We are witnessing a rapid decline in global biodiversity. International protocols and local conservation laws have been installed to counter such an unprecedented rate of decline. However, quantitatively evaluating how much biodiversity has been lost due to climatic and anthropogenic effects and how much biodiversity has been restored due to conservation efforts remain challenging. We applied a comparative conservation genomic approach to statistically and quantitatively address these questions using three geographical taxa from a stag beetle species complex. We found that the three sky-island taxa formed three independently evolving units without detectable post-divergence gene flow; furthermore, the three taxa, which have been divergent from each other since the mid-Pleistocene, have experienced episodes of demographic decline in the past.  More importantly, even though idiosyncratic anthropogenic exploitations have been hypothesized to impact the recent demographic history (< 100 years) differently, we found a shared pattern of continuous decline in effective population size among the three geographical taxa. We argue that future empirical studies should include more taxa, in addition to the focal species, that may or may not be affected by the focal historical events to avoid making biased conservation plans.