Selection‐delayed population dynamics in baleen whales and beyond

Abstract

AbstractWhile it is known that population cycles are driven by delayed density‐dependent feedbacks, the search for a common feedback mechanism in natural populations with cyclic dynamics has remained unresolved for almost a century. To identify the existence and cause of delayed feedbacks I apply six age‐ and sex‐structured population dynamics models to seven species of baleen whales (suborder Mysticeti) that were heavily depleted by past commercial whaling. The six models include a predator–prey model with killer whale (Orcinus orca) as the predator, and five singe‐species models based on (1) exponential growth, (2) density‐regulated growth, (3) density‐regulated growth with depensation, (4) delayed density‐regulated growth and (5) selection‐delayed dynamics. The latter model has a density‐regulated growth rate that is accelerated and decelerated by the intra‐specific natural selection that arises from the density‐dependent competitive interactions between the individuals in the population. Essential parameters are estimated by a Bayesian statistical framework, and it is shown that baleen whales have a delayed recovery relative to density‐regulated growth. The time‐lag is not explained by depensation, or by interactions with prey or predators. It is instead resolved by a selection‐delayed acceleration of the intrinsic growth rate. The results are discussed in relation to the literature on cyclic dynamics, and it is noted (1) that selection‐delayed dynamics is both theoretically and empirically sufficient for cyclic population dynamics, (2) that it is widespread in natural populations owing to the widespread occurrence of otherwise unexplained phenotypic cycles in populations with cyclic dynamics, and (3) that there is a lack of empirical evidence showing that predator–prey interactions is a sufficient cause for the cyclic dynamics of natural populations. The conclusion stresses the importance of intra‐specific delays in cyclic dynamics, and suggests that it is the acceleration of the growth rate, and not the growth rate itself, that is determined by the density‐dependent environment.