Optimal foraging theory and risk-sensitive foraging Matt Begley

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

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

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

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

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

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

# 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

# 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

# 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

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

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

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

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

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

References Molles, M.C. 2013. Ecology: Concepts and Applications. 6th edition. McGraw Hill, New York, NY. Raven 8th edition