Geoscience Reference
In-Depth Information
mice seemed not to select (based on a use-availability study) from among three
types of croplands (macrohabitats), but within each of these croplands they
chose microhabitats with a high abundance of certain plants (Tew et al. 2000;
Todd et al. 2000).
In sum, significant challenges in defining habitats include: partitioning
them in terms of the features that the animals are selecting for, which are not
necessarily the ones we most easily discern; delineating sufficient habitat cate-
gories to ensure that the truly important types are not lumped with and thus
diluted by less important types; and not diminishing the power to discern
selection by parceling out too many types.
MEASURING HABITAT USE
Sample bias is an obvious potential problem in measuring habitat use. Inter-
pretations of habitat use from visual observations of animals or their sign can
vary among observers (Schooley and McLaughlin 1992) and sightability can
vary among types of habitats (e.g., because of differing vegetative density; Neu
et al. 1974), both of which can introduce biases in the data. For example, Pow-
ell (1994) noted that fisher (
Martes pennanti
) tracks in snow were difficult to
follow in habitats with dense vegetation, especially where fishers followed trails
of snowshoe hares (
Lepus americanus
); in this case the bias against observing
tracks in dense vegetation merely detracted from the overall conclusion that
densely vegetated habitats were frequently used.
Counts of pellet groups (e.g., from ungulates or lagomorphs) may poorly
reflect habitat use because defecation rates often vary with the food source, and
hence the habitat type (Collins and Urness 1981, 1984; Andersen et al. 1992).
Capture locations may be a poor indicator of habitat use because baits and
other trap odors (e.g., from captures of other animals) may affect behaviors in
an unpredictable way (Douglass 1989).
Telemetry also may yield biased data on habitat use because the detection
of an animal's radio signal may depend on the habitat it is in (e.g.,
GPS
collars;
Moen et al. 1996), and location data obtained by triangulation have inherent
associated errors. Intuitively, and as shown in computer simulations by White
and Garrott (1986), errors in determining habitat use increase with increased
habitat complexity and decreased precision in the telemetry system. Errors do
not necessarily introduce bias, but can if patch size differs among habitats
(detected use would be underrepresented in habitat types that tend to occur as
small patches) or if the animal preferentially used the edge of some habitat
types but not others. Powell (1994) reported different perceptions of habitat
