title: Black-swan events in animal populations.
abstract: Black swans are improbable events that nonetheless occur-often with profound consequences. Such events drive important transitions in social systems (e.g., banking collapses) and physical systems (e.g., earthquakes), and yet it remains unclear the extent to which ecological population numbers buffer or suffer from such extremes. Here, we estimate the prevalence and direction of black-swan events (heavy-tailed process noise) in 609 animal populations after accounting for population dynamics (productivity, density dependence, and typical stochasticity). We find strong evidence for black-swan events in [Formula: see text]4% of populations. These events occur most frequently for birds (7%), mammals (5%), and insects (3%) and are not explained by any life-history covariates but tend to be driven by external perturbations such as climate, severe winters, predators, parasites, or the combined effect of multiple factors. Black-swan events manifest primarily as population die-offs and crashes (86%) rather than unexpected increases, and ignoring heavy-tailed process noise leads to an underestimate in the magnitude of population crashes. We suggest modelers consider heavy-tailed, downward-skewed probability distributions, such as the skewed Student [Formula: see text] used here, when making forecasts of population abundance. Our results demonstrate the importance of both modeling heavy-tailed downward events in populations, and developing conservation strategies that are robust to ecological surprises.
authors.affiliation: Earth to Ocean Research Group, Department of Biological Sciences, Simon Fraser University, Burnaby, BC V5A 1S6, Canada; sandrsn@uw.edu.|School of Aquatic and Fishery Sciences, University of Washington, Seattle, WA 98195.
authors.forename: Sean C
authors.identifier: 0000-0001-9563-1937
authors.initials: SC
authors.lastname: Anderson
authors.affiliation: School of Aquatic and Fishery Sciences, University of Washington, Seattle, WA 98195.
authors.forename: Trevor A
authors.initials: TA
authors.lastname: Branch
authors.affiliation: School of Resource and Environmental Management, Simon Fraser University, Burnaby, BC V5A 1S6, Canada.
authors.forename: Andrew B
authors.initials: AB
authors.lastname: Cooper
authors.affiliation: Earth to Ocean Research Group, Department of Biological Sciences, Simon Fraser University, Burnaby, BC V5A 1S6, Canada.
authors.forename: Nicholas K
authors.identifier: 0000-0002-4295-9725
authors.initials: NK
authors.lastname: Dulvy
mesh_terms: D000818:Animals; D046090:Anseriformes; D008322:Mammals; D008962:Models, Theoretical; D011157:Population Dynamics
keywords: die-offs; ecological risk; ecological surprises; mass mortality; population dynamics