Conservation of endangered species - Ecological Applications: Toward a Sustainable World - page 113

Uncertainty and the risk of extinction - the population dynamics of small populations
While the general theory of population dynamics has focused on central tendencies such as mean
population size, species at risk of extinction are almost always rare and more weight needs to be
placed on variability and the prediction of extreme values (such as extinction). The dynamics of
small populations are governed by a high level of uncertainty, whereas large populations can be
described as being governed by the law of averages (Caughley, 1994). Three kinds of uncertainty
are of particular importance to the fate of small populations.
1 Demographic uncertainty: random variations in the number of individuals that are born male or
female, or in the number that happen to die or reproduce in a given year, or in the genetic quality
of offspring in terms of survival/reproductive capacities can matter very much to the fate of small
populations. Suppose a breeding pair produces a clutch consisting entirely of males - such an
event would go unnoticed in a large population but would be the last straw for a species down to
its last pair.
2 Environmental uncertainty: unpredictable changes in environmental factors, whether large scale
(such as fl oods, storms or droughts of a magnitude that occurs very rarely) or small scale (year
to year variation in average temperature or rainfall), can also seal the fate of a small population.
Even where the average rainfall of an area is known accurately from historical records, we cannot
predict whether next year will be average or extreme, nor whether we are in for a number of years
of particularly wet conditions. A small population is more likely than a large one to be reduced by
adverse conditions to zero (extinction), or to numbers so low that recovery is impossible
(quasi-extinction).
3 Spatial uncertainty: many species consist of subpopulations that occur in more or less discrete
patches of habitat. Since the subpopulations are likely to differ in terms of demographic uncertainty,
and the patches they occupy in terms of environmental uncertainty, the patch dynamics of extinc-
tion and local recolonization can be expected to have a large infl uence on the chance of extinction
of the population as a whole (the so-called 'metapopulation').
Thus, chance events play a particularly large role for small populations. As a result, the models
that have been developed to predict their behavior often feature important elements of random
change or stochasticity (expressed as variation in reproduction and survival). This will become
clear in the various examples discussed later in the chapter.
One thing to note before proceeding is that the extinction of a population does not necessarily
signal the extinction of a species (global extinction). Global extinction occurs when the last surviv-
ing local population goes extinct.

5.2 Assessing
extinction risk from
correlational data

When dealing with species at risk and in the absence of detailed demographic data
(birth and death rates, etc.), a simple approach might be to search for correlations
between environmental factors and presence/absence or species density. If individu-
als are associated with habitat X but not habitats Y or Z, managers might decide to
protect or enhance habitat X. This is a risky assumption, however, because habitat
use of a species in decline may not equate to optimal niche conditions, as you saw
in Section 2.3.2 for a New Zealand bird, the takahe. A better approach would be to
look for correlations between environmental factors and population decline rather
than current distribution. The corncrake Crex crex , a bird from eastern Africa that
breeds in summer in the UK, has been declining for more than a century. Stowe
et al. (1993) identifi ed sites occupied by corncrakes during censuses in 1978-79 that
had retained or had lost singing males by 1988. Their analysis showed that locations
retaining corncrakes were hay meadows with tall vegetation. It seems that an agri-
cultural switch to early-season mowing had been responsible for reduced breeding
success. Management recommendations have since been made to reduce the impact
of mowing operations and some populations have started to recover (Green &
Gibbons, 2000).