ISOTOPIC ANALYSIS OF WOLF AND UNGULATE BONES SUGGESTS A LINK BETWEEN FAUNAL CHANGE AND NUTRIENT CYCLING IN YELLOWSTONE GRASSLANDS OVER THE PAST CENTURY

The grassland ecosystems of Yellowstone National Park (YNP) have persisted for at least the past 3,200 years, and today they support one of the highest concentrations of mammal megafauna in North America. Modern ecological conditions in YNP are the nearest to ‘natural' that the ecosystem has experienced in the past century. During the previous 150 years the large mammal fauna of YNP, including wolves (Canis lupus), elk (Cervus elaphus), and bison (Bison bison), have undergone multiple episodes of exploitation, extinction, and protection. We used carbon and nitrogen stable isotope values measured in faunal bone collagen to study species interactions among YNP wolves and ungulates from two ecologically important time periods; the first captures historic YNP (1902-1916) during the final years that wolves were present, prior to reintroduction, and the second captures modern YNP (1997-2004) in the first years after wolves returned to the ecosystem. Elk and bison population sizes were also different between these time periods, with many fewer elk and bison in the early 1900's than today. We investigated the feeding preferences of historic and modern wolves, and found that both populations were elk-specialists with relatively low levels of intra-populational isotopic variance. Historic elk and bison had similar diets that were more variable than elk and bison diets today. We also analyzed serial samples taken from modern elk hooves, and generated chronologies to quantify elk seasonal migration and foraging behaviors. We found that historic grassland fauna (wolf, elk, bison) have higher nitrogen isotope values than the modern grassland fauna. We propose that this difference reflects the effects of grazer abundances on YNP grassland nutrient processes, such as nitrogen cycling. Smaller grazer population sizes in the early 1900's, due to intense exploitation during the previous century, had ecosystem-wide repercussions that were recorded in the nitrogen isotope values of historic consumers, including wolves.