The diet breadth model (along with patch choice model) is heavily influenced by the works of Emlen and of MacArthur and Pianka. The diet breadth model is used specifically to predict whether a forager will take a resource if it is encountered while foraging. Primarily, the diet breadth model assumes a “fine grained” environment; that is, a forager will encounter possible food resources at the same frequency with which the prey type occurs in the environment (Smith 1983). In other words, resources are homogeneously distributed throughout the environment. For example, if prey type X constitutes 20% of the possible resources in a given area, the forager will encounter prey type X 20% of the time. Additionally, this model assumes that “a forager will decide on which foods to take based on a knowledge of the quality of different foods and on a knowledge of resource densities (hence search costs) and handling costs (hence return rates)”(Kelly 1995:83).
Thus, the diet breadth model breaks up the time spent acquiring a resource into two periods: search costs (aka encounter rates) and handling costs (aka post-encounter return rates). Search costs is the time spent locating the resource, while handling costs includes the time it takes to harvest (plants) or pursue (in hunting animals) the resource and process the resource. Search costs and handling costs are estimated using ethnographic and experimental data.
Winterhalder and Kennett (2006) offer a very succinct explanation of the costs and decisions that form the basis of the diet breadth model:
The decision [to pursue, not pursue, or to harvest, not to harvest] entails an immediate opportunity coast comparison: (a) pursue the encountered resource, or (b) continue searching with the expectation of locating more valuable resources to pursue. If the net return to (b) is greater that (a), even after allowing for additional search time, then the optimizing forager will elect to pass by the encountered resource, and will continue to do so no matter how frequently this type of resource is encountered. (pp 14)
Using this model, the researcher can adjust hypothetical variables (i.e. seasonality, resource depression, new technologies which lower search and/or handling costs, environmental changes, etc.) in order to predict how a forager’s diet would adjust to such changes.
One critique of the diet breadth model is in regards to its fine grained environment assumption. Due to conditions such as topography and microclimatic variations, resources are rarely distributed homogeneously across the landscape. This criticism is addressed using the patch choice model.