Seed banks, salmon, and sleeping genes: Effective population size in semelparous, age-structured species with fluctuating abundance

Previous studies reached contrasting conclusions regarding how fluctuations in abundance affect N-e in semelparous species with variable age at maturity: that N-e is determined by the arithmetic mean N among the T years within a generation (N-e approximate to T(N) over bar (t); monocarpic plants with seed banks) or the harmonic mean (N-e approximate to T (N) over tilde (t); Pacific salmon). I show that these conclusions arise from different model assumptions rather than inherent differences between the species. Sequentially applying standard, discrete-generation formulas for inbreeding N-e to a series of nominal generations accurately predicts the multigenerational rate of increase in inbreeding. Variability in mean realized reproductive success across years ((k) over bar (t)) is the most important factor determining N-e and N-e/N. When abundance is driven by random variation in (k) over bar (t), Ne <= T (N) over tilde (t) < (t).With random variation in N-t and constant per capita seed production (C), variation in (k) over bar (t) is low and N-e similar to T (N) over tilde (t); however, if C varies among years, N-e can be closer to T (N) over tilde (t). Because population regulation affects the genetic contribution of entire cohorts of monocarpic perennials, N-e for these species may be more closely approximated by than by T (N) over bar (t) than by T(N) over bar (t). With density-dependent compensation, Cov (k(t), N-t) < 0, and N-e is further reduced because relatively few breeders make a disproportionate contribution to the next generation.