Download Predation

Predation
• Competition (intra- and inter-) involve
interactions between same trophic level
• Predation involves interactions between trophic
levels
• One species negatively effects the other
• Very important for management goals
– Conservation
– Control
– Harvesting
Types of predation
• Herbivory
– Animals prey on green plants
– Death of plant not usual
• Parasitism
– Usually don’t kill host
• Carnivory
– Predator kills and eats prey
• Cannibalism
– Predator and prey same species
Predator behavior and prey
survival
• Herbivores gain nutrition from plants
– Have to deal with plant defenses
– chemical and physical
• Must also avoid predators
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Protective shapes and coloration
Active defense
Alertness and swiftness
Protective cover
morphology
• Predators must catch prey
– Speed, agility, claws, teeth, smell, vision, etc
Predation
• Many different techniques
• Normally involves large investment of time
and effort per unit food
• Usually target physically weakened or in a
vulnerable place
• Typically 4.5 to 10.8 % capture success
• e.g. 124 moose, 7% success rate by
wolves
Predation
• Often select prey by
oddity
– Color
– Behavior
– Location
Predation
• Conspicuousness and
crypsis
Predation
• Conspicuousness and crypsis
• Prey change behavior in absence of predators
Predation
• Conspicuousness and crypsis
• Prey change behavior in
absence of predators
• Marginal habitat
Effect of predators on prey
density
LOCATION
DENSITY PER KM2
Slate islands
4-8
Norway
3-4
Newfoundland
8-9
South Georgia
2
Finlayson
0.15
Little Rancheria
0.1
and frequent)
Central Alaska
0.2
FOREST-DWELLING
Quesnel Lake
0.03
Ontario
0.03
Saskatchewan
0.03
CATEGORY
PREDATORS ABSENT
MOUNTAIN-DWELLING
(predator more intense
(predation high and
constant)
Effect of predators on prey
density
• Predator numbers also correlated with
prey numbers
• Based on correlations
• Are predators keeping prey below food
supply, or killing only those that are
malnourished?
• Need to understand the behavior of
predators
The behavior of predators
How do predators respond to:
1. Changes in prey density?
2. Changes in predator density?
3. Differences in the degree of clumping of
prey?
Response to changes in prey
density
Response of predators to prey depends on:
1. Feeding behavior of individual predators
•
Functional response
2. Response of the predator population
through reproduction, immigration and
emigration
•
Numerical response
Functional response
1. Searches randomly for its prey
2. Has an unlimited appetite
3. Spends a constant amount of time searching
– Then the number of prey found will increase in
direct proportion to prey density
Percent of Prey
Population Eaten
Number of Prey Eaten
Per Predator
– Type I response
Prey Density
Prey Density
Functional Response
• Assumptions are usually unrealistic
• Some predators show an approximation of
a type 1 function response
– Reindeer feeding on lichens
• No animal has an unlimited appetite.
• Constant search time unlikely
• Handling time – more prey eaten per unit
time, more time is taken up with handling
and less for searching.
Functional response
Percent of Prey
Population Eaten
Number of Prey Eaten
Per Predator
•
•
Na = (aTtN) / (1 + ahN)
Type II response
Number eaten per time increases to an
asymptote as prey density increases
Prey Density
Prey Density
Functional response
Percent of Prey
Population Eaten
Number of Prey Eaten
Per Predator
• Type III response
• Prey switching
Prey Density
Prey Density
Functional response
• Success at catching depends on prey density
• Predators react to individuals of own species by
dispersing
– Territoriality
– Eviction
• Interference
• Interference reduces the searching efficiency of
the predator as predator density increases
• Interference stabilizes predator numbers
Functional response
• Prey live in small patches of high density
and low density in between
• Clumped distribution
• Predators concentrate on areas of high
density
• Predators thereby have a regulating effect
on prey numbers
Numerical response
•
Trend of predator numbers against prey
numbers
• Predators increase as prey density increases
• Due to
1. Increased rate of predator reproduction when
prey are abundant
•
Numerical response
2. Attraction of predators to prey aggregations
•
Aggregational response
Numerical response
• Reproduction and mortality rate of
predators depends on predation rate
• More prey, more energy
• Predator numbers increase to an
asymptote determined by interference
Total response
• Total number eaten = number eaten by one
predator multiplied by number of predators
• Can be plotted as Total response curves
• Need to also incorporate recruitment rate of
prey
• Different types of curves indicate different
types of relationships
– Predators regulate prey population
– Prey regulated by intraspecific competition for
food
– Multiple stable states
Behavior of prey
• How does the behavior of the prey
influence predation?
• Migration
• Herding and spacing
• Birth synchrony
Migration
• If a prey species can migrate beyond the range
of its predators the population can escape
predator regulation
• Predators have slow growing young and are
restricted to a small area to breed
• Ungulates have precocial young that can move
within a few hours
• Thus predators follow food resource, predators
cannot.
Herding and spacing
• Animals reduce risk of predation by
forming groups
• Group size predicted to increase with
increasing predator densities
• Can also leave group when most
vulnerable
– Leave group when give birth as predators are
concentrated around herds
Birth synchrony
• Synchronize births to reduce predation
rate
• Predator swamping
• Also influenced by seasonal availability of
resources
• Unlikely predation is cause of synchrony,
but likely intensifies it.
Conservation and management
• Predator and prey populations usually coexist
– Prey at low density by regulation
– Prey at high density by intraspecific competition for
food with predation depensatory
• Both systems can operate in one area
– Type III functional response or density-dependent
numerical response
– Disturbance moves from one state to other
– Explains outbreaks of pest species, decline of hunted
species
Conservation and management
• Prey population can become extinct
– Type II response with no prey refuge, no alternative
prey
– Important in management where there are habitat
changes
• Which situation occurs depends on
– Ability of predator to catch prey
– Ability of prey to escape predation
– Reproduction