Global assessment of effective population sizes: consistent taxonomic differences in meeting the 50/500 rule

Abstract

Effective population size (Ne) is a particularly useful metric for conservation as it affects genetic drift, inbreeding and adaptive potential within populations. Current guidelines recommend a minimum Ne of 50 and 500 to avoid short-term inbreeding and to preserve long-term adaptive potential, respectively. However, the extent to which wild populations reach these thresholds globally has not been investigated, nor has the relationship between Ne and human activities. Through a quantitative review, we generated a dataset with 4145 georeferenced Ne estimates from 3576 unique populations, extracted from 712 articles. These data show that certain taxonomic groups are less likely to meet 50/500 thresholds and are disproportionately impacted by human activities; plant, mammal, and amphibian populations had a [≤]52% probability of reaching[N] e = 50 and a <5% probability of reaching[N] e = 500. Populations listed as being of conservation concern according to the IUCN Red List had a lower[N] e than unlisted populations, and this relationship held true across all taxonomic groups.[N] e was reduced in areas with a greater Global Human Footprint, especially for amphibians and mammals, however relationships varied between taxa. We also highlight several considerations for future works estimating[N] e, including the role that gene flow and subpopulation structure plays in the estimation of[N] e in wild populations, and the need for finer-scale taxonomic analyses. Our findings provide guidance for more specific thresholds based on Ne and help prioritize assessment of populations from taxa most at risk of failing to meet conservation thresholds.