# Marginal value theorem

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In behavioural ecology, the marginal value theorem (MVT) considers an optimally foraging animal that is exploiting resources that occur in patches, and that must decide when to move on to the next patch. The theorem predicts that individuals will stay longer:

• in a more profitable patch,
• as the distance between patches increases,
• when the environment as a whole is less profitable.

One common image to have in mind is picking apples from trees in an orchard. When one first arrives at a new tree, the rate of apple picking is large but decreases with time, eventually falling to zero as apples becomes scarcer. A strategy of picking one or two apples from a tree then moving on to the next tree would not be optimal because you would spend too much time walking between trees. Similarly, a strategy of exhausting each tree would be inefficient because then you would spend too much time searching for the last few apples on a nearly exhausted tree. It is therefore better to find a compromise between these two strategies.

The marginal value theorem is useful to apply to many situations in which patchy resources are exploited and a decision must be made as to when to leave a patch and find a new one. Examples might include bees visiting flowers, birds eating berries, or even mobile shop owners (in which case the "resource" being exploited is paying customers).

The MVT has been criticised on the grounds that few foragers are optimal (typical limiting factors include inability to assess exploitation rate and lack of knowledge of distant patch existence). Nevertheless, it remains a useful theorem with which to assess many types of forager behaviour.

## ReferencesEdit

• Charnov, E. L. 1976. Optimal foraging: the marginal value theorem. Theoretical Population Biology 9:129-136.
• Charnov, E. L. 1976. Optimal foraging: attack strategy of a mantid. The American Naturalist 110:141-151.