More vegetation--more species--Beringia--

In the Late Pleistocene, Beringia was a mosaic of biological communities.[30][26][31] Commencing from c. 57,000 YBP (marine isotope stage [MIS] 3), steppe–tundra vegetation dominated large parts of Beringia with a rich diversity of grasses and herbs.[30][26][32] There were patches of shrub tundra with isolated refugia of larch (Larix) and spruce (Picea) forests with birch (Betula) and alder (Alnus) trees.[30][31][32][33] It has been proposed that the largest and most diverse megafaunal community residing in Beringia at this time could only have been sustained in a highly diverse and productive environment.[34]

Duration of snow cover in days, East Beringia, 20000 years ago. Chelsa Trace 21ka variable bio/scd 200.

Analysis at Chukotka on the Siberian edge of the land bridge indicated that from c. 57,000 – c. 15,000 YBP (MIS 3 to MIS 2) the environment was wetter and colder than the steppe–tundra to the east and west, with warming in parts of Beringia from c. 15,000 YBP.[35] These changes provided the most likely explanation for mammal migrations after c. 15,000 YBP, as the warming provided increased forage for browsers and mixed feeders.[36] At the beginning of the Holocene, some mesic habitat-adapted species left the refugium and spread westward into what had become tundra-vegetated northern Asia and eastward into northern North America.[27]