Life-history traits and effective population size in species with overlapping generations revisited: the importance of adult mortality

Abstract

The relationship between life-history traits and the key eco-evolutionary parameters effective population size (N e ) and N e /N is revisited for iteroparous species with overlapping generations, with a focus on the annual rate of adult mortality (d). Analytical methods based on populations with arbitrarily long adult lifespans are used to evaluate the influence of d on N e , N e /N and the factors that determine these parameters: adult abundance (N), generation length (T), age at maturity (α), the ratio of variance to mean reproductive success in one season by individuals of the same age (ϕ) and lifetime variance in reproductive success of individuals in a cohort (V k• ). Although the resulting estimators of N, T and V k• are upwardly biased for species with short adult lifespans, the estimate of N e /N is largely unbiased because biases in T are compensated for by biases in V k• and N. For the first time, the contrasting effects of T and V k• on N e and N e /N are jointly considered with respect to d and ϕ. A simple function of d and α based on the assumption of constant vital rates is shown to be a robust predictor (R 2 = 0.78) of N e /N in an empirical data set of life tables for 63 animal and plant species with diverse life histories. Results presented here should provide important context for interpreting the surge of genetically based estimates of N e that has been fueled by the genomics revolution.

Sweepstakes reproductive success is absent in a New Zealand snapper (Chrysophrus auratus) population protected from fishing despite “tiny” Ne/N ratios elsewhere

“…Estimates of census population sizes ( N ) in the two studies were as expected; there was several orders of magnitude more snapper in the Hauraki Gulf (55 × 10 6 ) compared to the marine reserve population (3 × 10 4 ). The finding of differing N e / N ratios for the same species in essentially the same location is important given the interest in this parameter for species with overlapping generations and the likely flood of new estimates of effective size facilitated by genomic‐scale analyses (Waples, ). The discrepancy could be due to a range of issues such as flaws in empirical data collection, inaccuracies in the estimation of N e and N , differences in techniques used to estimate N e , and in the precision of the N e estimates between studies.…”

“…The N 1 term specifies the numbers of individuals that reach their first year of sexual maturity and specifies the age‐cohort over which V k * is measured. More generally, N 1 is the number of recruits that survive to age at maturity (Waples, ). Following Waples et al (), a discrete‐generation formula was used to calculate N b (Caballero, ; Crow & Denniston, ).…”

“…Waples et al () point out an important distinction between the variance parameters in these N e and N b equations. In Equation (), the term V k * is the cumulative reproductive success over the lifetime of individuals (Waples, ). In contrast, the V k parameter in the N b Equation () is the variance in offspring number produced in one time period by individuals of age x and is referred to here as V kx .…”

“…For tuna, they concluded that SRS appeared not to be operating which implied that SRS was not an inevitable consequence of the life history of their study species, even though it had high fecundity and high mortality on early‐life history stages. Prior to this empirical study, two theoretical studies (Waples, , ) examined the likely contribution of life history features to “tiny” N e / N ratios (<10 –3 ). Waples () evaluated the effect of increases in longevity, changes to reproductive success over the lifetime of an individual and increases in egg quality with age.…”

Between semelparity and iteroparity: Empirical evidence for a continuum of modes of parity

“…In each of these systems, a distinct continuum of reproductive strategies despite the supervening effect of seasonality is readily observable. Additionally, new models are being developed that consider generation length independently from parity (Waples, ). Thus, we can easily tease apart the question of whether reproduction is concentrated in time—that is, whether a given species is semelparous—from the question of whether seasonality requires that, in temperate climates, late‐reproducing individuals should enter diapause rather than reproduce immediately.…”