Millennial-scale drivers of small mammal isotopic niche dynamics in southern California

Abstract

Quantifying the niche of contemporary and fossil organisms is key to identifying the primary factors driving species and community dynamics through time, in particular teasing apart abiotic and biotic drivers of change. However, niche quantification can be difficult due to short time spans (for contemporary systems), time averaging (for fossil systems), and incomplete information on different aspects of ecology and environment (for both contemporary and fossil systems). Here, we use stable isotope analyses coupled with specimen-level radiocarbon dating from the Rancho La Brea Tar Pits in Los Angeles, California to examine niche dynamics over the last >55,000 years. We sampled over 100 specimens of small and mid-sized mammals, mostly sciurids (squirrels) and leporids (rabbits), from Rancho La Brea to quantify their isotopic niche, track niche changes through time, and identify probable cause(s) of those changes. Individual specimens were radiocarbon dated and niches were quantified from stable carbon (δ13C) and nitrogen (δ15N) isotope analysis of fossil collagen to track individual isotopic niches at precise points in time. Overall, sciurids and leporids exhibit different isotopic signals, suggesting niche variation among taxa. Comparison of animals during the Pleistocene versus the Holocene reveals overall greater δ13C values and greater δ13C and δ15N isotopic niche breadth among Holocene individuals, suggesting that small mammal resource use changed from the last glacial to current interglacial period. Evaluating the isotopic data continuously through time against a high-resolution regional paleoclimate record shows that climate contributed to small mammal niche variation over the last 55,000 years in the Los Angeles Basin. These findings reveal the complexity of long-term abiotic and biotic forcings on organismal niches and emphasize the importance of scale and data resolution when quantifying and interpreting (paleo)ecological patterns and processes.