Test of Martin's overkill hypothesis using radiocarbon dates on extinct megafauna.

Following Martin [Martin PS (1973) Science 179:969-974], we propose the hypothesis that the timing of human arrival to the New World can be assessed by examining the ecological impacts of a small population of people on extinct Pleistocene megafauna. To that end, we compiled lists of direct radiocarbon dates on paleontological specimens of extinct genera from North and South America with the expectation that the initial decline of extinct megafauna should correspond in time with the initial evidence for human colonization and that those declines should occur first in eastern Beringia, next in the contiguous United States, and last in South America. Analyses of spacings and frequency distributions of radiocarbon dates for each region support the idea that the extinction event first commenced in Beringia, roughly 13,300-15,000 BP. For the United States and South America, extinctions commenced considerably later but were closely spaced in time. For the contiguous United States, extinction began at ca. 12,900-13,200 BP, and at ca. 12,600-13,900 BP in South America. For areas south of Beringia, these estimates correspond well with the first significant evidence for human presence and are consistent with the predictions of the overkill hypothesis.

Dietary and isotopic specialization: the isotopic niche of three Cinclodes ovenbirds.

By comparing the isotopic composition of tissues deposited at different times, we can identify individuals that shift diets over time and individuals with constant diets. We define an individual as an isotopic specialist if tissues deposited at different times have similar isotopic composition. If tissues deposited at different times differ in isotopic composition we define an individual as an isotopic generalist. Individuals can be dietary generalists but isotopic specialists if they feed on the same resource mixture at all times. We assessed the degree of isotopic and dietary specialization in three related Chilean bird species that occupy coastal and/or freshwater environments: Cinclodes oustaleti, Cinclodes patagonicus, and Cinclodes nigrofumosus. C. oustaleti individuals were both isotopic and dietary generalists. Tissues deposited in winter (liver and muscle) had distinct stable C (delta(13)C) and stable N isotope ratio (delta(15)N) values from tissues deposited in the summer (wing feathers) suggesting that birds changed the resources that they used seasonally from freshwater habitats in the summer to coastal habitats in the winter. Although the magnitude of seasonal isotopic change was high, the direction of isotopic change varied little among individuals. C. patagonicus included both isotopic specialists and generalists, as well as dietary specialists and generalists. The isotopic composition of the feathers and liver of some C. patagonicus individuals was similar, whereas that of others differed. In C. patagonicus, there were large inter-individual differences in the magnitude and the direction of seasonal isotopic change. All individuals of C. nigrofumosus were both isotopic and dietary specialists. The distribution of delta(13)C and delta(15)N values overlapped broadly among tissues and clustered in a small, and distinctly intertidal, region of delta space. Assessing individual specialization and unraveling the factors that influence it, have been key questions in animal ecology for decades. Stable isotope analyses of several tissues in appropriate study systems provide an unparalleled opportunity to answer them.

Should we use one-, or multi-compartment models to describe (13)C incorporation into animal tissues?

Understanding rates of isotopic incorporation and discrimination factors between tissues and diet is an important focus of ecologists seeking to use stable isotopes to track temporal changes in diet. We used a diet-shift experiment to measure differences among tissues in (13)C incorporation rates in house sparrows (Passer domesticus). We predicted faster incorporation rates in splanchnic than in structural tissues. We also assessed whether isotopic incorporation data were better supported by the one-compartment models most commonly used by ecologists or by multi-compartment models. We found large differences in the residence time of (13)C among tissues and, as predicted, splanchnic tissues had faster rates of isotopic incorporation and thus shorter retention times than structural tissues. We found that one-compartment models supported isotopic incorporation data better in breath, excreta, red blood cells, bone collagen, and claw tissues. However, data in plasma, intestine, liver, pectoralis muscle, gizzard, and intestine tissues supported two-compartment models. More importantly, the inferences that we derived from the two types of models differed. Two-compartment models estimated longer (13)C residence times, and smaller tissue to diet differences in isotopic composition, than one-compartment models. Our study highlights the importance of considering both one- and multi-compartment models when interpreting laboratory and field isotopic incorporation studies. It also emphasizes the opportunities that measuring several tissues with contrasting isotopic residence times offer to elucidate animal diets at different time scales.

Beyond the reaction progress variable: the meaning and significance of isotopic incorporation data.

Ecologists conduct isotopic incorporation experiments to determine the residence time of various stable isotopes in animal tissues. These experiments permit determining the time window through which isotopic ecologists perceive the course of diet changes, and therefore the scale of the inferences that we can make from isotopic data. Until recently, the results of these experiments were analyzed using first-order, one-compartment models. Cerling et al. (Oecologia 151:175-189, 2007) proposed an approach they named the reaction progress variable to: (1) determine how many compartments are needed to describe a pattern of istopic incorporation, and (2) to estimate the size and rate constant of each pool. We elaborate on the approach described by Cerling et al. (Oecologia 151:175-189, 2007) by providing a way to estimate average retention times for an isotope in a tissue (and its associate error) for multi-compartment models. We also qualify the interpretation of the parameters in multi-compartment models by showing that many possible mechanisms yield models with the same functional form. Multi-compartment models are phenomenological, rather than mechanistic descriptions, of incorporation data. Finally, we propose the use of information theoretic criteria to assess the number of compartments that must be included in models of isotopic incorporation.