Journal Article

Black-swan events in animal populations

Anderson, Sean C., Cooper, Andrew B., Branch, Trevor A., Dulvy, Nicholas K.

2017

Overview

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 ∼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 t 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.

• Publisher: National Academy of Sciences
• Subject: Animal populations / Animals / Anseriformes / Aquatic ecology / Biological Sciences / Climate science / Density dependence / Earthquakes / Life history / Mammals / Models, Theoretical / Parasites / Population Biology / Population Dynamics / Population forecasting / Predation / Predators / Risk factors / Seismic activity / Wildfowl / Wildlife conservation