Environments of Pleistocene Beringia: analysis of faunal composition using cenograms

Cenograms are graphs of body size for mammalian faunas. Cenogram analysis is based on correlation between the distribution of body mass in mammalian faunas and two important environmental factors. Aridity is inferred from the slope of medium-sized species and openness of plant formations is correlated with the size gap between medium and small species. Cenogram analysis does not rely on attributes of particular taxa in the community, and thus is relatively free from both taxonomic and taphonomic influences. Cenograms have been used to infer environmental parameters for Paleocene to Oligocene assemblages of mid-latitude North America and Europe, and here are applied to subarctic and arctic faunas. The present-day Yukon boreal forest assemblage produces cenogram statistics that indicate semiarid climate and plant formations about as open as in more southern forests. Taiga and tundra both fall into the arid portion of the moisture gradient, with tundra at an extreme end. These are also extremely open habitats. Cenograms for seven Quaternary faunas of Beringia, ranging from early Pleistocene to latest Wisconsinan, all indicate aridity comparable with taiga conditions and openness similar to tundra. No forest-habitat faunas were detected. Similar arid, open habitats probably appeared in Beringia by the earliest Quaternary and apparently persisted for a major part of the Pleistocene, even at times other than glacial maxima. Alate Wisconsinan fauna from Saskatoon, Saskatchewan occupied a semiarid, somewhat open environment comparable to the present-day parkland ecotone. Late Wisconsinan to late Holocene faunas from Charlie Lake Cave, British Columbia show little change through time and lived in an open area of boreal forest, comparable to present-day conditions. Mammalian diversity is very low in Beringia today, and was even more restricted in the Pleistocene among small and medium-sized groups. Pleistocene faunas show the same trends as the present-day mammalian community but express them more strongly. Low diversity in Pleistocene faunas of Beringia is not an artifact of inadequate sampling. Several faunas have been collected carefully enough to indicate actual diversity among small and medium-sized mammals. Interpretation of late Pleistocene Beringian faunas is consistent with weakness of the hydrologic cycle at last glacial maximum; perhaps this condition too was typical of much of the Pleistocene.