Ontogenetic patterns of density-dependent mortality: contrasting stability effects in populations with adult dominance

Several stage-structured population models are considered in which fecundity, mortality, and maturation to the next stage are allowed to depend on stage and within-stage age, and in which adults are assumed to be the dominant consumers of limiting resources. Local stability effects of changing the strength of regulation about a fixed equilibrium point are examined. The main result is that shifting the principal target of strongly density-dependent mortality from adults to progressively earlier phases of juvenile development reverses the stability effect of such mortality from locally stabilizing to destabilizing. This result holds for all the models studied and therefore appears to be fairly robust to the pattern of age-dependence in adult fecundity and the shape of the juvenile period’s durational distribution. A simple intuitive interpretation is developed which can account for these and several related results in the literature. The necessity of distinguishing between life-historical delays and regulatory delays is also discussed.