Late Quaternary Mammal Community Dynamics. Understanding the consequences of contemporary trophic downgrading and the ensuing loss of biodiversity and ecosystem function is a pressing environmental issue. Yet, such large-scale changes in ecological systems have occurred in the recent past. For example, about 13,000 years ago, most species of large-bodied mammals were extirpated in the Americas. While scientists have hotly debated the cause of the megafauna extinction for decades, only a handful of studies have moved the debate forward. Accordingly, along with my colleagues Kate Lyons (Smithsonian) and Seth Newsome (UNM), I have recently begun a major project that aims to examine the consequences of the loss of tens of millions of large-bodied mammals on the structure and functioning of ecosystems in the Americas. By employing stable isotopes, geometric morphometrics, species distribution and occupancy modeling, we are characterizing patterns of abundance, distribution, diet and morphology in the surviving mammals before and after the extinction event. Our study employs continental-wide databases such as NEOTOMA, as well as detailed work at a unique cave site in Texas, which has a comprehensive and continuous fossil record for the past 22 ka. Our results to date demonstrate a fundamental shift in the structure and functioning of the mammal community related to both the extinction and the climate shifts that occurred. Extinct species formed significantly more associations within the community than do modern animals. Moreover, we are finding shifts in the body size of small herbivores just after the megafauna extinction, which occur in the opposite direction as might be expected on the basis of Bergmann’s rule. Intriguigly, the size increase is accompanied by a highly significant expansion of the dietary isotopic niche as well as a time-transgressive shift towards a higher C4 diet. We believe our work will establish an important ecological baseline for the understanding of contemporary trophic downgrading.