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2017 ; 114
(12
): 3252-3257
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Black-swan events in animal populations
#MMPMID28270622
Anderson SC
; Branch TA
; Cooper AB
; Dulvy NK
Proc Natl Acad Sci U S A
2017[Mar]; 114
(12
): 3252-3257
PMID28270622
show ga
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.
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