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Download paper Released 2013-03-26

Whales reveal population dynamic laws

The population dynamics of large whales is explained by a selection induced growth rate acceleration

Are population cycles an exception caused by stronger than usual interactions between prey and predators, or is single species dynamics inherently cyclic? This has been debated for decades, with no general answer. Yet, the question reflects a more fundamental puzzle: Is it really possible to determine the growth rate of a population as a function of the environment (as assumed in density regulated growth, in predator-prey models, and by most biologists for more than a century), or is it only possible to determine the acceleration?

In a 2013 paper in Population Ecology I tried to resolve the issue from the largest population dynamic experiment on Earth; the worldwide over-exploitation of large whales by commercial whaling in past centuries. Modelling the dynamics of seven whale species it was found that density regulated growth was unable to reconcile past exploitation with current growth. Whales are either delayed in their recovery and/or they are increasing at a higher rate than expected for the current density of whales. The problem could not be resolved by inverse density regulation at low densities (depensation), and nor by predator-prey interaction with the underlying resources, or with the only real predator - the killer whale.

Density regulated growth assumes that it is possible to determine a fixed growth rate for fixed environmental conditions, but growth rates - even at fixed environmental conditions - are constantly evolving from natural selection by density dependent interactive competition. This leads to selection-delayed dynamics, and when applied to the seven species there was no longer a conflict between the historical catches and the current rates of increase. This suggests that selection induced changes in natural populations are sufficient to change density regulated growth into acceleration driven dynamic.

What was not determined, however, is the proximate mechanism behind the acceleration. Are the growth rate responses to natural selection entirely due to genetic changes between generations or, more likely, do they also include behavioural responses and epigenetic changes imposed by maternal effects, amongst others?

The widespread occurrence of selection-delayed dynamics beyond whale species is indicated by cyclic changes in the life histories of species with cyclic population dynamics. Such changes in traits like body mass seem to be the rule rather than the exception and, quite generally, they are expected only when a species respond to selection by density dependent interactive competition.