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1976 Marginal Value Theorem
Chapter 48
1976 Marginal Value Theorem THE CONCEPT Animals forage in order to maximize their net return from the environment. In patchy habitats, the marginal value theorem helps to predict when animals will depart a food patch and search for new foraging areas.
THE EXPLANATION Plants and animals are not randomly scattered in an environment. Generally speaking they occur in patches that align with preferred habitats. Competition for space with a patch may lead to territoriality and evenly spaced plants for animals, but zooming out to a bigger scale almost always reveals gaps between occupied areas. In 1976, Eric Charnov (Charnov, 1976) produced a set of important discoveries about patches. He focused on how the patchiness of prey items affects the behavior of a consumer. Essentially, when one prey is found, an experienced predator can form a prediction of how far it must travel, on average, and how long it will take, on average, to find the next prey item. These averages change as the predator exploits a patch; in patches of food that do not replenish themselves, the longer the predator works in the patch the harder it becomes to find food (Fig. 48.1). The predator is then faced with a critical question: when is it more profitable to stay in a patch, even though returns are diminishing, or when is it more profitable to search out another patch? Experienced predators will know how much work (time and effort) will go into searching out another patch, and how much risk may be involved if unfamiliar terrain needs to be traversed during the search. Charnov’s key prediction is that a predator should leave the patch when the yield rate from that patch dips below the average yield rate for the environment. In other words, an animal getting below-average returns should look for new foraging grounds. Charnov’s paper inspired a generation of ecologists and behaviorists who focused on testing the marginal value theorem and its relevance to real-life Conceptual Breakthroughs in Ethology and Animal Behavior. DOI: http://dx.doi.org/10.1016/B978-0-12-809265-1.00048-4 © 2017 Elsevier Inc. All rights reserved.
153
154
Conceptual Breakthroughs in Ethology and Animal Behavior
FIGURE 48.1 Information flow in making foraging decisions. Classical marginal value theory predicts that an animal will assess the current yield of the patch in which it is foraging and the mean yield of the environment. If only this information is considered, then the animal should search for a new patch when the yield of the current patch falls below the habitat mean. However, animals tend to be more conservative than this prediction, and this is where other factors are now understood to play key roles. Risk of failure—death due to starvation if a new patch is not found—is not necessarily correlated with mean habitat yield. Knowledge of predation risks between patches can play a key role in decisions. Finally, animals can vary in their risk aversion (see Chapter 77: 2004 Behavioral Syndromes—Personality in Animals), a factor that can cause responses to differ among animals.
predators. One interesting outcome is that animals tend to be a bit more conservative than the theorem dictates; faced with the choice between a certain but slightly less profitable outcome and the uncertainties of searching for a new patch, many animal species wait longer than expected before departing their patch. This leaves three intriguing dangling threads; the roles of patch replenishment, of the prey animal’s experience with predators, and of variation in risk aversion among animals. Nectar is a resource than can be replenished rapidly by a plant. For nectar foragers (its hard to think of them as predators, but the model fits this type of foraging) plants with high replenishment rates allow for continued foraging in a patch even as resource is being removed. For an inexperienced consumer there is much to be learned about resources and how they are distributed in an environment. It is not surprising that many animals are extremely good at accumulating this type of information even if they have limited general memory capacities. Under the marginal value theorem, experience is critical for foragers in their decision making. Finally, the assessment of the initial value of a patch has a major influence on subsequent decisions. This is easily observed when watching
Chapter | 48
1976 Marginal Value Theorem
155
bees sample flowers; if plants bear multiple flowers and each plant is viewed as a patch, how many flowers does the bee need to sample to make a determination about whether to visit many flowers on that same plant or to move onto the next plant? In most cases the answer is only one or a few; this demonstrates how powerful the application of the marginal value theorem can be in predicting efficient movements by foragers.
IMPACT: 5 The marginal value theorem gave a route for developing hypotheses about how evolution shapes the movement of foraging animals. The marginal value theorem played a major role in shaping optimal foraging theory. Perhaps early work resulting from the marginal value theorem and optimal foraging theory was overly optimistic about the ability of animals to assess yields, remember foraging results, and make calculations of potential future returns, but the revelations about the limitations that animal sensory and cognitive abilities place on optimal foraging have, in themselves, been fascinating.
SEE ALSO Chapter 55, 1980 The Risk Paradigm; Chapter 57, 1981 Producers and Scroungers; Chapter 63, 1990 Fear; Chapter 77, 2004 Behavioral Syndromes—Personality in Animals.
REFERENCES AND SUGGESTED READING Brown, J.S., 1988. Patch use as an indicator of habitat preference, predation risk, and competition. Behav. Ecol. Sociobiol. 22, 37 47. Charnov, E.L., 1976. Optimal foraging, marginal value theorem. Theor. Populat. Biol. 9, 129 136. McNair, J.N., 1982. Optimal giving-up times and the marginal value theorem. Am. Nat. 119, 511 529. Pyke, G.H., 1978. Optimal foraging in hummingbirds—testing the marginal value theorem. Am. Zool. 18, 739 752.
