numbers even further. Their effect increases with declining population size, modified
further by the sex and age structure of the population. The probability of population
extinction over a specified time span can sometimes be estimated using population
viability analysis (PVA). There are many different ways to develop PVAs, some of
which derive from diffusion approximation to stochastic population growth.
Extinction due to demographic or environmental stochasticity is less common than
habitat modification or the introduction, usually by people, of a new element into
the environment. This is commonly a new predator, competitor, or pathogen.
Sometimes the new factor is simply an unsustainable level of harvesting by humans.
The population is then driven to extinction rather than dropping out by chance. These
processes can be incorporated in a variety of modeling frameworks that usefully aug-
ment and extend the PVA approach.
brent
(Brent)
#1