Relationship between (a) observed heterozygosity (Ho) and elevation, and (b) effective population size (Ne) and elevation in meters for each site and species. Solid circles and dashed lines represent B. tricaudatus; and open circles and dotted lines represent B. bicaudatus. B. tricaudatus samples with n<5 were excluded from the estimation of Ne. B. bicaudatus had significantly lower levels of both Ho and Ne, and a decline in Ho with increasing elevation. In B. tricaudatus, Ne displayed a declining trend with increasing elevation.