Estimation of effective population size (Ne) in an island population of feral horses (Equus caballus L.)

Abstract

A concern of conservation biology is the viability of small, isolated populations because these are expected to lose genetic diversity at a greater rate than large populations due to genetic drift. Factors such as non-random mating systems and population fluctuations are known to reduce genetic diversity in natural populations. The Sable Island Horse (Equus caballus L.) population exhibits these characteristics; as such it is an interesting population in which to explore change in genetic diversity over time. Effective population size (N[subscript e]) is the size of a Wright-Fisher ideal population exhibiting the same level of genetic drift as the study population. Comparison of N[subscript e] with census population size can be informative about the rate at which genetic diversity is changing in the study population, and about what factors are likely influencing patterns of genetic diversity. This study aimed to use the Sable Island Horse population as a case study of how traits known to reduce genetic diversity affect estimates of N[subscript e]. Heterozygosity (±SE) of the population based on samples collected in 1987-88 was 0.560 (±0.020). By 2011 heterozygosity had declined to 0.536 (±0.018). The maximum range of N[subscript e] estimates was 20 to 244 individuals with 2011 estimates averaging 60.9 individuals. This is well below the census size of 150 to 400, which is consistent with expectations given the size, lack of migration, non-random mating system, and population fluctuations of the horse population. It seems that the population has not yet reached mutation-drift equilibrium. Areas for future investigation include developing a pedigree of the horses and deeper investigation of the mating system, particularly in regards to mate choice and relatedness in harem bands.