THE ISOTOPIC ECOLOGY OF VERTEBRATES: AN INTRODUCTION

Stable isotope analysis is a power approach that can be used to trace animal diet, the strength of species interactions, thermal and nutritional physiology, reproductive biology, habitat preferences, and migration. Stable isotope ratios of carbon, nitrogen, hydrogen, and oxygen are routinely used in studies of vertebrates. Isotopic variations for other elements, such as sulfur, lead, strontium, and calcium, are used as well, but less extensively. The isotopic composition of an animal is primarily determined by the isotopic composition of the food, water, and gas that enter its body and from which it makes tissues and minerals, offset by isotopic fractionations associated with metabolism, tissue or mineral synthesis, waste production, and outgoing fluxes. If the isotopic composition of the material entering an animal is known, some aspects of its metabolism can be estimated from the isotopic composition of its tissues, such as growth temperature or protein balance.

I will offer an overview of the carbon, nitrogen, and oxygen isotope gradients that vertebrates sample in marine and terrestrial ecosystems. On land, carbon isotope variations are strongly controlled by differences in plant photosynthetic physiology, with higher values in C4 or arid C3 ecosystems, and lower values in more mesic C3 ecosystems. Oxygen isotopes vary with temperature (due to an association between temperature and the composition of meteoric water), with lower values in cold regions/seasons, and higher values in warmer regions/seasons. Terrestrial nitrogen isotopes are highly variable, but values tend to be higher in dry regions and lower in wet regions. In marine systems, oxygen isotopes are relatively invariant, though values are lower in brackish waters and in high-latitude surface waters. There are significant differences in food web carbon isotopes related to types of primary producer, and strong onshore/benthic vs. offshore/pelagic gradients, as well as a meridional gradient, with lower values at high latitudes. Nitrogen isotopes also show spatial variations in marine systems, including a meridional gradient and regional differences associated with nutrient availability and the extent of nitrogen fixation and denitrification.