7842

• UCREFRP Catalog Number: 7842
• Author: Knopf, F. L.
• Title: Biological Diversity in Wildlife Management.
• USFW Year: 1992.
• USFW - Doc Type: 1992.
• Copyright Material: NO

100 or 1,000 years with a 95-percent probability. Population sizes ranged from
hundreds to millions of individuals, with corresponding minimum area requirements
of tens to millions of mi'-. As body size increased, the minimum viable populations
decreased. The disturbing result was that those small populations of larger bodied
species still required larger areas than their smaller bodied counterparts.
The minimum viable population is a useful concept only in that it provides non-
biologists who may be in positions of influence, such as politicians, with a single
number of individuals needed for a population to persist. As is the case for most
oversimplifications, there is danger in the misuse of minimum viable population
because there is no universal population number for a species. Also, a single number
diverts attention from the mechanistic processes accounting for population persistence
or extinction, and places the focus on the final outcome or product. It may be more
productive to analyze the population processes that result in the minimum viable
population rather than estimate this single number. This mechanistic approach has
been taken by Gilpin and Soule (1986), which they refer to as population vulnerability
analysis.
Metapopulation Dynamics and Population Persistence
In our discussion of minimum viable population, we ignored the effects of spatial
structuring on population persistence. Metapopulation dynamics provides a frame-
work for analyzing the persistence of species inhabiting patchy environments and
should prove useful in elucidating the conservation implications of fragmentation.
Following Levins (1980), Hanski and Gilpin (1991) defined a metapopulation as "a
set of local populations which interact via individuals moving among populations."
Most models of metapopulations incorporate local extinctions followed by recolon-
ization of individuals dispersing from extant populations (Holt 1985, Pulliam 1988).
Several generalities about metapopulation extinction emerge from simple diffusion
models (Harrison and Quinn 1989, Hanski 1989). Metapopulations may go extinct
when: (1) habitat patches are small, leading to low population density; (2) the number
of habitat patches is decreased, thereby increasing population isolation and decreasing
dispersal; (3) the population dynamics in different patches are correlated, leading to
a correlation of extinction probabilities.
Spatial heterogeneity in the environment may cause differences in habitat quality
among populations within a metapopulation. Populations in higher quality habitats
may contain a surplus of animals that might disperse to neighboring populations.
Thus, populations may persist in low quality habitats due to the colonization of
individuals from higher quality habitats. Holt (1985) and Pulliam (1988) described
populations that produce a surplus of dispersing animals as "sources," and popu-
lations in suboptimal habitat maintained by dispersal as "sinks."
Fragmented landscapes containing an array of different patch sizes may lead to
sources/sink population dynamics. For most species, there should be a minimum
patch size below which a population cannot persist without immigration. In the
following case study, we report on our experiments investigating source/sink pop-
ulation structure in a small mammal community and how this structure affects small
mammal biological diversity.
Population Processes ? 255