Optimal Foraging

E R Pianka, University of Texas, Austin, TX, USA © 2008 Elsevier B.V. All rights reserved.

Further Reading

Foraging tactics involve ways in which animals gather matter and energy. Matter and energy constitute profits gained from foraging used in growth, maintenance, and reproduction. Foraging has costs as well; a foraging animal may often expose itself to potential predators; much of the time spent in foraging is rendered unavailable for other activities, including reproduction.

An optimal foraging tactic maximizes the difference between foraging profits and their costs. Natural selection, acting as an efficiency expert, has often favored such optimal foraging behavior. Consider, for example, prey of different sizes and what might be termed 'catchability'. How great an effort should a foraging animal make to obtain a prey item with a given catchability and of a particular size (and therefore matter and energy content)? Clearly, an optimal consumer should be willing to expend more energy to find and capture food items that return the most energy per unit of expenditure upon them. Optimal foragers should also take advantage of natural feeding routes and should not waste time and energy looking for prey either in inappropriate places or at inappropriate times. What is optimal in one environment is seldom optimal in another, and an animal's particular anatomy strongly constrains its optimal foraging tactic. Considerable evidence suggests that animals actually do attempt to maximize their foraging efficiencies, and a substantial body of theory on optimal foraging tactics exists.

Numerous aspects of optimal foraging theory were concisely summarized by MacArthur. He made several preliminary assumptions: (a) Environmental structure is repeatable, with some statistical expectation of finding a particular resource (such as a habitat, microhabitat, and/ or prey item). (b) Food items can be arranged in a continuous and unimodal spectrum, such as size distributions of insects. (This assumption is clearly violated by foods of some animals, such as monophagous insects or herbivores generally, because plant chemical defenses are typically discrete.) (c) Similar animal phenotypes are usually closely equivalent in their harvesting abilities; an intermediate phenotype is best able to exploit foods intermediate between those that are optimal for two neighboring phenotypes. Conversely, similar foods are gathered with similar efficiencies; a lizard with a jaw length that adapts it to exploit 5-mm-long insects best is only slightly less efficient at eating 4- and 6-mm insects. (d) The principle of allocation applies, and no one phe-notype can be maximally efficient on all prey types; improving harvesting efficiency on one food type necessitates reducing the efficiency of exploiting other kinds of items. (e) Finally, an individual's economic

'goal' is to maximize its total intake of food resources. (Assumptions b, c, and d are not vital to the argument.)

MacArthur then broke foraging down into four phases: (1) deciding where to search; (2) searching for palatable food items; (3) upon locating a potential food item, deciding whether or not to pursue it; and (4) pursuit itself, with possible capture and eating. Search and pursuit efficiencies for each food type in each habitat are entirely determined by preceding assumptions about morphology (assumption c) and environmental repeatability (assumption a); moreover, these efficiencies dictate probabilities associated with search and pursuit phases of foraging (2 and 4, respectively). Thus, MacArthur considered only the two decisions: where to forage and what prey items to pursue (phases 1 and 3 of foraging).

Clearly, an optimal consumer should forage where its expectation of yield is greatest - an easy decision to make, given knowledge of the previous efficiencies and the structure of its environment (in reality, of course, animals are far from omniscient and must make decisions based on incomplete information). The decision as to which prey items to pursue is also simple. Upon finding a potential prey item, a consumer has only two options: either pursue it or go on searching for a better item and pursue that one instead. Both decisions end in the forager beginning a new search, so the best choice is clearly the one that returns the greatest yield per unit time. An optimal consumer should thus opt to pursue an item only when it cannot expect to locate, catch, and eat a better item (i.e., one that returns more energy per unit of time) during the time required to capture and ingest the first prey item.

Many animals, such as foliage-gleaning insectivorous birds, spend much of their foraging time searching for prey but expend relatively little time and energy pursuing, capturing, and eating small sedentary insects that are usually easy to catch and quickly swallowed. In such 'searchers', mean search time per item eaten is large compared to average pursuit time per item; hence, the optimal strategy is to eat essentially all palatable insects encountered. Other animals ('pursuers') expend little energy in finding their prey but a great deal of effort in capturing it (such as, perhaps, a falcon or a lion): these should select prey with small average pursuit times (and energetic costs). Hence, pursuers should generally be more selective and more specialized than searchers. Moreover, because a food-dense environment offers a lower average search time per item than does a food-sparse area, an optimal forager should restrict its diet to only the better types of food items in the former habitat. The most robust theorem of optimal foraging is that when food is abundant, diets contract, but they expand when food is scarce. Optimal foraging theory is usually formulated in terms of the rate at which energy is gathered per unit of time. Limiting materials such as nutrients in short supply and risks of predation could also be important.

Carnivorous animals forage in extremely different ways. In the 'sit-and-wait' mode, a predator waits in one place until a moving prey item comes by and then 'ambushes' the prey; in the 'widely foraging' mode, the predator actively searches out its prey. Wide foraging requires a greater energy expenditure than ambush foraging. The success of the sit-and-wait tactic usually depends on one or more of three conditions: a fairly high prey density, high prey mobility, and low predator energy requirements. The widely foraging tactic also depends on prey density and mobility and on a predator's energy needs, but the distribution of prey in space and the predator's searching abilities assume paramount importance. Although these two tactics are endpoints of a continuum of possible foraging strategies (and hence somewhat artificial), foraging techniques actually employed by many organisms do appear to be rather strongly polarized. The dichotomy of sit-and-wait versus widely foraging therefore has substantial practical value. Among snakes, for example, racers and cobras forage widely when compared with boas, pythons, and vipers, which are relatively sit-and-wait foragers. Among hawks, accipiters such as Cooper's hawks and goshawks often hunt by ambush using a sit-and-wait strategy, whereas most buteos and many falcons are relatively more widely foraging. Webbuilding spiders and sessile filter feeders such as barnacles typically forage by sitting and waiting. Many spiders expend considerable amounts of energy and time building their webs rather than moving about in search of prey; those that do not build webs forage much more widely.

Similar considerations can be applied in comparing herbivores with carnivores. Because the density of plant food almost always greatly exceeds the density of animal food, herbivores often expend little energy, relative to carnivores, in finding their prey (secondary chemical compounds of plants, such as tannins, and other antiherbivore defenses reduce palatability of plants or parts of plants, greatly reducing the effective supply of plant foods). Because cellulose in plants is difficult to digest, herbivores must expend considerable energy in extracting nutrients from their plant food. (Most herbivores have a large ratio of gut volume to body volume, harbor intestinal microorganisms that digest cellulose, and spend much of their time eating or ruminating.) Animal food, composed of readily available proteins, lipids, and carbohydrates, is more readily digested; carnivores can afford to expend considerable effort in searching for their prey because of the large dividends obtained once they find it. Efficiency of conversion of food into an animal's own tissues (assimilation) is considerably lower in herbivores than it is in carnivores.

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