CHANGES IN MAMMAL SPECIES ASSOCIATIONS ACROSS LOCALITIES ON THE EDWARDS PLATEAU FOLLOWING THE MEGAFAUNAL EXTINCTION

Biodiversity loss is having dramatic effects on mammalian communities worldwide. Previous paleo and modern studies have found that large changes in biodiversity at both regional and continental levels have cascading effects on the composition and interactions of species within these communities. However, work is continuing to understand the roles of abiotic and biotic factors on the response of species associations. Here we examine changes in a regional ecosystem following the megafaunal extinction of North America during the late Pleistocene. We focus on 19 localities across the fairly homogenous landscape of the Edwards Plateau in Texas, diminishing the possible effects of habitat filtering. We examine spatial patterns of species associations and quantify changes in the proportion of species aggregations and segregations before and after the extinction event. Our species list for the region dropped from 75 species of all body sizes (shrew to mammoth) to 53 species across 6 dietary (carnivore, grazer, etc.) categories, with the largest species being lost. We use a Fisher’s Exact t-test to quantify changes in the co-occurrence structure following the extinction. We found an increase in the proportion of significant associations after the extinction from 20% to 28% of possible pairs. Proportionally aggregations doubled from the pre- to post-extinction intervals (~8 to 16%), while negative associations (segregations) remained approximately the same (~12%). The strength of both aggregations and segregations were similar before and after the extinction, with segregations being 3 to 5 times stronger than aggregations, despite the fact that many pre-extinction segregations were between extant and extinct species. Species associations that included surviving individuals remained, with half of the species aggregations and segregations that were present pre-extinction found after the event. Because of their size and ability to travel, megafauna were found across all localities and thus were more likely to form segregations than aggregations at this spatial scale, all of which were lost. Our results suggest that the megafaunal extinction had significant effects on the regional structure of mammalian communities across the Edwards Plateau, but those effects were not consistent across all surviving species.