1. Optimal foraging theory and risk-sensitive foraging
Matt Begley

2. Outline of what will be covered
principle of allocation
trade-offs and foraging behavior
functional response curves

3. Organisms cannot simultaneously maximize all of life’s functions.
The Principle of Allocation
avoiding predators
energy
foraging
Principle of energy allocation says that when an organism allocates energy to one function, it reduces the amount of available energy that can be used for other functions (Molles 2013). For example, this frog can gain energy from eating the spider. Once the energy is acquired it must go to a variety of functions. If the frog spends all of its energy foraging it cannot allocate energy to other life functions.
reproduction
fighting disease

4. High energy demands for carnivores to catch prey
The Principle of Allocation
avoiding predators
vs.
energy
foraging
High energy demands for carnivores to catch prey. Some prey take more energy to capture (e.g., quick flight by the fly may make it more energy expensive). Predators have evolved a variety of anatomical and behavioral mechanisms to decrease amount of time and energy spent foraging.
reproduction
fighting disease

5. Mechanisms to decrease the costs of foraging
Search time: time spent looking for a prey item.
Search image: use of specific attributes in prey to scan their habitat for these prey items
Handling time: once prey have been found
Defining important terms. Discuss how search time and handling time could be minimized by great-horned owls. Also, discuss what search images a great-horned owl may use for white-footed mice and striped skunks.
Trade-offs and foraging behavior

6. Are foraging patches with more food more attractive to predators?
Question at the top for discussion.
Trade-offs and foraging behavior

7. Are foraging patches with more food more attractive to predators?
Functional response: An increase in animal feeding rate, which eventually levels off, that occurs in response to an increase in food availability.
Predators eventually become satiated and food intake levels off—called a functional response.
Functional response
Definition from Molles 2013

8. # Prey Consumed
Type 1
Type 2
Type 3
Prey Density
Type 1: feeding rate increases linearly as food density increases and then levels off abruptly at some maximum feeding rate
Functional response
Definition from Molles 2013

9. # Prey Consumed
Type 1
Type 2
Type 3
Prey Density
Type 2: feeding rate at first rises linearly at low food density, rises more slowly at intermediate food density, and then levels off a high densities
*most common in nature
Functional response
Definition from Molles 2013

10. # Prey Consumed
Type 1
Type 2
Type 3
Prey Density
Eventually predators become satiated. Even with unlimited food, prey consumed has a threshold. Energy limitation is an important assumption of the optimal foraging theory.
Type 3: feeding rate at low densities increases slowly, then rises steeply at intermediate food densities, eventually leveling off at higher densities
Functional response
Definition from Molles 2013

11. Optimal foraging theory
natural selection should favor individuals that forage as energetically efficient as possible with the assumption that energy supplies are limited
definition
Trade-offs and foraging behavior
Definition from Molles 2013

12. Optimal foraging theory
Prey should choose to forage in patches that are the most profitable until the risk of predation becomes too high.
Eventually a threshold is reached where the risk of predation outweighs the benefit of continued feeding and prey must move to another patch.
Similarly, predators that optimize energy intake should aggregate in patches that are most profitable.
When overcrowding of predators occurs, interference competition should cause habitat shifts in which predators switch to a different foraging patch.
Trade-offs and foraging behavior

13. Optimal Foraging Theory
Organisms should maximize energy gain while minimizing search and handling time; Small mussels are not worth the effort for the return, however, large mussels are difficult to open and too energy expensive for the return.
Trade-offs and foraging behavior

14. Optimal Foraging Theory and Predators
Keep in mind that these crabs are not living in a predator-free environment and that trade-offs between foraging and predation risk must also be made. Often, organisms will display habitat-shifts foraging in patches that are less profitable but have a decreased risk of predation. What would you expect to happen when a gull predator is introduced into the foraging patch?
Trade-offs and foraging behavior

15. Habitat-shifts, foraging, and predation risk
Giving-up density
critical point in which the density of food within a given patch is no longer profitable causing the animal to move to other food patches
This is often altered by predation risk.
Review data from the activity. What was the average giving-up density for striped skunks and great-horned owls?
Trade-offs and foraging behavior

16. References
Molles, M.C Ecology: Concepts and Applications. 6th edition. McGraw Hill, New York, NY.
Raven 8th edition