Conservation of endangered species - Ecological Applications: Toward a Sustainable World - page 135

survival is 0.90 and feeding interval is 2 days) only 0.132% or 1 in 757 carcasses are
needed. Proportions of vultures found dead or dying in the wild with signs of
diclofenac poisoning were similar to the proportions of deaths expected from
the model if the observed population decline was due entirely to diclofenac
poisoning.
The researchers recommended urgent action to prevent the exposure to livestock
carcasses contaminated with diclofenac and we can note, for example, that the
Punjab government has now banned its use. Green and his colleagues also high-
lighted the need for research to identify alternative drugs that are effective in live-
stock and safe for vultures. Clearly, on the one hand, the banning of a veterinary
option can be expected to have consequences for the agricultural economy. On the
other hand, the unmeasured economic benefi t of ecosystem services provided by
vultures is likely to be many times as great as the economic loss of banning this
particular drug. The social cost of vulture decline is the heightened disease risk
associated with slow decomposition of large mammal carcasses, while the cultural
cost to Parsees has been the disruption of funeral rites (Section 5.1). Finally, given
the depths to which the vulture populations have sunk, Green's team point to the
importance of holding and breeding vultures in captivity until diclofenac is under
control. This is a sensible precaution to ensure the long-term survival of the threat-
ened species and to provide for future reintroduction programs.

Summary Population dynamics

The individual organisms of a single species are collectively known as a population.
When confronted with a declining population, managers need to call on population
dynamics theory, which seeks to understand what determines population size and
the way this varies through time. A population's dynamics depend on the interplay
of processes that increase (birth and immigration) or decrease population size
(de at h and emig rat ion). Some processes act on rates of birth, death and migration
in a density-dependent manner (bigger percentage effect at higher density). However,
the abundance we observe is determined by the combined effects of all the processes
that affect the population, whether they are dependent or independent of density.

The population dynamics of small populations
Many species are naturally rare, but others have had rarity thrust upon them by
human actions. The probability of their extinction is enhanced because of features
of the population dynamics of small populations. The behavior of small populations
is governed by a high level of uncer t a i nt y, where a s l a rge populations can be described
as being governed by the law of averages. Three kinds of uncertainty are of particular
importance to small populations: demographic uncertainty (e.g. random var i at ion s
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), environmental uncertainty (unpredict able
changes in environmental factors such as storms, fl oods or droughts) and spatial
uncertainty (t he p atchy dy n a m ic s of extinction and local recolonization of popula-
tions that exist as a number of relatively discrete subpopulations).
Using an understanding of population dynamics, managers often need to decide
what constitutes the 'minimum viable population'. There are three basic approaches:
(i) a simple correlational approach to identify easily measured factors that are cor-
related with extinction risk; (ii) the use of general algebraic population models when