CHANGES IN MAMMALIAN NETWORKS ACROSS THE PLEISTOCENE-HOLOCENE TRANSITION AS A RESULT OF THE MEGAFAUNAL EXTINCTION

Current climate and human impacts have the potential to fundamentally alter the composition of mammal communities and the interactions of their members. The megafaunal extinctions of the Late Pleistocene represent an analogous period of mammal community disruption which may provide insight into how modern communities will respond to global anthropogenic change. Mammalian communities underwent dramatic changes at the terminal Pleistocene (~13 kya), which resulted in distinct changes in body size, diet, and species associations; however, the impact of these changes on species interactions remains poorly understood. Ecological networks are a representation of the interactions between species within a community and can be used to measure properties such as how communities are partitioned into clusters of interacting species (modularity), and how densely species are connected by consumer-resource interactions (connectedness). Past work has linked network connectedness to community resilience and modularity to ecological complexity. Using a robust data set of species composition, stable isotope information, and body size, we constructed and compared ecological networks of mammal paleocommunities across 9 sites in the Edwards Plateau, Texas before and after the Pleistocene-Holocene transition We find consistent increases in connectedness and decreases in modularity across all sites with food webs shifting from having multiple clusters in the Pleistocene to typically one cluster in the Holocene. Examination of individual contributions to modularity shows that particular species are responsible: in the majority of sites Canis lupus and sometimes Panthera onca have the largest contribution to the change in empirical modularity. These results suggest that the shift from being a mesopredator to an apex predator by these species had a significant impact on mammalian community structure and species interactions. Counterintuitively, the observed decrease in modularity and increase in connectedness imply an increase in resilience of the network after the extinction but comes at the expense of ecological complexity.