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

population would occur as a result of genetic drift. The increase in genetic hom-
ozygosity could then reduce individual survival and reproduction.
Population biologists have known for some time that the smaller the population,
the more susceptible it is to extinction (Shaffer 1981). However, what is required
by managers is a precise way to relate population size to the probability of extinction.
Attempts to understand and predict the relationship of population size to extinction
have spawned a burgeoning literature on the minimum viable population concept.
The basic premise of the minimum viable population concept is that a threshold
population density must be maintained for a population to persist. Shaffer (1981:132)
defined minimum viable population for any given species in any given habitat as
"the smallest isolated population having a 99 percent chance of remaining extant
for 1,000 years despite the foreseeable effects of demographic, environmental and
genetic stochasticity, and natural catastrophes."
Three approaches have been used to estimate minimum viable population sizes
and related area requirements: observational, experimental and theoretical. The ob-
servational approach examines biogeographic patterns of abundance and distribution
across a species' range. If populations occur in habitat patches of different sizes,
one can estimate the smallest patch inhabited by a species and the percent of the
patches of a certain size supporting that species. Additional information that is needed
to estimate minimum viable population size, but is most difficult to obtain, is species-
specific colonization and extinction rates for different-sized habitat patches. There
are three critical assumptions in this biogeographic approach: (1) communities are
at equilibrium in different patches; (2) population characteristics of a species are
solely a function of patch size and do not vary in different parts of its range; and
(3) there are no systematic differences in other patch attributes as a function of patch
size.
In the experimental approach, minimum viable population size and area require-
ments are assessed by creating patches of different sizes and monitoring population
parameters within them. For instance, Lovejoy et al. (1984, 1985, 1986) have studied
the rate of disappearance of populations in 2.4-acre (1 ha) and 24-acre (10 ha) reserves
in a Brazilian rain forest. Fragment size determined the persistence rates of different
tropical species. Other researchers also have found that fragmentation affects per-
sistence rates in a variety of species (Quinn et al. 1989, Paine 1989, Bergen 1990)
and distributions of various-sized animals (Bennett 1991). We are conducting an
experiment on habitat fragmentation on a small mammal community in eastern Kan-
sas, which we discuss below. The major drawback of the experimental approach is
that it requires long-term monitoring of populations. Unfortunately, the results of
these experimental studies often will be too late to be of use due to the high rate of
habitat destruction. Furthermore, such studies are impractical for many species and
ecosystems.
Theoretical models have been developed to predict the probability that a population
of a given size will go extinct and the time to extinction. Goodman (1987) used a
classical birth-and-death process model incorporating environmental fluctuations to
predict persistence times of different-sized populations. Persistence time strongly
depended on the magnitude of the variance in population growth rate. Belovsky
(1987) used Goodman's model to calculate the population size needed for mammalian
species, that ranged in body mass from 2 ounces to 99 tons (10' to 106 g), to persist
254 ? Trans. 57' N. A. Wildl. & Nat. Res. Conf. (1992)