Herbivore dispersal, extinction, and diet: What happens when large herbivores enter and exit North America?

Large mammalian herbivores can significantly impact the ecology of other mammals in particular ecosystems by creating and maintaining open habitats and competing for limited resources, for example.  Over time, large herbivores have repeatedly dispersed from Eurasia into North America, spread throughout the continent, and subsequently gone extinct. It can be difficult to understand the evolutionary consequences, if any, in mammalian ecology due to large mammal dispersal and extinction without, first, examining such events within a historical framework.  In North America, the Quaternary Period (1.8 Ma to present) provides an ideal time frame to examine and understand mammalian ecology before, during, and after megafaunal herbivore presence.  During this period, two large herbivore taxa, mammoths and bison, separately disperse into the continent.  Further, this epoch is punctuated by the loss of much of the mammalian megafauna with the end-Pleistocene extinction event.  This study aims to understand (1) the effect of the dispersal of mammalian megafauna on the diets of native North American herbivorous mammals, and (2) whether surviving herbivorous mammals experienced competitive release in diet after the end‑Pleistocene extinction.  Diet is examined through the analysis of stable carbon isotope values (δ¹³C values) obtained from mammalian body tissues such as tooth enamel and collagen.  Examination of δ¹³C values in herbivorous mammal tooth enamel from Florida shows no statistically significant differences in native taxa before and after either mammoth or bison dispersal.  Similarly, examination of collagen in white‑tailed deer (Odocoileus virginianus) from the late Pleistocene through the Holocene of New York shows no significant differences in δ¹³C values before or after the megafaunal extinction event.  While it seems that large herbivorous mammals can have a significant impact on modern ecosystems, their appearance, sustained presence, and finally disappearance appears to have had limited evolutionary effect on the ecology of other mammalian herbivores.