Assembly of Alaska-Yukon boreal steppe communities: testing biogeographic hypotheses via modern ecological distributions

Beringia (eastern Asia, Alaska, northwest Canada) has been a land-bridge dispersal route between Asia and North America intermittently since the Mesozoic Era. The Quaternary, the most recent period of exchange, is characterized by large, geologically rapid climate fluctuations and sea-level changes that alternately expose and inundate the land-bridge region. Insights into how Quaternary land-bridge geography has controlled species exchange and assembly of the North American flora comes from focusing on a restricted community with narrow ecological tolerances: species that are today restricted to isolated steppe habitats (dry grasslands) in the Subarctic. We evaluated i) potential controls over current spatial distributions of steppe plants and their pollinators in Alaska and Yukon and ii) their ecological distributions in relation to potential biogeographic histories. Taxa present in North America that are disjunct from Asia tended to have larger altitudinal ranges (tolerating colder temperatures) than taxa disjunct from farther south in North America, which were largely restricted to the warmest, lowest-elevation sites. Ecological findings support the following biogeographic scenarios. Migration from Asia via the land-bridge occurred during Quaternary glacial periods when conditions were colder and drier than today. While a corridor for migration of cold-tolerant species of cold steppe and tundra, the land bridge acted as a filter that excluded warmth-demanding species. Migration from North America occurred under warm, dry interglacial conditions; thermophilous North American disjuncts taking this route may have long histories in Beringia, or they may have migrated recently during the relatively warm and dry early Holocene, when forest cover was incomplete.