exploiter-victim relationships host-parasite: host death need not occur, and often does not; birth...
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Exploiter-Victim Relationships
Host-Parasite: Host death need not occur, and often does not; birth rate of host reduced by parasite
Host-Parasitoid: Host death always occurs
Predator-Prey: Death rate of prey increased by predators
Herbivore-Plant: May resemble predation or parasitism
Parasitoids
Weevils and wasps
Lynx and Snowshoe Hare
Orange Mites, simple universe
Orange Mites, increased patchiness
Orange Mites, complex habitat
Field Studies: Dingoes and kangaroos
Dingoes and Boars
Lamprey and Lake Trout
Fox and Rabbit
Plant-Herbivore
Herbivore-- positive effect?
N-fertilization effects
N-fertilization effects
Big HerbivoresBig Herbivores
Amboseli Elephants
Elephants not excluded
Elephants Excluded
Baobab
Baobab
Baobab, Elephant Damage
Functional ResponseChange in predator’s attack behavior
as prey density increasesBasic forms to consider:
Type I: Linear increase in # attacked with increasing # prey
(insatiable predator)
Type II: Gradual levelling off
As predators become satiated
Type III: Predators satiable as in Type II, but hunt inefficiently at low prey densities
# at
tack
e d/p
red/
tim
e
Prey density
I
II
III
Toxorhynchites
Toxorhynchites brevipalpus
Toxorhynchites Functional Response, sympatric & allopatric
prey:
NC (sympatric)
IL (allopatric)
Fraction killed per predator/timeType I Type II Type III
Prey Density
Type II and III: satiable predators become less effectiveat controlling prey as prey become more abundant.
Lotka-Volterra Predator-Prey Model:
Assume:
1) Random search, producing encounters between prey andpredators (and subsequent attacks) proportional to the product of their densities (attack rate = a’)
2) Exponential prey population growth in absence of predator, with constant growth rate, r
3) Death rate of predator is constant = q
4) Birth rate of predator proportional to #prey consumed
Prey growth equationPrey:
Without predator, dN/dt=rN
If predator searches with attack rate a’, and there are CPredators, then deaths due to predation = a’CN
dN/dt = rN - a’CN
Predator Growth Equation
dC/dt = (birth rate - death rate)C
Death rate assumed constant = qBirth rate: #prey consumed x conversion constant, f
= (#prey consumed)x f
# prey consumed = a’CN (see prey equation)births = a’CNfbirth rate = a’Nf
dC/dt = (a’Nf - q)C
Equilibrium Conditions, Prey
Prey:dN/dt = rN - a’CN = 0
r-a’C = 0C = r/a’
C
N
C = r/a’
Too many predators
Not enough predators
Equilibrium conditions, predatorsdC/dt = (a’Nf - q)C = 0
a’Nf - q = 0
N = q/a’f C
N
N =
q/a’f
More than enough preyN
ot e
noug
h pr
ey
Changes in both species:
C
N
The prey curve has a hump
Humped Prey curves
Rot
ifer
den
sity
Phytoplankton density
Change in phytoplankton density at different combinations of Rotifer density and phytoplankton density
Why the Prey curve has a Hump
1. Resource limits for prey at high densities
(fewer preds needed to keep in check)
2. But, predator is most effective at low prey densities
Effects of a humped prey curve:
Increasing oscillation(unstable)
Damped oscillation(stable point)
Neutralstability
C
N
Effects of a humped prey curve:
Increasing oscillation(unstable)
Damped oscillation(stable point)
Neutralstability
C
N
time
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