Geoscience Reference
In-Depth Information
Manly et al.'s (1993) technique can handle both use-availability and site
attribute study designs. Moreover, it can be performed on an individual animal
basis or with pooled data, it can be used to compare habitat selection among
groups (e.g., species, sex-age classes, seasons, times of day, times within sea-
sons), and it can incorporate both discrete and continuous variables. For these
reasons, it has been heralded as a unified approach.
Manly et al.'s (1993) approach generates a resource selection probability
function, giving the probability of a site being used as a function of various
habitat variables. Each habitat variable can be tested to determine whether it
contributes significantly to the probability of use. In the special case of only a
single categorical habitat variable (i.e., habitat type), the function reduces to
the Manly-Chesson selection index (Manly et al. 1972; Chesson 1978).
An advantage of this index, as discussed earlier, is that it is rather unaffected
by the inclusion or exclusion of seldom-used habitats. In this sense, Chesson
(1983:1297) suggested that the index is a measure of preference that “does not
change with [resource] density unless [the animal's] behavior changes” and
that it represents the expected use of the various resources if all were equally
abundant. I think it is doubtful that this is true.
Consider first the simple example presented by Chesson (1978) to demon-
strate the intuitiveness of the Manly-Chesson technique. The example deals
with choice of foods, but I will adapt it for habitat selection. Suppose habitats
A and B are equally available, and an animal spends 25 percent of its time in
habitat A and 75 percent in habitat B (table 4.1). Because the Manly-Chesson
selection index represents the expected use when resources are equally available,
the index for each habitat in this case simply equals their proportional use (0.25
and 0.75 for A and B, respectively). Now suppose that the same animal is placed
in an area composed of 80 percent habitat C and 20 percent habitat B, and it
uses C 40 percent of the time and B 60 percent. The Manly-Chesson index
would be 0.14 for habitat C and 0.86 for habitat B (table 4.1), suggesting that
if habitats C and B had been equally available, they would have been used in
these proportions. Because both A and C were compared against the same stan-
dard (habitat B), the results indicate that A would be preferred to C if those two
types were offered together. However, given that the animal used A only 25 per-
cent of the time but C 40 percent of the time, when in both cases the other
choice was habitat B, the higher standardized selection index for A is not intu-
itive; these results are clearly a function of the higher availability of habitat C.
A
f atal flaw
of habitat selection studies in general, especially use-availabil-
ity studies, is that they are based on the assumption that the more available a
resource is, the more likely an animal should be to use it. This may not be true
