All species populations are subjected to two broad classes of process: deterministic and stochastic. What are the relative contributions of these processes to the extinction risk of species populations? This is a fundamental question in ecology, and also a practical question for sustainable management and conservation. Some answers to this question are provided in our new theoretical study led by Tak and published in the journal Ecological Complexity.
We used a suite of population models to partition species extinction risk according to deterministic processes and two main types of stochastic process—demographic variance and temporal environmental variance. We found that if the intrinsic growth rate of a species population is moderately or far below zero, then deterministic processes are the dominant driver of species extinction risk, even when the population is small. This contradicts the intuition that demographic variance is always a dominant driver of species extinction risk for small populations. However, if the intrinsic growth rate of a species population is only slightly below zero, then stochastic processes are the dominant driver of species extinction risk, with demographic variance being the main driver for small populations and temporal environmental variance being the main driver for moderate to large populations, consistent with established theory. A surprising finding was that the effects of environmental and demographic variance on extinction times were sub-additive, i.e., the influence of the two combined was substantially less than the sum of their influences in isolation.
Mean time to extinction for species populations with different initial abundances, under different combinations of deterministic and stochastic processes. In panel (A), the intrinsic growth rate is far below zero, such that deterministic processes are the dominant driver of species extinction risk. In panel (B), the intrinsic growth rate is close to zero, such that stochastic processes are the dominant driver of species extinction risk.
Chisholm Lab 