A spatial energetics model of cadmium accumulation by diving ducks

Abstract

Food-chain transfer of contaminants is often predictable below the level of birds, but relations between contaminant concentrations in birds and their foods are often weak and variable. Much of this variation probably results from the energetics of foraging in fluctuating, heterogeneous environments, which has not been accounted for in past models of contaminant uptake by birds. Because birds are highly mobile with varying lengths of stay in different areas, collecting birds and analyzing their tissues yields unclear information on the importance of local contaminants. Likewise, toxicity assays with captive birds ignore changes in food intake with varying activity and weather experienced by wild birds. Neither of these approaches alone can predict maximum allowable contaminant levels in foods that avoid toxic effects under different field conditions, or what body burdens accumulate during varying lengths of stay that might affect the birds' biology at other places and times. To allow such predictions, an individual-based computer model was developed to simulate the intake of contaminated foods by diving ducks for varying conditions such as weather, water depth, food dispersion, and the size and digestibility of food. Food-intake estimates are combined with laboratory data on contaminant uptake as a function of food consumption and contaminant content. As an example, cadmium uptake is estimated for Canvasback ducks (Aythya valisineria) foraging on belowground winter buds of the submerged plant Vallisneria americana. Results indicate that relations between cadmium content of food and cadmium uptake by kidneys can be quite different depending on field conditions, and that cadmium content of food can be less important than food dispersion and resulting search costs in determining cadmium uptake. Models that estimate food intake based on energetic profitability can yield very different predictions than models that assume constant daily intake, or assume that food intake simply increases as energy costs of thermoregulation or other factors go up. Such profitability effects might explain much of the confounding variation in deriving bioaccumulation factors for birds.