Optimal foraging theory - AbsoluteAstronomy.com

Optimal foraging theory is an idea in ecology

Ecology

Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...

based on the study of foraging

Foraging

- Definitions and significance of foraging behavior :Foraging is the act of searching for and exploiting food resources. It affects an animal's fitness because it plays an important role in an animal's ability to survive and reproduce...

behaviour and states that organism

Organism

In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

s forage in such a way as to maximize their net energy

Energy

In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

intake per unit time. In other words, they behave in such a way as to find, capture and consume food

Food

Food is any substance consumed to provide nutritional support for the body. It is usually of plant or animal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals...

containing the most calorie

Calorie

The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule...

s while expending the least amount of time possible in doing so. The understanding of many ecological concepts such as adaptation

Adaptation

An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....

, energy flow and competition

Competition (biology)

Competition is an interaction between organisms or species, in which the fitness of one is lowered by the presence of another. Limited supply of at least one resource used by both is required. Competition both within and between species is an important topic in ecology, especially community ecology...

hinges on the ability to comprehend what food items animals select, and why.

MacArthur

Robert MacArthur

Robert Helmer MacArthur was an American ecologist who made a major impact on many areas of community and population ecology....

and Pianka

Eric Pianka

Eric Rodger Pianka is an American biologist, whose work includes herpetology and evolutionary ecology. His textbook, Evolutionary Ecology is considered a classic, and his writings for the general public and television appearances have made him an influential figure.-Youth:Pianka was born in...

(1966) developed a theoretical and empirical

Empirical

The word empirical denotes information gained by means of observation or experimentation. Empirical data are data produced by an experiment or observation....

construct, the optimal foraging theory (OFT), which led to a better understanding of foraging behavior. Emlen (1966) published a paper on foraging behavior at the same time [indeed, in the same issue of American Naturalist]. Although it was different in detail, it demonstrated the need for a model where food item selection of animals could be understood as an evolution

Evolution

Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

ary construct which maximizes the net energy

Net energy gain

Net Energy Gain is a concept used in energy economics that refers to the difference between the energy expended to harvest an energy source and the amount of energy gained from that harvest. The net energy gain, which can be expressed in joules, differs from the net financial gain that may result...

gained per unit feeding time. Since its original conception, there have been many papers and books published mentioning OFT which have made important contributions to a number of disciplines including ecology

Ecology

, psychology

Psychology

Psychology is the study of the mind and behavior. Its immediate goal is to understand individuals and groups by both establishing general principles and researching specific cases. For many, the ultimate goal of psychology is to benefit society...

and anthropology

Anthropology

Anthropology is the study of humanity. It has origins in the humanities, the natural sciences, and the social sciences. The term "anthropology" is from the Greek anthrōpos , "man", understood to mean mankind or humanity, and -logia , "discourse" or "study", and was first used in 1501 by German...

. Some of these additions include papers from Earl Werner (1974), Eric Charnov (1974, 1976, 1977), Krebs

John Krebs

John Richard Krebs, Baron Krebs FRS is a world leader in zoology and more specifically bird behaviour. He is currently the Principal of Jesus College, Oxford University...

(1972, 1974, 1977, 1985), Smith (1966, 1974), Ricklefs (1973), Schoener (1969, 1971, 1974, 1983), Pyke (1977, 1984), Krebs, Stephens & Sutherland (1983) and Stephens & Krebs (1986). Recently, scholars have connected optimal foraging theory to prospect theory

Prospect theory

Prospect theory is a theory that describes decisions between alternatives that involve risk i.e. where the probabilities of outcomes are known. The model is descriptive: it tries to model real-life choices, rather than optimal decisions.-Model:...

, noting that survival thresholds might be responsible for human attitudes towards risk (McDermott et al. 2008).

The following is an outline of MacArthur and Pianka’s model.

The functional classes of predators

Optimal foraging theory uses predators as the object of analysis. There are four functional classes of predators:

True predators attack large numbers of their prey throughout their life. They kill their prey immediately, or shortly after the attack. They may eat all or only part of their prey. True predators include tigers, lions, plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

-eating whales, seed-eating
Seed predation
Seed predation, often referred to as granivory, is a type of plant-animal interaction in which granivores feed on the seeds of plants as a main or exclusive food source, in many cases leaving the seeds damaged and not viable...

birds, ants and humans.
Grazers attack large numbers of their prey throughout their lifetime and eat only a portion of their prey. They harm the prey, but rarely kill it. Grazers include locust
Locust
Locusts are the swarming phase of short-horned grasshoppers of the family Acrididae. These are species that can breed rapidly under suitable conditions and subsequently become gregarious and migratory...

s, deer
Deer
Deer are the ruminant mammals forming the family Cervidae. Species in the Cervidae family include white-tailed deer, elk, moose, red deer, reindeer, fallow deer, roe deer and chital. Male deer of all species and female reindeer grow and shed new antlers each year...

