Predation: Definitions and Models

Predation Concepts

• Lecture covers the introductory concepts of predation, models of predation, and predator responses.

Definition of Predation

• Broadly, predation defined as one living organism consuming another, where the prey is alive at the initial attack
• This definition excludes scavengers and decomposers

Functional Definitions of Predation

• True predators kill their prey more or less immediately after attacking.
• Grazers remove only part of the plant (prey) they feed on, without entirely destroying it.
• Parasites consume part of their prey (host), harming but rarely killing them in the short term, exhibiting concentrated attacks on few individuals, and specialization.
• Parasitoids are insects that lay eggs in, on, or near a host, which the larvae then consume, eventually killing the host.
• When "predators" are referenced, it is often a reference to true predators.

Models of Predation

• The models, including Lokta-Volterra models, Rosenzweig-MacArthur models, and Nicholson-Bailey's model, predict oscillations of predator and prey populations.

The Lokta-Volterra Model

• It involves paired equations for predator and prey, considering their interaction.
• Initially, prey population is assumed to increase exponentially in the absence of predators, represented as dN/dt = rN.
• Predator numbers are assumed to decline exponentially in the absence of prey, represented as dP/dt = -dP.
• Combined zero isoclines result in a spiraling of the combined population size.
• The model results in cyclic oscillation of the two populations, with the predator population lagging behind the prey.
• This model assumes mutual regulation of the prey and predator populations.
• The basic Lotka-Volterra model includes unrealistic assumptions such as exponential or logistical prey population growth in the absence of predators and linear linear function of prey density even with insatiable predators.

Nicholson-Bailey Model

• Accounts for some deficiencies of the Lokta-Volterra model.
• Based on a Host-Parasitoid relationship model with assumptions such as predators sampling a proportion of prey and discrete time intervals.
• Results in unstable coupled oscillations.

Rosenzweig-MacArthur Model

• Adds more realism than previous models.
• The zero growth curve of the prey is convex rather than horizontal because it factors in density dependent growth
• It is based on a series of graphs considering a range of predator-prey interactions and curves that display various predator-prey interaction types.
• The predator-prey interaction can be neutral, stable, damped, or unstable.
• Density increase causes the own rate of increase of prey to be limited below isoclines because of scarce food, disease, or social interactions.
• Predator are to the left of the highest point of the hump.
• The curve's shape depends on prey's demographic traits and carrying capacity.
• Intersecting both predator and prey isoclines results in four possible states which are a stable cycle (intersection of the lines), damped, unstable, or a stable cycle with refuge.
• Stable cycle occurs if prey increases if the intersection is below the isocline of the prey, prey decreases outside this area, and moderately efficient predator presence leads to stable oscillations.
• Damped oscillations occur when intersection is not at a right angle or the predator isocline is moved right or left, with inefficient predators requiring high prey densities.
• An unstable system contains a predator in the isocline. Such a system, the predator exploits a prey leading the prey to near extinction
• Stable with refuge occurs if prey have a safe space leading them to escape predators and the immigrants that settle in the exploited area.

Predator Responses

• Relationships between predator and prey populations result in two responses: functional and numerical.
• Functional response means predators take more prey as prey density increases.
• Numerical response means predators become more numerous.

Functional Responses Types

• Type I: Individuals consume more prey as prey density increases.
• Type II: Intake of taken prey, raises toward a maximum It occurs in situations of varying densities of one-species prey.
• Type III: The number of prey taken is low at first and then increases rapidly in a sigmoid fashion, approaching an asymptote. It involves two or more species of prey.
• Type III responses involve development of a search image and switching to more profitable alternative prey.
• The threshold of security is the prey population level when hunting a prey species is unprofitable.

Numerical Response

• The increase of predators resulting from an increased food supply (prey).
• Includes short-term responses like aggregative response and possible delayed responses based on density.

Heterogeneity

• Risk of predation can be varied between patches so predators may aggregate in response to prey density.
• Spatial refuges may allow prey to influence predator-prey dynamics.
• Environmental factors should be considered.

Conclusions from Models

• Simpler mathematical representations mimic predator and prey populations
• It provides stabilization when it comes to density
• Predators can have different functional responses to prey.
• Functional responses Types II and III can destabilize predator-prey models

Intraguild Predation

• Intraguild predation occurs when two predator species compete for the same prey, and one also preys on the other predator species.