Population Viability Analysis
303
PROCESS VARIATION
The combined effect of demographic, temporal, spatial, and individual variation is called process varisation. That is, each of these sources of variation
affects population processes. Process variation is used as a general term for the
inherent stochasticity of changes in the population level. Process variation is in
contrast to sampling variation, which is the variation contributed when biologists attempt to measure population processes. That is, researchers are unable
to measure the exact survival rate of a population. Rather, they observe realizations of the process, but not the exact value. Even if the fate of every animal
in the population is observed, the resulting estimate of survival is only an estimate of the true but unknown population survival rate. The concept of sampling variation is explained later in this chapter, where methods of separating
sampling variation from process variation are developed.
Several lessons should be learned from this simple exercise. Persistence is a
stochastic phenomenon. Even though the expected outcome for a particular
model is to persist, random variation prevents this outcome from always
occurring. Small populations are much more likely to go extinct than larger
populations because of demographic variation. Increased temporal variation
results in decreased persistence. Increased individual variation results in
increased persistence.
Components of a PVA
As demonstrated earlier in this chapter, many factors affect the persistence of a
population. What components are needed to provide estimates of the probability that a population will go extinct, and what are the tradeoffs if not all
these components are available?
• A basic population model is needed. A recognized mechanism of population regulation, density dependence, should be incorporated because no population can grow indefinitely. “Of course, exponential growth models are
strictly unrealistic on time scales necessary to explore extinction probabilities”
(Boyce 1992:489). The population cannot be allowed to grow indefinitely, or
persistence will be overestimated. Furthermore, as discussed later in this chapter, the shape of the relationship between density and survival and reproduction can affect persistence, and density dependence cannot be neglected for
moderate or large populations (Ludwig 1996b). Density dependence can provide a stabilizing influence that increases persistence in small populations.
303
PROCESS VARIATION
The combined effect of demographic, temporal, spatial, and individual variation is called process varisation. That is, each of these sources of variation
affects population processes. Process variation is used as a general term for the
inherent stochasticity of changes in the population level. Process variation is in
contrast to sampling variation, which is the variation contributed when biologists attempt to measure population processes. That is, researchers are unable
to measure the exact survival rate of a population. Rather, they observe realizations of the process, but not the exact value. Even if the fate of every animal
in the population is observed, the resulting estimate of survival is only an estimate of the true but unknown population survival rate. The concept of sampling variation is explained later in this chapter, where methods of separating
sampling variation from process variation are developed.
Several lessons should be learned from this simple exercise. Persistence is a
stochastic phenomenon. Even though the expected outcome for a particular
model is to persist, random variation prevents this outcome from always
occurring. Small populations are much more likely to go extinct than larger
populations because of demographic variation. Increased temporal variation
results in decreased persistence. Increased individual variation results in
increased persistence.
Components of a PVA
As demonstrated earlier in this chapter, many factors affect the persistence of a
population. What components are needed to provide estimates of the probability that a population will go extinct, and what are the tradeoffs if not all
these components are available?
• A basic population model is needed. A recognized mechanism of population regulation, density dependence, should be incorporated because no population can grow indefinitely. “Of course, exponential growth models are
strictly unrealistic on time scales necessary to explore extinction probabilities”
(Boyce 1992:489). The population cannot be allowed to grow indefinitely, or
persistence will be overestimated. Furthermore, as discussed later in this chapter, the shape of the relationship between density and survival and reproduction can affect persistence, and density dependence cannot be neglected for
moderate or large populations (Ludwig 1996b). Density dependence can provide a stabilizing influence that increases persistence in small populations.