, leeches and mosquitoes.
Parasites, like grazers, eat only a part of their prey (host
Host (biology)
In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...

) but rarely the entire organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

, and spend all or large portions of their life cycle living in/on a single host. This much more intimate relationship is typical of tapeworms, liver flukes
Trematoda
Trematoda is a class within the phylum Platyhelminthes that contains two groups of parasitic flatworms, commonly referred to as "flukes".-Taxonomy and biodiversity:...

and plant parasites such as the potato blight.
Parasitoid
Parasitoid
A parasitoid is an organism that spends a significant portion of its life history attached to or within a single host organism in a relationship that is in essence parasitic; unlike a true parasite, however, it ultimately sterilises or kills, and sometimes consumes, the host...

s are mainly typical of wasps (order Hymenoptera
Hymenoptera
Hymenoptera is one of the largest orders of insects, comprising the sawflies, wasps, bees and ants. There are over 130,000 recognized species, with many more remaining to be described. The name refers to the heavy wings of the insects, and is derived from the Ancient Greek ὑμήν : membrane and...

), and some flies
Fließ
Fließ is a municipality in the Landeck district and is located5 km south of Landeck on the upper course of the Inn River. It has 9 hamlets and was already populated at the roman age; the village itself was founded around the 6th century. After a conflagration in 1933 Fließ was restored more...

(order Diptera
Diptera
Diptera , or true flies, is the order of insects possessing only a single pair of wings on the mesothorax; the metathorax bears a pair of drumstick like structures called the halteres, the remnants of the hind wings. It is a large order, containing an estimated 240,000 species, although under half...

). Eggs are laid inside the larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

e of other arthropods which hatch and consume the host from the inside, killing it. This intimate predator-host relationship is typical of about 10% of all insects. Many virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

es that attack single-celled organisms (such as bacteriophage
Bacteriophage
A bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid...

) are also parasitoids, in that they reproduce inside a single host that is inevitably killed by the association.

Basic variables of OFT

The OFT attempts to explain predator behavior since no predator eats everything available. This is typically due to habitat

Habitat

* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...

and size constraints, but even within habitats, predators eat only a proportion of what is available.

E is the amount of energy (calories) from a prey item. h is the handling time which includes capture, kill

KILL

KILL is the sixth album by Detroit rock band Electric Six.In initial press releases, the band described the album as being a return to a sound more akin to their debut album, but this was later revealed by front-man Dick Valentine to be more gimmick than truth.An explicit video was released for...

ing, eating and digesting. h starts once the prey has been spotted. E/h is therefore the profitability of the prey item.

Animals typically eat the most profitable prey types more than would be expected by chance since it will appear in the diet at a higher proportion than it is encountered in the environment. Predators do not, however, eat only the most profitable prey types. Other prey types may be easier to find, and E is not the only nutritional requirement. Toxin

Toxin

A toxin is a poisonous substance produced within living cells or organisms; man-made substances created by artificial processes are thus excluded...

s may be present in many prey types, therefore variability of diet prevents any one toxin from reaching dangerous levels. There are also other essential nutrient

Essential nutrient

An essential nutrient is a nutrient required for normal body functioning that either cannot be synthesized by the body at all, or cannot be synthesized in amounts adequate for good health , and thus must be obtained from a dietary source...

s in all organism's diets, so it is clear that an approach focusing only on energy intake will not provide an adequate model.

Optimal foraging and limitations of digestive rate

The Digestive rate model

Digestive rate model

The digestive rate model is related to optimal foraging theory in that the model describes the diet selection that animals should perform in order to maximize the energy available to them...

(DRM) of optimal foraging makes the same basic assumption as do other models, i.e. animals select food in order to maximize their rate of energy intake. Therefore, it belongs to Optimal foraging theory. However, it makes the point that some animals should select food on the basis of its 'digestive quality' (net energy content/digestive turnover time) rather than its 'profitability' (net energy content/handling time). Food or prey can come at the expense of indigestible bulk that takes up capacity in the digestive tract. The DRM states that animals under some circumstances will maximize their digestive rate instead of their food ingestion rate or foraging behaviour as predicted by the Contingency Model, the basic and most commonly used model of OFT. The DRM is a useful alternative to the foraging models that maximize ingestion rate only, as it explains some behaviour currently looked on as deviation from OFT.

