Lecture 28

Topics

• Optimal Foraging

• Optimal Patch Use

Study Sources

• Chapter 31 in Ricklefs; especially pp. 649-656.

• Study Guide: Predation

The Main Points

• Last time we discussed the assumption that predators harvest prey in direct proportion to the number of encounters between the two populations. Work on the "functional response" behavior of predators indicates that the assumption is best used in the model and not in the real world. But, then, how do predators go about seeking, capturing, processing and eating prey?

• In 1966, MacArthur and Pianka presented some formal arguments (see Figs. 31-17 and 31-18) about what has come to be known as "optimal foraging theory." They started by considering the "decisions" that have to be made by a predator when encountering any potential prey in the habitat.

• The encounter produces a choice. 1) the predator could choose the pursue the prey and eat it, or 2) it could let it go and search for a more "desirable" prey. Each of the choices has costs and benefits.
  1. The Costs associated with pursuing and eating the prey are The time and energy expended in catching, processing and eating the prey. The Benefits are the food energy gained by consuming the prey.

  2. The Costs associated with letting the prey go and continuing the search are Time, Energy and Loss of the Potential prey. The Benefit is the Potential for Better Food.

• In order to maximize food-gathering potential, the question is, "How many species should the predator include in its diet?"

• Study Figure 31-17 and associated text. I presented this figure in the Lecture. The overall strategy would seem to be that:
  1. If encounters with prey are infrequent compared with the time required to subdue and consume prey, the predator should Eat All Prey Types.

  2. If encounters with prey are frequent, the predator should pass-up less desirable types because it will soon find better ones.

• The Diet Optimization Model which is presented in Figure 27-9 has the following assumptions:
  1. The best diet selection minimizes the average time required to find and consume prey.

  2. Prey species have identical nutritional value.

  3. Prey species vary in abundance and escape tactics.

  4. Prey species are added to the diet in descending order of suitability.

• The Best Strategy would seem to be to broaden the diet until the decrease in average suitability of prey more than offsets any decrease in searching time due to greater abundance of prey types in the diet. The curves in Figure 31-17 show the optimization of the diet in terms of Search Time vs Pursuit and Handling Time.

• The last item I presented has to do with the question, "How long should the predator remain in any particular habitat (patch) before going on to another patch?" Ricklefs presents his analysis of this problem on pp. 652-654 and in Figs. 31-20 through 31-22.

• Two predictions that have been put forward are:
  1. Time in a patch ("giving-up time") should decrease as travel time between patches decreases. Or, when a new patch with high resource levels can be reached quickly, one cannot gain by staying long in a patch of diminishing quality.

  2. Optimal "giving-up" time, then, should decrease as the rate of depletion increases and it should increase as travel time between patches increases.

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