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INCORPORATING CONCENTRATION DEPENDENCE IN STABLE ISOTOPE MIXING MODELS: A REPLY TO ROBBINS, HILDERBRAND AND FARLEY (2002)
Citation:
Koch, P. L. AND D L. Phillips. INCORPORATING CONCENTRATION DEPENDENCE IN STABLE ISOTOPE MIXING MODELS: A REPLY TO ROBBINS, HILDERBRAND AND FARLEY (2002). OECOLOGIA. Springer-Verlag, New York, NY, 133:14-18, (2002).
Description:
Phillips & Koch (2002) outlined a new stable isotope mixing model which incorporates differences in elemental concentrations in the determinations of source proportions in a mixture. They illustrated their method with sensitivity analyses and two examples from the wildlife ecology literature. One of these examples involved using stable isotope analysis to determine the proportions of N-rich salmon and terrestrial meat, and N-poor terrestrial plants on which Alaskan bears were feeding. They concluded that the failure to consider the difference in N concentration among the food sources could skew the estimated proportions of each food source utilized. In a comment on this example, Robbins et al. (2002) noted the importance of using more realistic plant diets, especially leafy plants versus fruit, and of considering differential digestibility of plant and animal foods in the Phillips & Koch (2002) paper. In this paper, we examine the effect of these changes on the dietary estimates from stable isotope mixing models for the bear diet example. The standard concentration-independent model shows a significant proportion of brown bear and allopatric black bear diets derived from salmon. With the concentration-dependent model, changing from fruit to leafy plant diets and incorporating the greater digestibility of salmon and meat compared to plant material had compensatory effects, and the results were similar to the standard concentration-independent model. However, there is no a priori reason to expect that such factors will cancel each other in general, and use of the concentration-dependent model of Phillips & Koch (2002) is recommended whenever elemental concentrations differ significantly among sources in order to properly adjust for these effects.