Conservation of endangered species - Ecological Applications: Toward a Sustainable World - page 115

reductions in fertility, survivorship, growth rates and resistance to disease. Evidence is also accu-
mulating of a deleterious effect of inbreeding depression in wild populations, although this is not
invariably the case. For example, there is some evidence that island populations, which may be
naturally small, can persist despite high levels of inbreeding. Ironically, frequent inbreeding may
allow the more lethal of recessive deleterious alleles to be exposed to natural selection so that they
are 'purged' from the population.
It has been suggested that an effective population size above 50 individuals would be unlikely
to suffer from inbreeding depression, whilst 500-1000 might be needed to maintain high genetic
diversity and thus long-term evolutionary potential (Franklin & Frankham, 1998). But such rules
of thumb should be applied cautiously. Conservation management action should ideally be based
on specifi c genetic information. But the objectives are usually straightforward, whether managers
are concerned with captive rearing programs or translocations of individuals from vulnerable to
safe habitats - maximize genetic diversity and minimize the risks of inbreeding depression. You
will see in Section 5.5 how conservation managers set out to achieve these objectives.

The relative importance of genetic and demographic risks for small populations
Despite the genetic risks, no example of extinction due to genetic problems has been reported.
Perhaps inbreeding depression has occurred undetected as part of the 'death rattle' of some declin-
ing populations (Caughley, 1994). Thus, a population may have been reduced to a very small size
by one or more of the processes illustrated in Figure 5.2, leading to an increased frequency of
matings among relatives and the expression of deleterious alleles in offspring, leading to reduced
survivorship and fecundity, and causing the population to become smaller still - the so-called
extinction vortex.
Another reason for the dearth of examples of genetics-related extinctions could be that the
potential pathways to extinction (demographic or genetic) operate at contrasting rates. Habitat
loss or the introduction of an exotic predator might, through demographic changes, drive a popula-
tion extinct within years. By contrast, severe inbreeding problems might conceivably lead (or con-
tribute) to extinction within a few generations. Then again, any effect of loss of genetic diversity,
via a reduction in the ability to adapt to changed circumstances, might take decades or centuries
to become obvious - if and when a dramatic change in environmental circumstances takes place.
Thus, genetic problems in small populations follow a very long and winding road (Jamieson, in
press), while demographic problems are more akin to a high-speed crash on the freeway. But this
does not mean that we can afford to ignore the warnings from population genetics theory.

• Environmental variation
• Catastrophic events

More
genetic
drift: loss
of ability
to adapt

Population
more
subdivided
by
fragmentation

More
inbreeding
depression

More
demographic
variation

Lower
population
size ( N a )

Extinction

• Habitat destruction
• Environmental degradation
• Habitat fragmentation
• Overharvesting
• Effects of exotic species

Fig. 5.2 Extinction vortices may progressively lower population sizes leading inexorably to
extinction. (After Primack 1993.)