1976 Marginal Value Theorem THE CONCEPT Animals forage in order to maximize their net return from the environment. In patchy habitats, the marginal value theorem helps to predict when animals will depart a food patch and search for new foraging areas.
THE EXPLANATION Plants and animals are not randomly scattered in an environment. Generally speaking they occur in patches that align with preferred habitats. Competition for space with a patch may lead to territoriality and evenly spaced plants for animals, but zooming out to a bigger scale almost always reveals gaps between occupied areas. In 1976, Eric Charnov (Charnov, 1976) produced a set of important discoveries about patches. He focused on how the patchiness of prey items affects the behavior of a consumer. Essentially, when one prey is found, an experienced predator can form a prediction of how far it must travel, on average, and how long it will take, on average, to find the next prey item. These averages change as the predator exploits a patch; in patches of food that do not replenish themselves, the longer the predator works in the patch the harder it becomes to find food (Fig. 48.1). The predator is then faced with a critical question: when is it more profitable to stay in a patch, even though returns are diminishing, or when is it more profitable to search out another patch? Experienced predators will know how much work (time and effort) will go into searching out another patch, and how much risk may be involved if unfamiliar terrain needs to be traversed during the search. Charnov’s key prediction is that a predator should leave the patch when the yield rate from that patch dips below the average yield rate for the environment. In other words, an animal getting below-average returns should look for new foraging grounds. Charnov’s paper inspired a generation of ecologists and behaviorists who focused on testing the marginal value theorem and its relevance to real-life Conceptual Breakthroughs in Ethology and Animal Behavior. DOI: http://dx.doi.org/10.1016/B978-0-12-809265-1.00048-4 © 2017 Elsevier Inc. All rights reserved.
153
154
Conceptual Breakthroughs in Ethology and Animal Behavior
FIGURE 48.1 Information flow in making foraging decisions. Classical marginal value theory predicts that an animal will assess the current yield of the patch in which it is foraging and the mean yield of the environment. If only this information is considered, then the animal should search for a new patch when the yield of the current patch falls below the habitat mean. However, animals tend to be more conservative than this prediction, and this is where other factors are now understood to play key roles. Risk of failure—death due to starvation if a new patch is not found—is not necessarily correlated with mean habitat yield. Knowledge of predation risks between patches can play a key role in decisions. Finally, animals can vary in their risk aversion (see Chapter 77: 2004 Behavioral Syndromes—Personality in Animals), a factor that can cause responses to differ among animals.
predators. One interesting outcome is that animals tend to be a bit more conservative than the theorem dictates; faced with the choice between a certain but slightly less profitable outcome and the uncertainties of searching for a new patch, many animal species wait longer than expected before departing their patch. This leaves three intriguing dangling threads; the roles of patch replenishment, of the prey animal’s experience with predators, and of variation in risk aversion among animals. Nectar is a resource than can be replenished rapidly by a plant. For nectar foragers (its hard to think of them as predators, but the model fits this type of foraging) plants with high replenishment rates allow for continued foraging in a patch even as resource is being removed. For an inexperienced consumer there is much to be learned about resources and how they are distributed in an environment. It is not surprising that many animals are extremely good at accumulating this type of information even if they have limited general memory capacities. Under the marginal value theorem, experience is critical for foragers in their decision making. Finally, the assessment of the initial value of a patch has a major influence on subsequent decisions. This is easily observed when watching
Chapter | 48
1976 Marginal Value Theorem
155
bees sample flowers; if plants bear multiple flowers and each plant is viewed as a patch, how many flowers does the bee need to sample to make a determination about whether to visit many flowers on that same plant or to move onto the next plant? In most cases the answer is only one or a few; this demonstrates how powerful the application of the marginal value theorem can be in predicting efficient movements by foragers.
IMPACT: 5 The marginal value theorem gave a route for developing hypotheses about how evolution shapes the movement of foraging animals. The marginal value theorem played a major role in shaping optimal foraging theory. Perhaps early work resulting from the marginal value theorem and optimal foraging theory was overly optimistic about the ability of animals to assess yields, remember foraging results, and make calculations of potential future returns, but the revelations about the limitations that animal sensory and cognitive abilities place on optimal foraging have, in themselves, been fascinating.
SEE ALSO Chapter 55, 1980 The Risk Paradigm; Chapter 57, 1981 Producers and Scroungers; Chapter 63, 1990 Fear; Chapter 77, 2004 Behavioral Syndromes—Personality in Animals.
REFERENCES AND SUGGESTED READING Brown, J.S., 1988. Patch use as an indicator of habitat preference, predation risk, and competition. Behav. Ecol. Sociobiol. 22, 37 47. Charnov, E.L., 1976. Optimal foraging, marginal value theorem. Theor. Populat. Biol. 9, 129 136. McNair, J.N., 1982. Optimal giving-up times and the marginal value theorem. Am. Nat. 119, 511 529. Pyke, G.H., 1978. Optimal foraging in hummingbirds—testing the marginal value theorem. Am. Zool. 18, 739 752.