Dimensionless life histories and effective population size

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The effective size (N(c)) of a population can be estimated from demographic information. We evaluated a recent model, showing that N(c) depends strongly on the relationship between age at reproductive maturity (M) and average adult lifespan (A). N(c) 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, convergrence of N(c) 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(c). We conclude that N(c) is expected to approach N/2, provided that M/A is (unusually) small, and that N(c)/N among poikilotherms may often exceed that of mammals and especially birds.

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Conservation Biology