Geoscience Reference
In-Depth Information
These examples demonstrate cases in which the activity requires a fixed
amount of time, so increasing availability of the preferred setting for that activ-
ity has no effect on how much time is spent there. This situation is just a spe-
cial case demonstrating the point that use and availability are not inexorably
linked. In the example of the house, the renovated kitchen might entice the
person to spend more time there, but only up to a point (one certainly would
not sleep there). Conversely, if the dining room had been remodeled at the
expense of room in the kitchen, the person might not eat in the kitchen any-
more, but, no matter how small it was, still prepare food there. Each room
might thus have its own functional relationship between area and use. Simi-
larly, if an animal prefers a certain habitat for resting because it offers protec-
tion from predators, it might spend more time resting in a larger patch of that
habitat because it offers greater safety than a small patch. Enlarging a patch
that offers virtually no predator protection to a size yielding some predator
protection might thus cause significantly increased use of the patch; however,
additional enlargements might have progressively lesser effects on use because
they do not add much predator protection, and eventually further enlarge-
ments do nothing, or might even attract a different predator, thus deterring
use. Various scenarios and corresponding relationships between patch size and
use are plausible (figure 4.2). Considering that the relationship between patch
size and use probably varies among habitat types and the mathematical rela-
tionship between use and availability also differs among the various selection
indices (e.g., Manly-Chesson, Ivlev, and others; Lechowicz 1982), it seems
doubtful that one could assess selection just by comparing relative use to the
relative area of different habitats.
Mysterud and Ims (1998) proposed a logistic regression model to compare
use:availability ratios among study subjects that had differing habitat compo-
sitions available to them. This model thus provides a test of the assumption
that use increases with increased habitat availability. Their method is applica-
ble to cases in which habitats can be categorized into two discrete types (e.g.,
forested vs. nonforested, oak vs. nonoak). They reexamined two data sets that
Aebischer et al. (1993b) had analyzed using compositional analysis. In one, use
increased with increased availability of a habitat for 9 of 12 ring-necked pheas-
ants (
Phasianus colchicus
); however, three individuals did not fit this trend. In
the second example, gray squirrels (
Sciurus carolinensis
) showed an inverse rela-
tionship between use and availability of open habitats within their home
ranges (the same unexpected relationship therefore existed for the alternate,
forested habitat). It was surmised that size and interspersion of habitat patches
greatly affected the choices that these animals made, more so than just total
