Behavioral Ecology I - Foraging

Foraging refers to a species eating habits - what to eat, when to eat, where to eat, and when to stop feeding in a certain area. Foraging is molded by natural selection. Essentially, energy gained by feeding affects reproduction and without food there is no survival.

Energy consumed either gets absorbed or is voided as feces. From the energy absorbed, some of it is lost through respiration, digestion, tissue maintenance and movement. The energy left is used for growth. The energy is divided into energy used for somatic growth and energy used for reproductive output. (There are allocation tradeoff, but more on that later.)

Consumer types

There are several consumer types. Although they can be grouped into specialists and generalists, these are not discrete categories, but rather a continuum.

• Monophagous: 1 prey type
• Oligophagous: few prey types
• Polyphagous: many prey types

However, for comparison purposes, we will classify consumer types into two broad categories.

1. Specialists: Feed on one/few type(s) of prey.
• Advantages include being adapted to one single prey, which means they can overcome prey defenses. There is less competition.
• Disadvantages include the risk of prey extinction, making them vulnerable. There may also be nutrition problems due to unmixed diet. In areas of low prey population density, there is an increased search time.
2. Generalists: Feed on multiple types of prey.
• Advantages include low search time and diffused effect of prey toxins.
• Disadvantages include a high or strong interspecific competition. Generalists are also vulnerable to prey defense due to lack of local adaptation.

Optimal Foraging Theory

Optimal Patch Use Model

This model describes the ideal pattern a species should follow to obtain the maximum energy gain. This model takes into account certain assumptions:

1. Food is found in discrete patches
2. No energy is gained when traveling from patch to patch
3. Consumers can assess food's energy value in a patch

The model is represented as an energy gain curve. There's an optimal time to remain in a given patch and an optimum energy gain described by the tangent (with the steepest slope) to the energy gain curve.

Marginal value theorem: Optimum time to reside is defined by the rate of energy gain at the time of leaving the patch.

[Foxglove flower example]

Optimal Diet Model

Maximum foraging is calculated by E/T where E is the energy content and T is the total time spent searching and handling prey. 

T = s + h (searching + handling)

Organisms can behave as time minimizers or energy maximizers.

1. Time minimizers
• Minimize time to gain specific amount of energy
• Mouse - high risk of predation
• Antelopes - males minimize foraging to defend females against other males.
• Snails feeding on barnacles - eat small barnacles quickly, whereas eating large barnacles takes longer
• Prefer small barnacles due to less risk of predation
• Large barnacles expose snails to predators
2. Energy maximizers
• Focus on increasing prey profitability
• Bison, deer, penguins, birds, sunfish

Prey profitability

The profitability of a specific prey can be obtained by dividing its energy content by its handling time (E/h). Handling time refers to the time it takes a predator to attack, kill, and consume a prey once it has encountered it. This is assuming a simple system where there are only two prey types.

Rules of thumb:

(assuming prey type 1 is more profitable than prey type 2)

• If a predator encounters prey type 1 eat it on the spot. Always eat.
• If predator encounters prey type 2, eat if the gain from eating it exceeds the gain from rejecting it and searching for prey type 1.
• if E2/h2 > E1/(s1+h1) where s1 is the additional search time.

Other important notes:

1. A predator will specialize on a prey type only if its search time is low
2. A predator will switch from a specialist to a generalist as average search time for prey 1 increases

Simplifying assumptions of OFT Models:

1. A predator can sense the energetic value of a prey
2. Foraging behaviors are heritable
3. The model only considers 2 prey types
4. Energy content is the only influence on prey choice. (Does not consider other factors such as salt, H2O, etc.)

It is a simplistic theory, but there is a great amount of evidence that supports it.