Predation and Optimal Foraging Theory: Diet, Prey Choice and Energy

Predation and Predator Diet

Predation is an interaction in which one organism (the predator) feeds on another (the prey). Similarities and differences with other interactions are often discussed. Some consumers, such as detritivores, do not kill their prey, and it is not always clearly defined whether a consumer is a parasite or a predator. Predators typically kill prey, and they show a strong metabolic dependence on the prey, unlike parasites.

Types of Predators

Predators can be classified in several ways:

• According to prey type: carnivores, herbivores, or omnivores.
• According to hunting strategy: pursuers, filter feeders, deposit feeders, or ambush hunters.
• According to diet variability: monophagous, oligophagous, or polyphagous.
• According to variety versus abundance: generalists, opportunistic predators that feed on whatever is abundant, and specialists that select particular prey.

Carbon and Nitrogen Content

The C/N ratio is an important factor in nutrition. Diet composition influences predator nutrition, and carnivores generally obtain more nutritive value per unit of food than herbivores.

Compensation Options

Predators can compensate for nutritional imbalances through several options:

• Eating more quantity.
• Combining different foods to compensate for toxins.
• A combination of both.

Another factor is energy. The energetic content of the diet is fundamental, and the size of the energy item also influences predator performance, as seen in carnivores that eat herbivores such as mussels and large small prey.

Optimal Foraging Theory

Assumptions

1. Foraging behavior is driven by natural selection.
2. The foraging strategy selected is efficient.
3. Strategies that yield a greater rate of energy intake are optimized.

Optimal Diet

Suppose each prey type i provides an energy value E_i, requires a handling time h_i, and has a search time s_i before detection. The energetic return is E_i/h_i. Predators should rank prey by E_i/h_i, and a predator should always swallow prey i if its profitability exceeds the overall average rate.

Marginal Value Theorem

The best time to leave a patch is when the rate of energy gain in that patch falls to the average rate for the environment. Predators should move between subareas when the rate of energy gain falls below the average, accounting for travel time between patches.

Predictions

• Under abundant prey conditions, a generalist predator ignores its less profitable prey.
• If prey are scarce, a specialist predator takes only the most profitable prey.
• If the predator's search time is short, expanding the diet does not add much value.

Critiques of Optimal Foraging Theory

The theory has been criticized for its assumptions. Other factors, such as diet quality and prey availability, also determine foraging strategies, and it does not evaluate organisms' energy costs fully.

Functional Responses

The functional response describes how predator consumption changes with prey abundance.

• Type I: consumption increases linearly with prey density, n/t = fP.
• Type II: consumption rises with prey density but levels off due to handling time, n/t = fP/(1 + hfP).
• Type III: consumption accelerates at low prey densities before leveling off, n/t = jP²/(1 + hjP²).

Here T_s is search time, T_h is handling time, N is the number of prey captured, h is handling time per prey item, f is capture efficiency, and P is prey density.

Numerical Responses

The numerical response describes how predator population density changes with prey abundance. Predator fecundity and reproduction are often linked to food intake: high-quality food increases reproduction, while low-quality food reduces egg production. Predator aggregations can also form in response to prey density, and predator mortality may vary with predator abundance.

Predator Development

Development refers to the rate of growth, the size attained, and the age at which individuals begin to reproduce.

Evidence for the Importance of Predation

Experimental Evidence

Natural experiments and manipulation experiments, such as those on ctenophores in the Black Sea, demonstrate predation effects.

Specialized Adaptations

The existence of specialized adaptations supports the importance of predation. An evolutionary arms race occurs between predators and prey:

• Prey often evolve to increase the distance between themselves and the predator.
• Predators and prey develop offensive and defensive adaptations, which favor phenotypes that are more likely to escape predation.

There is also a geological record of predator-prey interactions dating back a long time.