Frequency content of environmental variability and extinction risk of age-structured populations: Chinook salmon (Oncorhynchus tschawytscha) as an example

Abstract

Investigations into the influence of environmental variability on population extinction have assessed the role of noise “color” (e.g., “white” or “reddened”), but the influence of spectral sensitivity of age-structured populations on extinction risk is not well understood. Recent findings indicate that age-structured populations are more sensitive to environmental variability that occurs on time scales near their mean age of spawning and on long time scales, particularly when populations are at low abundance. This raises the question of how variability at those sensitive time scales would influence extinction risk. We used an age-structured population model with density-dependent recruitment based on the life history of spring-run Chinook salmon, as an example, to investigate how the spectrum of environmental variability influences (1) variability of spawning female abundance, (2) the probability of quasi-extinction, and (3) the distribution of times to quasi-extinction. Population simulations were run with different spectra to investigate the influence of: (a) low frequencies, (b) frequencies equal to the inverse of the mean age of spawning (generational time scales), (c) both of these at once, and (d) “reddened” white noise. Results showed that even though variability on generational time scales causes greater population variability than other time scales, by itself, it does not increase probability of quasi-extinction. However, environmental variability at the other peak in sensitivity at low frequencies increased extinction in a way similar to the reddened signal. Environmental variability distributed over both sensitive frequency bands (i.e., low frequency and cohort frequencies) actually had less effect on extinction than low frequency variability alone.