THE MISSING PIECE I: CONSEQUENCES OF TERMINAL PLEISTOCENE MEGAFAUNAL EXTINCTION ON THE ISOTOPIC AND BODY SIZE NICHE OF LARGE HERBIVORE COMMUNITIES IN TEXAS

For millions of years, North America housed diverse mammalian communities that included mammals ranging from several grams to 15 tons. Approximately 13,000 years ago—subsequent to the arrival and spread of humans—North America lost the majority of its large-bodied predators and herbivores. Today, the last of the world's megafauna are threatened or endangered. In the Pleistocene, large-bodied species performed important ecosystem services by reducing herbivore populations (e.g. saber-toothed cats) or modifying vegetation structure (e.g. mammoths), yet more data are needed to quantify the ecological roles these species played within regional mammal communities. Here we report on the isotopic and morphological space occupied by large mammals within Pleistocene North America. Specimens (n=11 genera, 545 specimens) were sampled from 40 localities on the Edwards Plateau, Texas before (~100ka to 13ka) and after the megafaunal extinction (13ka to present). We measured bone collagen carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values, as well as bioapatite carbon and oxygen (δ¹⁸O) isotope values to quantify isotopic niche space. Body size was also estimated for large-bodied (>30 kg) herbivores before and after the extinction. Pre-extinction, large mammals span the δ¹³C spectrum, where Mammuthus and Bison consume primarily C₄ and some C₃ grasses, Mastodon and Paleolama specialize on C₃ browse, and Equus and Camelops are mixed feeders. After the extinction, the surviving large-bodied grazers (e.g, Bison) have lower δ¹⁵N and δ ¹⁸O isotope values than their pre-extinction counterparts, while browsers show no change. The shift in δ¹⁵N may reflect a shift in primary producer (baseline) isotope values associated with changes in biogeochemical cycling across this interval of rapid climate change. Further, none of the surviving large mammals measured (Odocoileus and Bison) show changes in δ¹³C values after the extinction, though both Odocoileus and Antilocapra increase in body size in the Holocene. These analyses reveal that the loss of a major portion of ecological diversity in North America did not affect the surviving large herbivore carbon isotope niche, while body size changes among browsers may reflect reduced competition and decreased nitrogen isotope values suggest ecosystem-wide changes biogeochemical cycling.