His team uses multiple data types (fossil record, bioclimatic simulations) to generate a genetic hypothesis that can be tested with DNA data. Tested on American Bison (Metcalf et al 2014) - best hypothesis for current genetic make-up of bison was a combination of climate combined with historical human impacts.
The next step is to synthesise across biomes and taxa. This is work in progress. He feels that both climate change and humans played a role in Eurasia.
Declining species paradigm with factors contributing to the general decline of species before their populations become rare
The small population paradigm - genetic drift and stochastic extinction of small populations
Humans and climate change sit on a spectrum of influences. For some species humans may be able to collapse continent-wide ranges. For others climate change may have caused the broad decline and human finished off refugial populations.
Difficult to to this in areas with poor fossilisation (e.g. much of tropics).