Although the concepts of trophic transfer and bioaccumulation are outlined elsewhere, it is worth noting here in the discussion of estimating exposure. The key to a useful exposure model is to derive a realistic link between exposure and uptake. Up to this point this article has focused on the landscape level and behavioral parameters associated with exposure models. However, most exposure models assume that the diets and the proportion of food items that the animal ingests are known. Contaminant exposure is strongly linked to the kind of material an organism occupying lower trophic levels ingest - for example, leaf versus fruit, or particular invertebrate species. Contaminant studies have relied on comparisons among a variety of target species, which confound interpretations due to dietary variations and differences in interspecific physiologies. It has been suggested that animals which show a higher diversity of food items better represent the extent of contamination and trophic transfer within a system, especially when they occupy the uppermost trophic levels. For some vertebrates, trophic level rather than body size (which is the usual parameter used) appears to be one of the most important factors.
However, accurately quantifying an animal's diet down to specific food items and quantities is extremely difficult. This is often achieved through fecal and stomach analyses, which are at best snap shots in time and do not capture the animals trophic position over a specified duration (such as weeks to seasons). These difficulties have biased exposure studies to animals with specialized diets, which is extremely unrealistic and may not adequately represent the dynamics of the ecosystem being studied.
One of the most promising techniques available that can minimize some of this variation is through analyzing tissues and food items using stable isotopic analyses. The stable isotope composition of biological materials provides insights into the life histories of fish and wildlife species. It has been demonstrated that animal tissues are enriched in 1 N in relation to their diet. Also, owing to differences in photosynthetic pathways, C3, C4, and CAM plants have different C/ C ratios (—32%o to —22%o and —23%o to —9%o, respectively, with CAM overlapping) and can be used to identify sources of primary productivity in the diet. The differences in isotopic composition between any tissue compartment of an animal and diet is represented by a tissue-diet enrichment factor etissue-diet, where £rissue-diet ~ tissue — ¿diet and is the delta value for the isotope ofinterest. This technique allows researchers to determine what levels of the food chain target species within an ecosystem are occupying and potentially where an animal is foraging. This enables a more sophisticated understanding of food web structure and spatial foraging patterns thus allowing exposure estimates to be better parametrized.
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