Geoscience Reference
In-Depth Information
nothing about habitat importance or substitutability in terms of fitness. Cor-
respondingly, infrequent use may not be indicative of lack of suitability. A habi-
tat may be used infrequently because it serves little value, because its value can
be extracted in a short amount of time, because it is not readily available, or
because access is constrained by threats (social pressures, competition, preda-
tion) or physical barriers. A high use:availability ratio might suggest (correctly)
that such a habitat is more important than indicated by its infrequent use.
Studies of habitat use would benefit greatly from replication. Significant
insights might be gained from comparisons of habitat use and use:availability
among individuals, among groups of individuals in different portions of a
study area, among study areas, among time periods, and so on. If individuals
are disparate in their use or apparent selection of habitats (Holbrook et al.
1987; Ehlinger 1990; Donázar et al. 1993; Boitani et al. 1994; Macdonald and
Courtenay 1996), inferences regarding habitat quality become more equivo-
cal. In contrast, if the data are partitioned and the subsets show consistent pat-
terns (e.g., use:availability ratios for each habitat are similar among individuals
despite large differences in availability within individual home ranges) or if
curvilinear relationships between use and availability can be ascertained (figure
4.4), inferences are strengthened; even so, studies of this sort provide only a
superficial understanding of the effects of habitat on population dynamics.
Certainly no strong prescriptions for habitat manipulation are warranted from
interpretations of selection based solely on observed patterns of habitat use.
Site attribute studies tend to provide stronger inferences about habitat
selection. However, in identifying myriad habitat characteristics that are
apparently preferred, even for activities that impinge on the animal's survival
and reproduction, these studies still cannot assume that population growth
would be significantly higher in more “ideal” settings. Controlled experiments
can help sort out the factors important in the animal's selection processes
(Danell et al. 1991; Parrish 1995) and can thus provide a better understanding
of its behavior, but without corresponding demographic measurements, the
importance of various habitat components in terms of their contribution to
the animal's fitness cannot be appraised. I agree with Kirsch (1996:37-38):
“Unfortunately, proximate habitat features may not indicate habitat suitabil-
ity, nor do they reveal the possible selective pressures that influence habitat
selection in a system. One must measure components of fitness, determine fac-
tors that influence fitness, and relate fitness and factors influencing fitness to
habitats or habitat features.”
Demographic response studies are the only means of truly evaluating the
relative importance and suitability of habitats for supporting animal popula-
