Diet optimization with a nutrient or toxin constraint.

The diet optimization model with nutrient constraints proposed by Pulliam and a similar model with a toxin constraint by Stephens and Krebs are not properly formulated. The appropriate version of the model is formulated and solved for the optimal diet. The model reveals that partial preference could occur only for one prey type, while all other prey types follow the zero-one rule. The model also indicates that the prey selection criterion changes as the intensity of the constraint is changed, so that we cannot rank prey types. The solution of the model for the two-prey-type case shows different qualitative predictions from those of Pulliam and of Stephens and Krebs.