Conservation of endangered species - Ecological Applications: Toward a Sustainable World - page 127

Fig. 5.9 Levels of threat
as a function of time
and probability of
extinction. Note that
the time axis is an a log
scale. (After Akçakaya,
et al., 1999. Reproduced
with permission of
Applied
Biomathematics .)

1.0

0.8

Critically
endangered

0.6

Endangered

0.4

Vulnerable

0.2

Lower
risk

0.0

1 2 5 10 20 50 100 200 500

Time to extinction (years)

the Conservation of Nature!) From the point of view of extinction probability a
species is categorized vulnerable if there is considered to be a 10% probability of
extinction within 100 years, endangered if the probability is 20% within 20 years or
fi ve generations, whichever is longer, and critically endangered if within 10 years or
three generations the risk of extinction is at least 50% (Rodrigues et al., 2006)
(Figure 5.9). However, because assessment of extinction risk is only one of the Red
List criteria, the correlation between population viability predictions and Red List
classes is relatively weak (but positive - O'Grady et al., 2004). Moreover, population
viability models have only been used for a minority of species recognized to be at
risk. Morris et al. (2002) believe that the value of population modeling needs to be
more widely recognized by managers. They recommend that conservation agencies
routinely use population viability analyses in their future recovery planning.

5.5 Conservation
genetics

You saw in Box 5.2 that small populations can be expected to have lower genetic
diversity so that their ability to adapt to environmental change may be compromised
over the long term. This may provide another reason, beyond the demographic
vagaries discussed in Section 5.4, for an increased risk of extinction when popula-
tions have been driven to small numbers. In the shorter term, the genetic risk with
the greatest potential to ease a small population over the edge to oblivion is inbreed-
ing depression. In Sections 5.5.1-5.5.3 I present case studies of a mammal, a bird
and a plant, all of which depend on a process of genetic mixing to reduce inbreeding
effects and thus increase the likelihood of persistence of the population. In Section
5.5.4 you will see how molecular technology can be used to uncover patterns of
genetic variation within a species, identifying evolutionary signifi cant units (ESUs)
that are worthy of conservation effort. Now the focus is on keeping genetically dis-
tinct populations apart because they represent meaningfully distinct genetic units
that could generate new species.