Genetic estimates of contemporary effective population size in an endangered butterfly indicate a possible role for genetic compensation

The effective population size (N(e)) is a critical evolutionary and conservation parameter that can indicate the adaptive potential of populations. Robust estimates of N(e) of endangered taxa have been previously hampered by estimators that are sensitive to sample size. We estimated N(e) on two remaining populations of the endangered Miami blue butterfly, a formerly widespread taxon in Florida. Our goal was to determine the consistency of various temporal and point estimators on inferring N(e) and to determine the utility of this information for understanding the role of genetic stochasticity. We found that recently developed ‘unbiased estimators’ generally performed better than some older methods in that the former had more realistic N(e) estimates and were more consistent with what is known about adult population size. Overall, N(e)/N ratios based on census point counts were high. We suggest that this pattern may reflect genetic compensation caused by reduced reproductive variance due to breeding population size not being limited by resources. Assuming N(e) and N are not heavily biased, it appears that the lack of gene flow between distant populations may be a greater genetic threat in the short term than the loss of heterozygosity due to inbreeding.