Geoscience Reference
In-Depth Information
tural lands, when in reality turkeys used agricultural lands extensively, but only
those near hardwood forests. In essence, the turkeys viewed the edge between
field and forest as a separate habitat type. Similarly, Neu et al. (1974) posited
that moose might feed preferentially in a recent burn, but not too far from the
surrounding forest. Thus they defined four habitat types—the interior of the
burn, the burn periphery, the forest edge adjoining the burn, and the remain-
der of the forest—and through a simple chi-square analysis showed selection
for the edge ( just inside or just outside the burn). Most situations probably are
not this simple.
Many authors have admitted to the importance, but difficulty, of incorpo-
rating spatial aspects of habitats in use-availability analyses. Porter and Church
(1987) proposed a method whereby the study area is gridded into cells and an
assortment of habitat variables within those cells are examined through multi-
variate analyses to find those that best explain differential use of cells. Litvaitis
et al. (1986) did just that in a study of bobcats (
Felis rufus
), which predated the
paper by Porter and Church (1987). Litvaitis et al. (1986) looked for associa-
tions (using regression and
DFA
) between the number of radiolocations within
25-ha cells inside home ranges and measurements of several habitat variables
sampled there; however, they found that these habitat variables poorly ex-
plained variation in frequency of use. Servheen and Lyon (1989) used a similar
approach in assessing habitat selection by caribou (
Rangifer tarandus
). They
measured habitat variables in 40-ha circles around telemetry locations and
sought to find those that best differentiated the areas that the animals used sea-
sonally. Although they had no real measure of juxtaposition or interspersion of
habitats, their 40-ha circles contained habitats neighboring the one actually
occupied, so the composition of these circles gave an indication of habitat com-
binations that corresponded with seasonal use. In another similar approach,
Clark et al. (1993) used grid cells that could encompass several habitat types
near the locations of radiocollared black bears. A suite of habitat characteristics
(including the number of different habitat types) within each cell used by bears
were combined to form what they called an ideal habitat profile. The habitat
quality of each cell in the study area was then assessed by comparing it to this
hypothetical ideal cell. Each of these studies looked at differential use, rather
than use in terms of availability, and thus avoided the fatal flaw of habitat selec-
tion studies.
Site attribute studies are like the habitat use studies just discussed, except
that instead of comparing cells with varying degrees of use, they categorize
cells (sites) simply as used or unused; based on this, important habitat variables
are identified. Interspersion and juxtaposition of habitats can thus be investi-
