Foraging

A squirrel foraging for food. Source: Peggy Notebaert Nature Museum

Many animals forage in the soil, looking for food such as plants or smaller organisms. The optimal foraging theory and optimal diet model are used to predict the decisions animals will make while foraging. Both microorganisms and macroorgansims can forage.

Optimal Foraging Theory

The optimal foraging theory predicts how a animal foraging will behave when presented with a choice in prey. This theory takes into account the energy the organism receives from the prey, and also the energy and time it costs to forage for the prey. Animals want to receive the greatest benefit of energy while expending the least amount of their own time and energy. The goal of this theory is to find the foraging strategy that maximizes the energy the species receives under the constraints of its environment. These constraints include how long it takes for the animal to travel to the foraging sites, how long it takes to search for the prey, how long it takes for the animal to prepare its foraged prey for eating, along with other factors. The optimal diet model can be used to find the optimal foraging strategy.

Optimal Diet Model

Functional Response Curves. Source: Staddon, J.E.R., 1983.

In this model, predators have to decide whether to eat the prey they find or look for another more profitable source of prey. Animals have to choose between small prey and large prey. They do this by considering the search time, handling time (how long it takes to prepare the prey for eating), and energy they would gain. To determine the profitability in this model, the value of energy the animal will receive should be divided by the handling time. The prey with the higher value is more profitable. However, if the predator comes across one prey and has to decide whether to eat it or look for another source of prey, search time for that second prey has to be taken into consideration. If the the energy value divided by the handling time plus the search time of the second prey is greater than the energy value divided by the handling time of the first prey, then the animal should search for the second of prey.

Shown in the following equation, the animal should only search for the second source of prey if E2/(h2+S2) > E1/h1. In this equation E1 and E2 are the energy values benefited from prey 1 and prey 2, respectively, in calories. h1 and h2 are the handling time of prey 1 and prey 2, and S2 is the search time for prey 2. If S2 is in a certain value threshold, the animal will eat both prey. These animals that eat both prey are often considered generalists, while animals that do not are considered specialists. It should also be noted that the animal searching for prey will not be consciously doing this equation or thinking of the benefits or lack of for certain prey. This equation/model quantifies natural unconscious behavior.

The search time depends on the density of prey. Functional response curves are used to plot the rate of prey capture over the prey density. There are type 1, type 2, and type 3 response curves. In type 1, there is a linear relationship between rate of prey captured and prey density. As the rate of prey capture increases, so does prey density. In type 2 response curves, the rate of prey captured increases with prey density to a point, and then flattens out because the predators become satiated. In type 3 response curves, rate of prey capture is high at low prey densities because the predators are more generalists and eat whatever is most abundant. At high prey densities, the predators will become specialists and pick the prey that is the most beneficial, not just the most abundant.

Citations

• Staddon, J.E.R. "Foraging and Behavioral Ecology." Adaptive Behavior and Learning. First Edition ed. Cambridge UP, 1983.
• Sinervo, Barry (1997). "Optimal Foraging Theory: Constraints and Cognitive Processes", pp. 105–130 in Behavioral Ecology. University of California, Santa Cruz.
• Jeschke, J. M.; Kopp, M.; Tollrian, R. (2002). "Predator Functional Responses: Discriminating Between Handling and Digesting Prey". Ecological Monographs. 72: 95.
• Stephens, D. W. and Krebs, J. R. (1986) "Foraging Theory". 1st ed. Monographs in Behavior and Ecology. Princeton University Press.
• Stephens, D.W., Brown, J.S., and Ydenberg, R.C. (2007). Foraging: Behavior and Ecology. Chicago: University of Chicago Press.
• Pulliam, H. Ronald (1974). "On the theory of optimal diets". American Naturalist. 108 (959): 59–74.
• Hughes, Roger N, ed. (1989), Behavioural Mechanisms of Food Selection, London & New York: Springer-Verlag, p. v, ISBN 0-387-51762-6
• Danchin, E.; Giraldeau, L. & Cezilly, F. (2008). Behavioural Ecology. New York: Oxford University Press. ISBN 978-0-19-920629-2.