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 4610 georeferenced N_e estimates from 3829 populations, extracted from 723 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 <54% probability of reaching ${\hat{N}}_e$ = 50 and a <9% probability of reaching ${\hat{N}}_e$ = 500. Populations listed as being of conservation concern according to the IUCN Red List had a smaller median ${\hat{N}}_e$ than unlisted populations, and this was consistent across all taxonomic groups. ${\hat{N}}_e$ was reduced in areas with a greater Global Human Footprint, especially for amphibians, birds and mammals, however relationships varied between taxa. We also highlight several considerations for future works, including the role that gene flow and subpopulation structure plays in the estimation of ${\hat{N}}_e$ 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 prioritise assessment of populations from taxa most at risk of failing to meet conservation thresholds.