18 Conservation in practice
In Chapter 17 we examined the ways in which demographics and genetics contribute,
at least potentially, to the risk that a population will go extinct. The extinction of a
species does not differ in kind. The species goes extinct because the last population
of that species goes extinct. Here we review actual extinctions or near extinctions to
show what are the commonest causes of extinction in practice. We then describe how
to detect such problems and how to treat a population in danger.Extinctions may be divided into two categories, driven extinctions and stochastic
extinctions.
1 Driven extinction: whereby a population’s environment changes to its detriment and
rate of increase falls below zero. The population declines. Perhaps this lowering of
density frees up resources to some extent, or lowers the rate of predation, but this
is not sufficient to counteract the force of the driving variable, and the population
finally goes extinct. Included in this category are extinctions caused by environmental
fluctuation and extinctions caused by catastrophes. The latter are viewed here as
simply large environmental fluctuations.
2 Stochastic extinction: whereby a population fails to solve the “small population prob-
lem.” The effect of chance events, which would be trivial when numbers are high,
can have important and sometimes terminal consequences when numbers are low.
(a)Extinction by demographic malfunction: whereby a population goes extinct by
accident (chance) because it is so small that its dynamics are determined critically
by the fortunes of individuals rather than by the law of averages. In those cir-
cumstances a population is quite capable, by chance configuration of age distri-
bution or sex ratio, of registering a steep decline to extinction over a couple of
years even though its schedules of mortality and fecundity (see Sections 6.3 and
6.4) would result in an increase if the age distribution were stable.
(b)Extinction by genetic malfunction: whereby a population at low numbers for sev-
eral generations loses heterozygosity to the extent that recessive semi-lethals are
exposed, average fitness therefore drops, and the population declines even further
and ultimately to extinction. The loss of an allele from the genotype is an event
resulting from the lottery of random mating. Although each individual loss is unpre-
dictable, the average rate of loss, as a function of population size, can be predicted
fairly accurately.
These mechanisms do not exclude each other entirely but they are sufficiently
distinct that we treat them separately. Although the relative contribution of these
mechanisms is unknown, enough anecdotal information is available to suggest that
the driven extinction is by far the most prevalent. Extinction by demographic312