of the food there and searches for alterna-
tive foods in other habitats. The same sort of relationship might occur for an animal that forages
mainly near the edge of the patch, if size (x-axis) is in units of area but use increases with the perime-
ter. (B) Relationship between use and size of a patch used primarily for cover. In this case a very
small patch offers virtually no benefit, so it is not used at all; use increases with increasing patch size,
but then declines when the patch becomes large enough to attract another type of predator. (C)
Relationship between density (a reflection of use) and cover (which in this case provides protection
from predators, is used for food, and influences microclimatic conditions) that was shown (and partly
hypothesized) for voles (Microtusspp.) (Birney at al. 1976). At low levels of cover, the area is occu-
pied only by transients searching for a better place to live. The first threshold represents the point at
which cover is adequate to attract residents. The second threshold represents a level of cover suffi-
cient to enable the population to surge and eventually cycle. Although this second threshold was
shown empirically, it is not well understood.
