Geoscience Reference
In-Depth Information
sured density and various habitat characteristics for five species of grassland
birds; the habitat models developed to explain species-specific density in one
study area were inexplicably poor predictors of density in a nearby area with
similar habitat. Kellner et al. (1992) found that density was positively related
to reproductive success in only 7 of 17 bird studies that they reviewed; more
studies showed a negative relationship. Sherry and Holmes (1996) felt that
density should be relied on as an indicator of habitat quality only if it is cor-
roborated by other data, as was the case in their study, in which population
density and weight loss of wintering migrant birds were correlated (high
weight loss in areas with low densities) and both were related to habitat type.
Reproduction and survival data may be more apt to reflect real influences
of habitat on demographics. However, reproduction and survival are probably
also tied to habitat in a complex manner. For example, a number of studies
observed a direct relationship between cover and the survival (and thus den-
sity) of voles (
Microtus
spp.), but a lower threshold exists below which reduc-
tions in cover have little effect on vole density; above the threshold, survival
and density increase but eventually reach an upper asymptote (Birney et al.
1976; Adler and Wilson 1989; Peles and Barrett 1996; figure 4.2C). The vole
studies found that cover provides food as well as protection from predators,
and also may affect microclimate, activity patterns, and interactions among
conspecifics, all of which affect the cover-density relationship. Moreover, male
and female voles have different responses to varying cover (Ostfeld et al. 1985;
Ostfeld and Klosterman 1986), and cover-demographic relationships tend to
be different for other small grassland rodents (Kotler et al. 1988). Each of the
various habitat components that relate to an animal's fitness probably has
thresholds, asymptotes, and inflection points, and these limits may vary with
the mix, shape, size, and juxtaposition of habitat components available; how-
ever, few attempts have been made to assess any of these factors individually
(Harper et al. 1993; Whitcomb et al. 1996), let alone in combination.
Several studies also found that density-dependent effects may reduce repro-
duction or survival independent of habitat quality (Kaminski and Gluesing
1987; Clark and Kroeker 1993; Clark 1994) or may even result in higher fit-
ness in low-quality, less crowded habitats (Pierotti 1982; Fernandez 1999).
Zimmerman (1982) found that nesting success of dickcissels (
Spiza americana
)
did not differ between habitats and was unrelated to density, but females nested
preferentially in the habitat preferred by males; males chose this more hetero-
geneous habitat because they could sequester more nest sites and thus mate with
more females. In other studies, reproduction and survival were found to be
unrelated to measured habitat variables, despite evidence of habitat selection,
