Conservation of endangered species - Ecological Applications: Toward a Sustainable World - page 110

and pollution. I will return to the likely causes of the demise of Jijanji's vultures in
Section 5.6.
It should be noted that many, probably most, species on earth are naturally rare.
In the absence of human interference there is no reason to expect that the rarer
types would be substantially more at risk of extinction. However, while some species
are born rare, others have rarity thrust upon them. The actions of humans have
undoubtedly reduced the abundance and range of many species (including naturally
rare species) and the probability of their extinction may be enhanced for reasons
related to the population dynamics of small populations (Box 5.1) and/or their
population genetics (Box 5.2).
Given the environmental circumstances and life-history characteristics of a
species of concern, what is the chance it will go extinct in a specifi ed period?
Alternatively, how big must its population be to reduce the chance of extinction to
an acceptable level? These are frequently the crunch questions in conservation
management.
So how are managers to decide what constitutes the 'minimum viable population'?
Three approaches will be highlighted in this chapter: a simple correlational approach
that seeks to identify easily measured factors that are correlated with extinction risk
(Section 5.2); the use of general algebraic population models when detailed popula-
tion information is lacking (Section 5.3); and the use of specifi c population viability
analyses, involving simulation models designed for particular species at risk
(Section 5.4). All the approaches have their limitations, which I will explore by
looking at particular examples. But fi rst it should be noted that attempts to defi ne
the minimum viable population, because of inherent constraints, are not so much
aimed at the precise estimation of extinction probability or the predicted time to
extinction, but to allow managers to compare the likely outcomes of alternative
management scenarios. In Section 5.5, I discuss how our knowledge of the genetics
of small populations can assist in conservation efforts. Then in Section 5.6, I broaden
the scope to consider not only the ecological aspects of population viability, but also
the economic and social aspects of actions based on the ecological theory.

Box 5.1 Population
dynamics theory 1

Some basics
In simple terms, the way that the number of individuals changes in a population is given by:

N now = N then + B − D + I − E

In other words, the number now ( N now ) equals the number there previously ( N then ; a year ago, say),
plus the number of births between then and now ( B ), minus the number of deaths ( D ), plus the
number of immigrants ( I ), minus the number of emigrants ( E ). Each of these components may be
expressed as absolute numbers or as densities (numbers per unit area). Note also that the pro-
cesses that add ( B , I ) or subtract individuals from the population ( D , E ) may usefully be expressed
as rates (per head of population per year).
Put another way:

N now = λ N then

Where λ is the fundamental net per capita rate of increase . Clearly, the population will increase

when 2, for example, means that on average every

individual in the population will give rise to two individuals in the next generation (either by produc-

λ > 1, and decrease when λ < 1. A value of λ =