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

Effective population size ( N e ) is a particularly useful metric for conservation as it affects genetic drift, inbreeding and adaptive potential within populations. Current guidelines recommend a minimum N e 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 N e and human activities. Through a quantitative review, we generated a dataset with 4145 georeferenced N e 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 N̂ e = 50 and a <5% probability of reaching N̂ e = 500. Populations listed as being of conservation concern according to the IUCN Red List had a lower N̂ e than unlisted populations, and this relationship held true across all taxonomic groups. N̂ 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 N̂ e , including the role that gene flow and subpopulation structure plays in the estimation of N̂ e in wild populations, and the need for finer-scale taxonomic analyses. Our findings provide guidance for more specific thresholds based on N e and help prioritize assessment of populations from taxa most at risk of failing to meet conservation thresholds.