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 new insights:

Predators with long handling times and short search times should be generalists and include a wide range of items. For example, a bird may find any number of insects palatable, and can hunt the first insect it encounters, subdueing it rather non-efficiently.
Specialists have a shorter handling times and longer search times; they are choosy. For example, a bird may find a limited number of insects palatable, so will spend longer periods of time searching for those specific prey items. These specialists have specific techniques in handling their selective prey, thus can minimise the handling time.
Predators should be generalists in unproductive environments and specialists in productive environments.
Predator-prey co-evolution
Co-evolution
In biology, coevolution is "the change of a biological object triggered by the change of a related object." Coevolution can occur at many biological levels: it can be as microscopic as correlated mutations between amino acids in a protein, or as macroscopic as covarying traits between different...

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
Porcupine
Porcupines are rodents with a coat of sharp spines, or quills, that defend or camouflage them from predators. They are indigenous to the Americas, southern Asia, and Africa. Porcupines are the third largest of the rodents, behind the capybara and the beaver. Most porcupines are about long, with...

quills, the palatability and digestibility of the poison dart frog
Poison dart frog
Poison dart frog is the common name of a group of frogs in the family Dendrobatidae which are native to Central and South America. These species are diurnal and often have brightly-colored bodies...

, crypsis
Crypsis
In ecology, crypsis is the ability of an organism to avoid observation or detection by other organisms. It may be either a predation strategy or an antipredator adaptation, and methods include camouflage, nocturnality, subterranean lifestyle, transparency, and mimicry...

and other predator avoidance behaviors.
The Digestive Rate Model
Digestive rate model
The digestive rate model is related to optimal foraging theory in that the model describes the diet selection that animals should perform in order to maximize the energy available to them...

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.

Response curves

Since there is a search time for each item, when predator density

Population density

Population density is a measurement of population per unit area or unit volume. It is frequently applied to living organisms, and particularly to humans...

increases the search time depends on the density of the prey. There is also a handling time which is species specific. This is best understood with a response curve

Functional response

A functional response in ecology is the intake rate of a consumer as a function of food density. It is associated with the numerical response, which is the reproduction rate of a consumer as a function of food density. Following C. S...

At low prey densities the predator is searching most of the time and eating every prey item it finds
At high prey densities, each new prey item is caught almost immediately. The predator spends almost all of its time catching, eating or digesting the prey. It chooses only those individuals with the highest E/h.

Handling time may not be the only reason why the slope levels off:

Confusion
ConFusion
ConFusion is an annual science fiction convention organized by the Stilyagi Air Corps and its parent organization, the Ann Arbor Science Fiction Association. Commonly, it is held the third weekend of January. It is the oldest science fiction convention in Michigan, a regional, general SF con...

effects at very high prey densities occur.
Handling time may vary between individual prey.

With a Type II functional response

Functional response

curve:

At low prey densities, intake rate increases linearly with prey density, there is no handling time.
At high prey densities, the predator has simply eaten enough.

With a Type III functional response

Functional response

curve:

At high prey densities, each new prey item is caught almost immediately. The predator spends almost all of its time catching, eating or digesting the prey. It chooses only those individuals with the highest E/h.
At low prey densities the intake rate increases faster than linearly. The predator will start to switch to this prey type
Prey switching
Prey switching is frequency-dependent predation, where the predator preferentially consumes the most common type of prey. The phenomenon has also been described as apostatic selection, however the two terms are generally used to describe different parts of the same phenomenon. Apostatic selection...

as it becomes more abundant. This implies the existence of a search image.

Most predators can probably only hold a few search images at one time. Hunter-gatherer

Hunter-gatherer

A hunter-gatherer or forage society is one in which most or all food is obtained from wild plants and animals, in contrast to agricultural societies which rely mainly on domesticated species. Hunting and gathering was the ancestral subsistence mode of Homo, and all modern humans were...

s, however, may hold numerous search images, yet focus on the most profitable prey types. As the prey density increases, they become less limited by search time and more limited by handling time.

Application to biology

Predators obviously do not solve OFT equations, and most animals will deviate from the aforementioned models. Application of this model to hunter-gatherer systems has helped ecological anthropologists empirically quantify predator-prey relationships.

The functional classes of predators

Basic variables of OFT

Optimal foraging and limitations of digestive rate

Optimal foraging and diet breadth

Response curves

Application to biology

See also