New aspects of the asymmetry of the Great American Biotic Interchange based on an analysis of the spatial and temporal structure of immigrant taxa at local scale

The Great American Biotic Interchange (GABI) happened after the connection between North and South America was established, forming the Isthmus of Panama. The GABI comprised a well documented pattern characterised by an asymmetrical movement of mammalian migrants, with a higher proportion of North American taxa being found in South America than vice versa. Using mammal fossil occurrences from The Paleobiology Database (PBDB) and Generalized Linear Mixed Models, we tested if the proportion of immigrants in local assemblages presents a spatial structure with respect to the distance from the point of entrance to the new continent. We focused on two Pleistocene intervals, to evaluate if such spatial structures varied with time. We show that during early stages of the Pleistocene there was a strong negative relationship between the proportion of immigrants and distance to the point of continental connection in both North and South Americas. During the later stages of the Pleistocene a strong negative relationship was still recovered in North America; however in South America this relationship is weak, characterizing a new type of asymmetry between the continents. We conducted sensitivity analyses related to fossil dating uncertainty, the spatial definition of local assemblages and data subsetting. Our results are qualitatively robust to all sources of uncertainty. We contrast our spatially explicit results to the classical hypotheses proposed to explain the asymmetry in the proportion of immigrants at the regional scale. We advocate that the time for full expansion of immigrant ranges is a relevant aspect in structuring the spatial pattern of immigrants at the local scale but that interspecific interactions, climate and the difference between pools of temperate and tropical species might also be relevant and more explicitly incorporated in future studies of GABI's results at local scale.