Dimensionless Life Histories and Effective Population Size

The effective size ( N _e ) of a population can be estimated from demographic information. We evaluated a recent model, showing that N _e depends strongly on the relationship between age at reproductive maturity ( M ) and average adult lifespan ( A ). N _e converges on half the number of potentially reproducing adults ( N/2 ) as M decreases relative to A , but it increases linearly as M increases for a given value of A . Therefore, convergence of N _e on N/2 is more likely in organisms with a short sexual maturation period scaled to adult lifespan. To assess the generality of this convergence we asked whether most organisms are characterized by this requisite relationship between M and A . The dimensionless number M/A is approximately invariant within taxa, but it is markedly different across taxa. Previous work focused on birds and mammals, taxa with unusually small M/A (0.4 and 0.75). Other animal taxa take longer than most birds and mammals to reach maturity for a given reproductive lifespan, so they are characterized by larger M/A (e.g., fish, 2.0). In theory, these taxon-specific life histories strongly influence N _e . We conclude that N _e is expected to approach N/2 , provided that M/A is (unusually) small, and that N _e / N among poikilotherms may often exceed that of mammals and especially birds.