Genetic estimates of contemporary effective population size: what can they tell us about the importance of genetic stochasticity for wild population persistence?

Abstract

Genetic stochasticity due to small population size contributes to population extinction, especially when population fragmentation disrupts gene flow. Estimates of effective population size (N e ) can therefore be informative about population persistence, but there is a need for an assessment of their consistency and informative relevance. Here we review the body of empirical estimates of N e for wild populations obtained with the temporal genetic method and published since Frankham's (1995) review. Theoretical considerations have identified important sources of bias for this analytical approach, and we use empirical data to investigate the extent of these biases. We find that particularly model selection and sampling require more attention in future studies.We report a median unbiased N e estimate of 260 (among 83 studies) and find that this median estimate tends to be smaller for populations of conservation concern, which may therefore be more sensitive to genetic stochasticity. Furthermore, we report a median N e /N ratio of 0.14, and find that this ratio may actually be higher for small populations, suggesting changes in biological interactions at low population abundances. We confirm the role of gene flow in countering genetic stochasticity by finding that N e correlates strongest with neutral genetic metrics when populations can be considered isolated. This underlines the importance of gene flow for the estimation of N e , and of population connectivity for conservation in general. Reductions in contemporary gene flow due to ongoing habitat fragmentation will likely increase the prevalence of genetic stochasticity, which should therefore remain a focal point in the conservation of biodiversity.

Fluctuating fire regimes and their historical effects on genetic variation in an endangered shrubland specialist

“…The question is what is the magnitude of recent population declines? Assuming the average ratio of N e to census population sizes ( N c ) of 0.1 reported in empirical wildlife studies (Frankham 1995; Palstra and Ruzzante 2008), our results suggest the potential existence of 12,000,000 BCVIs, 1000 times more than currently estimated (Wilkins et al. 2006).…”

Temporal patterns of genetic diversity in Kirtland’s warblers (Dendroica kirtlandii), the rarest songbird in North America

“…The utility of our N e estimates for evaluating the short-term genetic status of the Kirtland’s warblers depends on the accuracy and precision of our contemporary N e estimates Simulations suggest that it is difficult to obtain a precise estimate of N e when the true N e is >400 because at those sizes of N e , sampling error is large compared to the strength of the drift induced shifts in heterozygosity and linkage disequilibrium [21,29]. Our direct estimate of a maximal N e of 700 is approaching the parameter space where N e is difficult to estimate [30]. Therefore, the large confidence limits around our N e estimates, limit our ability to definitively state whether the current size of Kirtland’s warbler populations can meet conservation genetic objectives such as maintaining 90% of the initial diversity for a minimum of 100 years [31], but the lower range of our N e estimates are too low for genetic variability to be retained in the long-term.…”

“…Based on our indirect estimates of contemporary N e in the 100–300 range, Kirtland’s warbler populations may not be large enough to safeguard against the loss of evolutionary potential [30,32,33]. Population bottlenecks can downwardly bias N e estimates [34,35], but our contemporary sample is more than 20 generations away from the lowest recorded population size, so we assume that this bias is not a major influence on our estimates [35].…”

“…Among a variety of common and rare species, N e /N C estimates range from 0.1 to 0.5 [30,37,38] so that genetic management could still be necessary at surprisingly large census sizes. Our lowest estimate of the N e /N C ratio for the contemporary Kirtland’s warbler population was approximately 0.1, in which case, target population sizes would need to exceed 5000 individuals in order to meet the N e =500 recommendation as per Franklin and Frankham (1998), or even larger population targets would be needed if the recommended N e  = 5000 of Lynch and Lande (1998) is adopted.…”

Long-term effective population size dynamics of an intensively monitored vertebrate population

“…However, an influential contribution of evolutionary theory to conservation biology has been the development of a framework for predicting the fate of small populations (Palstra and Ruzzante, 2008). Central parameters to this framework are the population census size (N c ) and effective population size (N e ).…”