Geoscience Reference
In-Depth Information
Advantages and Problems of the Demographic Response Design
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The best measure of habitat quality would be a test of its effects on demo-
graphic parameters such as population growth and carrying capacity. Such
tests are extraordinarily difficult in most situations, as evidenced by the scant
published studies of this nature. Most demographic response studies I re-
viewed examined potential relationships between habitat and animal density.
Because habitat-specific density is actually a reflection of differential habitat
use, investigations of habitat-related density suffer the same drawbacks as stud-
ies of habitat use.
Density tends to be an ineffective measure of habitat quality because it may
fluctuate widely, is subject to sizable errors in estimation, and may be largely
influenced by social factors. Van Horne (1983) gave several examples of situa-
tions in which juveniles were restricted from settling in the best habitats and
thus accumulated in large numbers in poorer quality habitats. She indicated
that such circumstances are likely to be common among generalist species with
high reproductive rates and a social hierarchy. For these species in particular,
then, habitat-specific density would probably be a poor indicator of habitat
quality unless the population is well below carrying capacity. A good example
was provided by Messier et al. (1990), who showed that density of muskrats
(
Ondatra zibethicus
) during a general population increase swelled 30- to 90-
fold in low-quality habitats but much less in high-quality habitats.
Considering that, in general, animals in poor-quality habitats should be
trying to leave and ones in high-quality habitats trying to stay (and keep com-
petitors out), Winker et al. (1995) posited that turnover rate would be a better
index of habitat quality than density. They measured turnover rates for wood
thrushes (
Catharus mustelinus
) by examining recapture rates and telemetry
movement data; low-quality habitat was defined as that in which recapture
rates were low and many radiotagged birds were transient visitors. They found
density and habitat quality, assessed in terms of turnover rates, to be inversely
related. Notably, Winker et al.'s (1995) turnover rate model may not be appli-
cable to other species, even other territorial, noncolonial songbirds, some of
which are preferentially attracted to habitats occupied by conspecifics, which
they use as a cue to habitat quality (Muller et al. 1997). Conspecific attraction
tends to perpetuate use of the same areas across generations, so even if habitat
quality deteriorates, high densities may be maintained through tradition.
Other competing species or unidentified confounding variables also may
weaken the linkage between habitat quality and density. Maurer (1986) mea-
