Brett A. Melbourne & Alan Hastings (2008)

**Extinction risk depends strongly on factors contributing to stochasticity**

Nature 454(7200): 100-103

*Key words*: extinction risk, stochasticity, demographic variance,
environmental variation, discrete temporal stochastic model

**Abstract**

Extinction risk in natural populations depends on stochastic factors that affect
individuals, and is estimated by incorporating such factors into stochastic models.
Stochasticity can be divided into four categories, which include the probabilistic
nature of birth and death at the level of individuals (demographic stochasticity),
variation in population-level birth and death rates among times or locations
(environmental stochasticity), the sex of individuals, and variation in vital rates
among individuals within a population (demographic heterogeneity). Mechanistic
stochastic models that include all of these factors have not previously been
developed to examine their combined effects on extinction risk. Here we derive a
family of stochastic Ricker models using different combinations of all these
stochastic factors, and show that extinction risk depends strongly on the
combination of factors that contribute to stochasticity. Furthermore, we show that
only with the full stochastic model can the relative importance of environmental
and demographic variability, and therefore extinction risk, be correctly determined.
Using the full model, we find that demographic sources of stochasticity are the
prominent cause of variability in a laboratory population of *Tribolium castaneum*
(red flour beetle), whereas using only the standard simpler models would lead to the
erroneous conclusion that environmental variability dominates. Our results
demonstrate that current estimates of extinction risk for natural populations could
be greatly underestimated because variability has been mistakenly attributed to the
environment rather than the demographic factors described here that entail much
higher extinction risk for the same variability level.