Foraging Tactics and Feeding Efficiency of Ecological Niche

Foraging, as we have seen, has both profits and costs. Its profits are the matter and energy gathered, which can be used in growth, maintenance, and reproduction.

Costs of foraging are perhaps more elusive, but a foraging animal must expend energy in foraging and must expose it to potential predators. Further, much of the time it spends foraging is unavailable for other activities, including reproduction. Natural selection should favour foraging behaviours that maximize the difference between foraging profits and their costs.

Carnivorous animals forage in two extreme ways. In the “sit-and- wait tactic,” a predator waits in one place until a moving prey item comes by and then “ambushes” the prey; in the “widely-foraging tactic,” the predator actively searches out its prey (Pianka, 1966b: Schoener, 1969a, 1969b.)

The second strategy normally requires greater energy expenditure than the first. 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 the predator’s energy needs, but here the distribution of prey in space and the predator’s searching abilities assume paramount importance. Although these two tactics are end-points of a continuum of possible foraging strategies (and hence somewhat artificial) foraging techniques actually employed by many organisms are rather strongly polarized.

The dichotomy of sit-and-wait versus widely foraging therefore has substantial practical value. Among snakes, for exmaple, 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 Goskhawks often hunt by ambush using as sit- and-wait strategy, whereas most buteos and many falcons are relatively more widely foraging.

Web-building spiders and sessile filters feeders such as barnacles typically forage by sitting and waiting. Many spiders expend considerable amounts of energy and time building their webs rather than in moving about in search of prey; those that do not build webs forage much more widely.

Similar considerations can be applied in comparing herbivors 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 (to the extent that secondary chemical compounds of plants, such as tannins, and other ant herbivore defenses- reduce palatability of plants or parts of plants, effective supply of plant foods may be greatly reduced). Because cellulose in plants is difficult to digest, however, herbivores must expend considerable energy in extracting nutrient from their plant food.

Most herbivores have a large ratio of gut volume to body volume, harbor intestinal organisms that digest cellulose, and spend much of their time eating or ruminating- envision a cow chewing its cud.)

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.

As would be expected, efficiency of conversion of food into an animal’s own tissues assimilation) is considerably lower in herbivores than it is in carnivores.

Many carnivores have extremely efficient prey-caturing devices; often the size of prey object markedly influences this efficiency. Using simple geometry Holling (1964) estimated the diameter of prey item that should be optimal for a praying mantid of a particular size.

He then offered five hungry mantids prey objects of various sizes and recorded percentages “attacked. Mantids were noticeably reluctant to attack prey that was either much larger or much smaller than the estimated optimum! Thus natural selection has resulted in efficient predators both by producing efficient prey- capturing devices and by programming animals so that they are unlikely to attempt to capture decidedly suboptimal items.

Larger predators tend to take larger prey than smaller ones. It may in fact be better strategy for a large predator to overlook prey below some size and to spend the time that would have been spent in capturing and eating small items in searching out larger prey.

Similarly, the effort a predator will expend on any given prey item is proportional the expected return from that item (which often increases with prey size).

Thus a lizard waiting on a perch will not usually go far for a very small prey item but will often move much greater distances in attempts to obtain larger prey.

Because small preys are generally much more abundant than large prey, most animals encounter and eat many more small prey items than large ones. Small animals that eat small prey items encounter prey of suitable size much more frequently than do larger animals that rely on larger prey items; as a result, larger animals tend to eat a wider range of prey sizes. Because of such increased foodniche breadths of large animals, size differences between predators increase markedly with increasing predator size.