A carbon and nitrogen isotopic analysis of Pleistocene food webs in North America: implications for paleoecology and extinction

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Carbon and nitrogen isotopic reconstructions of North American Pleistocene trophic relationships were used to examine the extinction within terrestrial mammals ~10,000 years ago and distinguish between two potential causal mechanisms - human over-hunting and climate change. Additionally, individual animals were examined for unique isotopic signatures associated with feeding specializations, digestive strategies, and juveniles.

Bones representing a comprehensive set of Pleistocene mammalian genera were obtained from three fossil localities:  McKittrick Brea, California; Saltville, Virginia; and several sites in Florida.  Collagen, a durable bone protein whose carbon and nitrogen isotopic composition reflects dietary input, was extracted from specimens and analyzed for delta-13C, delta-15N, % collagen, %C, %N, and C:N.  Radiocarbon dating and amino acid analyses were performed on select sample sets.

Results indicated that several specimens contained well preserved collagen, the isotopic values of which indicated both trophic position and vegetation preference.  Those samples that contained residual diagenetic proteinaceous material exhibited increased hydrolysis of collagen with time and leaching of disassociated amino acids.  Trophic relationships were reconstructed from well preserved specimens for Aucilla River, Florida and the herbivores of Saltville, Virginia, with a less complete reconstruction established for McKittrick Brea, California.  The following notable trends emerged:  
  1. absence of nitrogen isotopic distinction between ruminants and non-ruminants,

  2. enriched juvenile nitrogen isotopic signature,

  3. distinction of giant ground sloths as omnivores,

  4. C4 grass grazers and open C4 grasslands restricted to southern North American latitudes,

  5. generalized and opportunistic feeding habits of herbivores,

  6. potential prey specializations of carnivores.

    A noticeable lack of competition and feeding specialists among herbivores suggested a stable base to these late Pleistocene ecosystems, which argues against climatically induced stress on plants. While carnivore specimens were fewer, the apex trophic levels appeared to exhibit a similar lack of competition, which would be expected in a human-driven extinction where carnivores were stressed due to rapidly over-hunted herbivores. The ultimate cause of the late Pleistocene mammalian extinction in North America can not be exclusively attributed to either of these two mechanisms at this point in time; rather, a combination of factors should be considered.