Geoscience Reference
In-Depth Information
lower viability over a narrow range in the presence of a fatal disease transmit-
ted by contact. He demonstrated the possibilities with a model, but had no
data to support his case. However, the point he made seems biologically
sound, and the issue can be resolved only by optimizing persistence between
these two opposing forces.
Spatial variation, that is, variation in habitat quality across the landscape,
affects population persistence. Typically, extinction and metapopulation theo-
ries emphasize that stochastic fluctuations in local populations cause extinc-
tion and that local extinctions generate empty habitat patches that are then
available for recolonization. Metapopulation persistence depends on the bal-
ance of extinction and colonization in a static environment (Hanski 1996;
Hanski et al. 1996). For many rare and declining species, Thomas (1994)
argued that extinction is usually the deterministic consequence of the local
environment becoming unsuitable (through habitat loss or modification,
introduction of a predator, etc.); that the local environment usually remains
unsuitable following local extinction, so extinctions only rarely generate
empty patches of suitable habitat; and that colonization usually follows
improvement of the local environment for a particular species (or long-dis-
tance transfer by humans). Thus persistence depends predominantly on
whether organisms are able to track the shifting spatial mosaic of suitable envi-
ronmental conditions or on maintenance of good conditions locally.
Foley (1994) used a model to agree that populations with higher repro-
ductive rates are more persistent. However, mammals with larger body size can
persist at lower densities (Silva and Downing 1994) and typically have lower
annual and per capita reproductive rates. Predicted minimal density decreases
as the -0.68 power of body mass, probably because of less variance in repro-
duction relative to life span in larger-bodied species.
The last item on the list—that environmental conditions that reduce car-
rying capacity or increase variance in the growth rates of populations decrease
persistence probabilities—suggests that increased variation over time leads to
lower persistence (Shaffer 1987; Lande 1988, 1993). One reason that in-
creased temporal variation causes lowered persistence is that catastrophes such
as hurricanes, fires, or floods are more likely to occur in systems with high tem-
poral variation. Populations in the wet tropics can apparently sustain them-
selves at densities much lower than those in temperate climates, probably
because of less environmental variation. The distinction between a catastrophe
and a large temporal variance component is arbitrary, and on a continuum
(Caughley 1994). Furthermore, even predictable effects can have an impact.
Beissinger (1995) modeled the effects of periodic environmental fluctuations
