More from the Proceedings of the National Academy of Sciences
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Apparently sudden population declines happen more frequently than a Gaussian distribution would predict.
These are "black swan events"
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The logic seems to be that there are lots of different ways that things can go wrong, and no limit to the number of individuals in the population who can end up dead.
Sudden population increases can happen with equally improbably causes, but there is a limit to the rate of increase in population per breeding season.