Global assessment of effective population sizes reveals deficiencies in meeting 50/500 criteria across taxa

Background/Question/Methods

Integrating genetic measures into extinction risk assessments is imperative for reducing biodiversity loss and ensuring the viability of wild populations. Effective population size (Ne) is particularly useful for conservation, as it predicts rates of genetic drift, inbreeding, and adaptive potential of a population. Current guidelines recommend minimum Ne of 50 and 500 to avoid short-term inbreeding and long-term adaptive potential, respectively. However, there is still debate over how to incorporate these guidelines into conservation management and risk assessment. To help inform the integration of Ne into management, we conducted a quantitative review of Ne estimates in wild populations. Specifically, we assessed whether taxonomic groups differ in their effective size or their likelihood of meeting the 50/500 thresholds, and whether Ne is correlated with the Global Human Footprint. To create our dataset, we screened 4513 articles from a Web of Science search to ensure they contained an Ne estimate from a wild population using a single-sample estimation technique. We extracted data from the remaining articles, filtered the dataset to remove estimates that could not be used in analyses (e.g. infinite estimates), created a map of estimates in ArcGIS, and used generalized linear mixed models to address our objectives.

Results/Conclusions

Here, we show that certain taxonomic groups have lower Ne, are less likely to meet the 50/500 thresholds, and are disproportionately impacted by human activities. Our final dataset included 4145 Ne estimates from 3576 populations, extracted from 712 articles. We found consistent taxonomic differences in Ne, with marine fishes having the largest Ne and plants, mammals, and amphibians having the smallest. Taxonomic groups also differed in their likelihood of meeting the Ne thresholds; plant, mammal, and amphibian populations had a ~50% or less chance of reaching Ne = 50 and a < 5% chance of reaching 500. Across all taxonomic groups, Ne was reduced in areas of greater human footprint, however this relationship varied in strength and direction between groups. Mammals and amphibians had the strongest negative relationship between Ne and footprint, while in plants and anadromous fishes Ne increased with increasing footprint. These results suggest that previous reviews reporting average or median Ne or Ne/Nc were not representative for certain taxonomic groups. Overall, these results may help inform changes to listing criteria, or provide a framework for prioritizing taxa that are least likely to meet thresholds or are most impacted by human activities.