COMMUNITY STRUCTURE OF PLEISTOCENE MAMMALS: THE EFFECTS OF BIOTIC AND ABIOTC FACTORS ON THE RESPONSE TO GLACIATION

Whether species remain together or change association repeatedly through time is crucial to questions about community structure and co-evolution as well as to understanding how species respond to climate change. Quantitative assessment of the range shifts of Pleistocene mammals indicates that the responses of species to climate change are inconsistent with a strict interpretation of individualistic range shifts. Moreover, community composition in a particular locality is more similar over time than would be expected given individualistic range shifts. Given this continuity in community composition, the next step is to examine the relative impacts of biotic and abiotic factors on the community structure of Pleistocene mammals. Data were divided into the following 4 time periods: Wisconsinan (20 to 40 kya), Glacial (9.5 to 20.4 kya), Holocene (0 to 10.5 kya), Modern (0 to 500 kya). Species that occurred in each time transition were split into groups: those that were expected to occur together because of ecological interactions and those that were not. For the abiotically interacting species, decisions to include a species in this group was entirely dependent on habitat descriptions. For the group of biotically interacting species, decisions to include a species in this group was based upon predator prey relationships. Overlap between the ranges of species pairs was calculated as the proportion of the area of the smaller range that overlapped the larger range. Change in overlap was calculated for each species pair as the change in proportional overlap from an older to a younger time period. Because species pairs can increase their overlap for reasons other than a similar reaction to glaciation, the angle between the direction of the shift for overlapping species pairs was calculated. Angles were calculated as the difference between the direction of the centroid shifts for each species pair. Angles could not be larger than 180°. Distributions were compared using Mann-Whitney U tests. Biotic and Abiotic factors were only important in the transition from the Glacial to the Holocene. Likely two types of range shifts are occurring in the three transitions. Regular background shifting during the oldest and youngest transition and a concerted response to climate change during the middle transition.