Conservation of endangered species - Ecological Applications: Toward a Sustainable World - page 136

detailed population information is lacking; and best of all (iii) the use of specifi c
population viability analyses, involving simulation models designed for particular
species at risk. Population viability analyses appear to produce good predictions of
population performance, but the results need to be used cautiously. They are not so
much aimed at precise estimation of extinction probability or the predicted time to
extinction, but to allow managers to compare the likely outcomes of alternative
management scenarios.

The population genetics of small populations
Populations vary in the amount of genetic variation they contain, and in small
populations this can have profound implications for their persistence. Genetic vari-
ation in a population is determined by the joint action of natural selection (where
the frequency of a particular form of a gene in a population is related to the evolu-
tionary advantage it confers) and genetic drift (where t he frequency is determined
simply by chance). The infl uence of genetic drift is greater in small isolated popula-
tions, which as a consequence are expected to lose genetic variation. This may prove
to be a problem because rare genetic forms that confer no immediate advantage
might turn out to be well suited to changed environmental conditions in the future.
A more immediate potential problem is inbreeding depression . When populations are
small there is a tendency for related individuals to breed with one another, increas-
ing the likelihood of expression of harmful forms of genes.
The objectives of managers, whether they are concerned with captive rearing
programs or translocations of individuals from vulnerable to safe habitats, are
straightforward - maximize genetic diversity and minimize the risks of inbreeding
depression. This usually involves a process of genetic mixing, based on knowledge
of relatedness among the breeding population, to reduce inbreeding effects and thus
increase the likelihood of persistence of the population.
A benefi t of modern molecular technology is that it can be used to uncover pat-
terns of genetic variation within a species, identifying evolutionary signifi cant units
(possibly subspecies) that are worthy of special conservation focus.

The extinction vortex
There may be an interaction between the population dynamics and population
genetics of declining populations. Thus, a population may be reduced to a very small
size by habitat loss, pollution, disease or overexploitation, leading to an increased
frequency of matings amongst relatives and the expression of deleterious forms of
genes in offspring, leading to reduced survivorship and birth rates, and causing the
population to become smaller still - the so-called extinction vortex.

The fi nal word

It's a great relief to Jijanji (Section 5.1) that the reason for the vulture collapse in
India is now understood and that action has begun to be taken. It has occurred to
Jijanji that it might be helpful if the economic value of an ecosystem service, like
that performed by vultures, could actually be calculated. ' Wouldn't this enable our
government to view the wonderful service provided by vultures in the wider context of
economic activities that put the service at risk ?'
Check out the website http://ecovalue.uvm.edu/evp/doc_economic_methods.asp
to discover the various ways that ecosystem services might have a value put on them.