Optimal Foraging

Organisms must acquire and use resources in their environment to survive. While food is one of the primary resources organisms must search for, individuals also need to seek habitats, shelter, and mates. This process of searching for resources is known as foraging, which involves a series of costs and benefits. More specifically, acquiring a resource provides the organism with a benefit, however, searching and capturing the resource requires expenditure of time and energy. Thus, organisms adopt foraging strategies that maximize the net gain to an optimal level by providing the most benefit for the lowest cost. Ecologists developed the “Optimal Foraging Theory” to model the situations under which an organism meets the optimum balance between foraging costs and benefits.

The Marginal Value Theorem

Resources are not uniformly distributed in the environment. For instance, in a forest ecosystem, it is much more likely to find species of trees clumped together rather than evenly distributed throughout the forest. This heterogeneity in resource availability creates “patches” of resources. Therefore, foragers must consider both the payoff of a patch as well as the distance between patches, thus the cost of moving to another patch. The Marginal Value Theorem (MVT) describes how optimal foragers utilize and move between patches of resources in their habitat ¹. There are five main predictions of the MVT:

1. Foragers should capture more prey in patches of high prey density than patches of low prey density.
2. Foragers should spend more time foraging in patches of high prey density than in those with low prey density.
3. Foragers should have a higher prey capture rate in dense patches than those that are spread farther apart.
4. Foragers should spend more time foraging in dense environments, where patches are clustered, than in sparse environments.
5. Foragers should leave a patch when the capture rate has declined to the average rate of all the patches.

The interval of time between when the last prey is captured and when the forager leaves the patch is known as the Giving-Up-Time, or GUT. In the context of the MVT, the GUT should be the same in all patches for each individual forager. This was tested in an experiment with black-capped chickadees in a large aviary foraging for mealworm hidden in artificial pine cones, where the birds had the same GUT for all types of patches, which was inversely related to the average capture rate for the entire environment ².

The longer an organism spends foraging, the more energy it spends, and the longer it exposes itself to predators and the elements, therefore developing an optimal foraging strategy for resource-acquisition is essential for most organisms. The principles of the MVT has many real-life applications to promoting diversity, conservation, and even human behavior.

Optimal Foraging in a Changing Environment

From the point of diversity and conservation, the MVT would predict that species can only exist if there are patches to adequately serve an organism’s energy needs, thus the availability and the distance between resources is important for organisms that depend on them for survival. Habitat loss, pollution, and other changes in the environment can interfere with an organism’s ability to forage optimally, therefore reducing its fitness. Thus, when habitats in an ecosystem are destroyed, patches or entire communities are removed. This removal limits the resources available to inhabitants of the rest of an ecosystem. Fewer patches with resources creates more intense competition and can eliminate certain species. This is particularly important for species under threat of extinction due to habitat loss who will be unable to meet their foraging needs if most of their habitat is destroyed.

Humans have developed sophisticated food storage to limit foraging needs, however there are many other instances in which foraging is necessary, such as searching for affordable housing, a well-paying job, or finding a suitable date among others. A recent study tested the predictions of the MVT using General Practitioners’ information-seeking abilities. They found that general practitioners optimally foraged for information when diagnosing a patient; they moved between multiple information sources quickly, and utilized both high-density sources, such as the internet, and easily-attainable sources, including their colleagues frequently ³. Similarly, Optimal Foraging Theory also applies to economics in how consumers shop, or “forage” for products and information when making a purchase ⁴. These studies demonstrate how humans adhere to the predictions of optimal foraging theory in a variety of contexts.

References

1. Charnov, E. L. ptimal foraging: the marginal value theorem. Theoretical Population Biology. 1976, Vol. 9, (129-36).
2. John Krebs, John Ryan, Eric Charnov. Hunting by expectation or optimal foraging: A study of patch use by chickadees. Animal Behaviour. 1974, Vol. 22, (953-64).
3. Mai Dwairy, Anthony C Dowell, Jean-Claude Stahl. The application of foraging theory to the information searching behaviour of general practitioners. BMC Family Practice. 2011, Vol. 12, 90.
4. Wells, V.K. Foraging: an ecology model of consumer behaviour? Marketing Theory. 2012, Vol. 2, 12 (117-136).