Geoscience Reference
In-Depth Information
A major inherent but generally unstated (maybe unrecognized) assump-
tion of habitat suitability models is that high-quality habitats (i.e., habitats
that confer high fitness) are in fact suitable (i.e., able to sustain a population;
Kellner et al. 1992). Explicit tests of this assumption are rarely conducted, yet
counterexamples exist. In one case, a habitat suitability model for Florida
scrub-jays (
Aphelocoma coerulescens
) correlated well with demographic per-
formance (reproduction, survival, and density), but most of the area was found
to be a population sink where mortality exceeded reproduction (Breininger et
al. 1998). Apparently, as birds competed to occupy the best habitats, they were
less alert to predators, and thus suffered high mortality. In another example,
Kirsch (1996) found that interior least terns (
Sterna antillarum,
least terns
nesting in noncoastal areas) selected high-quality nesting habitats, but possibly
because of disturbance, their productivity was not sufficient to maintain pop-
ulation size (i.e., the nesting habitats were unsuitable). Lomolino and Chan-
nell (1995, 1998) observed that remnant populations of endangered mammals
often occur near the periphery of their former ranges; because the periphery of
the range represents the edge of suitable habitat, studies of habitat suitability
of endangered mammals based on habitat use in existing populations are likely
to be misleading. These are not gratifying results.
It has not been for lack of effort, expense, or analytical developments that
relationships between habitats and population growth often elude detection. It
is simply the complexity of the interactions between animals and their envi-
ronment that make such relationships deceptively difficult to understand. A
case in point is the spotted owl, which has undergone intense scrutiny because
of its threatened status and apparent proclivity for old-growth forest in a
region where the economy is tied largely to timber; despite a plethora of habi-
tat studies, significant debate persists among ecologists as to the critical habi-
tat requirements of this species, and why it prefers old-growth forest (Forsman
et al. 1984; Carey et al. 1992; Rosenberg et al. 1994; Carey 1995).
I am not suggesting that ecologists have not been creative in their efforts.
However, substantive flaws in the most commonly used techniques for study-
ing wildlife-habitat relationships apparently have not been widely recognized.
I believe it is time to reconsider the ways these techniques are used in evaluat-
ing habitat quality.
Studies of the use of habitat have merit, but also many limitations. Habitats
in which an animal spends a large proportion of its time are clearly selected
among others available. Even if widely available, frequently used habitats are
certainly not “selected against” or “avoided” (except maybe among life forms
that have no memory of where they were). However, frequent use suggests
