Transcript Predator-prey theory

Predation – what is it?
One animal kills
another for food
( + - interaction )
Parasitism / Parasitoidism
Herbivory
Predation
Any ecological process in which energy
and matter flow from one species to another.
One organism consumes another in the
classic “+ -” interaction:
carnivore
herbivore
parasite / parasitoid
(virus, bacteria, etc)
‘cannibalism’
Why do we care?
1. Predation can act as a force to structure communities.
Keystone predator
Trophic cascades
2. Predation can act as a mechanism for natural selection.
3. Predation can control species distribution or density.
Regulation
Lotka-Volterra: classic 2-species model
Primary Assumptions
● all individuals represented by the average
● linear relationship between predator and prey
● no time lag in response
● exponential growth, no K
Reality
(Begon et al. 1996 p. 118)
Gause 1934 – rotifer vs. protist
unmanipulated
prey refuge added
“immigration”
Huffaker 1958 – mites and oranges
Eotetranychus sexmaculatus
vs.
Typhlodromus occidentalis
Initial results – similar to
Gause, simple systems
were incapable of
sustaining coexistence
Coexistence was only
possible with extreme
environmental
heterogeneity
“hide and seek”
Adding reality to model predictions:
C.S. Holling 1959
Individual predation rate
I
II
Prey density
III
IV
2 types of predator response:
1. Functional:
Change in the rate
of predation by an
individual predator
- ‘search image’
- ‘prey switching’
2. Numerical:
Increase in the total
number of predators
- reproduction
- aggregation
Holling 1959
2 types of predators:
Generalists
Specialists
Specialist numerical response:
Krebs et al. 2001
Quick review
Simple dynamics (L-V) predicts a steady cycle
This cannot be re-created empirically without
interference by the investigator
In reality, we observe a wide variety of
dynamics
- cycles
- stability
- extinction
- chaos
- periodic fluctuations
What factors control whether predator/prey
interactions are stable?
Factors influencing stability
Stabilizing
Destabilizing
Low predator efficiency
High predator efficiency
Generalist predator
Refugia
Complex systems
multiple predator spp.
multiple prey spp.
prey switching
Specialist predator
Type III functional response
Time lags
Simple systems
Summary
● Predation involves energy and matter
flowing from one species into another (+ -).
● Factors that influence coexistence:
predator efficiency, system complexity
refugia, time lags
● Holling’s Type I, II, III curves
● Generalist vs. specialist, numerical vs. functional
● Predation can:
act as an agent of selection
structure communities
regulate populations