Optimal Foraging Theory - Optimal Foraging and Diet Breadth

Optimal Foraging and Diet Breadth

The predator attempts to maximize E/(h+s), where s is the search time involved. For a range of prey, the predators average intake rate is E_average/(h_average+s_average). Where E_average is the average energy of all prey items in the diet, h_average is the average handling time and s_average is the average search time.

When the predator has found an item it doesn’t currently eat, it has two choices. It can eat the new item, in which case the profitability is E_new/h_new or it can leave it and search for an item already in its diet, in which case we use E_average/(h_average+s_average). The predator should eat this new item when E_new/h_new ≥ E_average/(h_average+s_average) because the new item increases its energy intake per unit time.

This leads to a number of insights:

• Predators with short handling times relative to their search times should be generalists and include a wide range of items in their diet. They will hunt and eat every prey they encounter since the prey is far apart and the handling time is short.
• Predators with long handling times relative to their search time should be specialists and only include the most efficient items in their diet. They will maximize their efficiency by only eating prey with high energy contents since the handling time is long and the search time is short.
• Predators should be generalists in unproductive environments and specialists in productive environments.
• Predator-prey co-evolution often makes it non-profitable for a prey item to be included in the diet, since many anti-predator defenses increase handling time. Examples include porcupine quills, the palatability and digestibility of the poison dart frog, crypsis and other predator avoidance behaviors.
• The Digestive Rate Model suggests that species can avoid predation by increasing the amount of indigestible bulk, e.g. shell material for molluscs, seed husks or fibre content in plants.