Predation– Chapter 13

Types of Predators

• Herbivores – animals that prey on green plants or their seed and fruits.– Plants are usually damaged but not killed

• Carnivores – animals that eat herbivores or other carnivores.

• Insect Parasitoids – lay eggs on or near host insect, which is subsequently killed and eaten.– Phorid fly

• Parasites – plants or animals that live on or in their hosts and depend on their host for nutrition.

• Cannibalism – predator and prey are the same species

P1 P2

H

P1 P2

H1 H2

Indirect Competition via exploitation

competition

Indirect Competition without competition

Predators can interact with one another by competition.

Predator populations may also be affected by indirect effects.

Three Important Predation Processes

1) Predation on a population may restrict distribution or abundance of the prey• If affected animal is pest – then good• If affected animal is valuable – then bad

2) Predation is another major type of interaction (like competition) that can influence the organization of communities.

3) Predation is a major selective force.• Many adaptations we see in organisms, such as

warning coloration, have their explanation in predator-prey coevolution

Predators Can Affect Prey Populations

Mathematical Models of Predation – discrete generations

Small prey population will increase without predation according to:

Nt+1 = [1.0 – B(zt)]Nt

If prey abundance is determined by predator abundance, then the whole predator population must eat proportionately more prey when prey densities are high. We can subtract a term from the above equation:

Nt+1 = [1.0 – B(zt)]Nt - CNtPt

Pt = population size of predators in generation t

C = a constant measuring the efficiency of the predator

Accounts for predation

Predator Population Growth

Pt+1 = QNtPt

If we assume that the reproductive rate of predators is dependent on prey abundance, then:

Pt = population size of predator

N = population size of prey

t = generation number

Q = a constant measuring the efficiency of utilization of prey for reproduction for predators

Nt+1 = [1.0 – B(Neq)]Nt

With predators absent and population low, prey growth is approximated by:

Rearrange equation:

= [1.0 – B(Neq)]Nt

Nt+1R =R = maximum finite rate of prey population increase

When prey are at equilibrium and predators are scarce, predator growth is approximated by:

Pt+1 = QNtPt or S =Pt+1

Pt

= QNeq

S = maximum finite rate of predator population increase

Example:

For Prey;

If R = 1.5; Neq = 100; absolute vale of B = 0.005; C = 0.5:

Nt+1 = [1.0 – 0.005(zt)]Nt – 0.5NtPtNt+1 = [1.0 – B(zt)]Nt - CNtPt

Prey population sizeFor Predator; If S = 2:

S = QNeq 2 = Q(100) Q= 0.02

Then Pt+1 = 0.02NtPt

Predator population size

We predict a predator-prey population cycle

Lynx and Snowshoe Hare

• Both lynx and snowshoe hare populations oscillate through a 9-year period.

How Do Predators Respond to a Change in Prey Density?

• Numerical Response – an increase in number due to an increase in reproduction.

• Aggregative Response – Predators tend to aggregate where the prey is at a high density.

• Functional Response – the number of prey eaten by an individual predator increases as the number of prey increases.

• Developmental Response – individual predators eat more or fewer prey as the predator grows.

Aggregate Response

Predators tend to aggregate where the prey is at a high density:

Three Functional ResponsesType 1 – Prey consumed increases with prey density.

Type 2 – Prey consumed increases rapidly with prey density, then levels off.

Type 3 – Prey consumed follows a logistic pattern as prey density increases.

Type 2 Functional Response

Optimal Foraging Theory-predicting behavior of predators in choosing prey

• Assume the predator makes a conscience decision when selecting prey when simultaneously faced with two or more choices.

• Assume the predator will maximize the net rate of energy gain while foraging.

• More energy is better for the predator because it will be able to meet its metabolic demands and still have energy for:– Defending a territory

– Fighting

– Reproducing

– Moving

Maximizing Daily Energy Uptake• Search time – the time it takes a predator to search

for a prey.

• Handling time – the time it takes a predator to kill and eat a single prey.

• Energy Value – the amount of energy available to the predator from the prey.

• Profitability – the amount of surplus energy a predator gets from a prey:

Profitability = Handling Time

Energy value

h

E =

If a predator has two prey types to choose from. Prey 1 is large and has a greater handling time than the smaller prey 2. However, assume the profitability is greater for prey 1, such that:

h1

E1

h2

E2 >

If a predator encounters a prey it must decide to eat it or ignore it. Two rules:

1. If the predator encounters prey 1, it should always eat it because it is the most profitable.

2. If it encounters prey 2, it should eat it if the gain from eating it exceeds the gain from rejecting it and searching for a more profitable prey 1.

Define S1 as the average search time to find a prey 1 individual then:

S1 + h1

E1

h2

E2 >

This model suggests that a predator will consume prey species 2 if the search time for prey 1 is large (energetically costly).

Predators will maximize profitability.

Size of Prey -Optimal Foraging Theory

• Predators tend to eat medium size prey– If the prey is too small, the energy value is not

great enough even though the handling time is small

– If the prey is too large, the handling time may be so great that it consumes too much of the prey’s energy value

– Medium size prey have maximum profitability

• Generalists predators tend to stabilize prey numbers– Once a prey population gets too small, the

predator will feed on something else– If a prey population becomes very abundant,

predators will feed on them

• Specialist predators tend to cause instability in prey numbers– Because a specialists feeds on only one

species, the predator-prey populations tend to oscillate (lynx-snowshoe hare).

Evolution of Predator-Prey Systems

• Coevolution – evolutionary change in two or more interacting species.– For this chapter, the coevolution of predator

and prey

• Prey that are best able to escape predators are strongly selected for.– Those that get caught die

• Predators that are better able to catch prey are selected for.– If a predator misses a prey, it only loses its meal,

not its life

Do Predators Only Eat The Weak?

Prey Species Capture Difficulty

% failed attacks

% substandard

Eastern Chipmunk Easy 72 8

Cottontail Rabbit Moderate 82 21

Gray Squirrel Hard 88 33

Predators do tend to capture more substandard prey of difficult to catch species, but not necessarily easy to catch species.

Evaluation of prey quality in predation by a trained red-tailed hawk.

Anti-predator Defense Strategies

• Warning Coloration – widespread correlation between conspicuous coloration (usually red or some other bright color) and the presence of aversive qualities.– If a predator samples one from a group and decides that

it is not a good prey, then the rest are protected.– Some prey species have evolved to mimic dangerous

animals

• Group Living – Safety in numbers.– More eyes can lead to early detection of predators.– If prey are not much smaller than predator, the prey can

gang-up on the predator.– Predator may become confused when the prey group

flees in several directions.