Transcript Chp13

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
Predators can interact
with one another by
competition.
P1
P2
H
Indirect Competition
via exploitation
competition
Predator populations
may also be affected
by indirect effects.
P1
P2
H1
H2
Indirect Competition
without competition
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 predatorprey 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
Accounts for predation
Pt = population size of predators in generation t
C = a constant measuring the efficiency of the predator
Predator Population Growth
If we assume that the reproductive rate of predators is
dependent on prey abundance, then:
Pt+1 = QNtPt
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
With predators absent and population low, prey growth is
approximated by:
Nt+1 = [1.0 – B(Neq)]Nt
Rearrange equation:
R=
Nt+1
Nt
= [1.0 – B(Neq)]
R = maximum finite rate of
prey population increase
When prey are at equilibrium and predators are scarce,
predator growth is approximated by:
S = maximum finite
Pt+1
Pt+1 = QNtPt or S =
= QNeq rate of predator
Pt
population increase
Example:
For Prey;
If R = 1.5; Neq = 100; absolute vale of B = 0.005; C = 0.5:
Nt+1 = [1.0 – B(zt)]Nt - CNtPt
Nt+1 = [1.0 – 0.005(zt)]Nt – 0.5NtPt
Prey population size
For Predator; If S = 2:
S = QNeq
2 = Q(100)
Then Pt+1 = 0.02NtPt
Predator population size
We predict a predator-prey
population cycle
Q= 0.02
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 Responses
Type 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:
Energy value
Profitability =
Handling Time
=
E
h
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:
E1
E2
>
h1
h2
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:
E2
E1
>
h2
S1 + h1
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?
Evaluation of prey quality in predation by a trained red-tailed hawk.
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.
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.