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

Amboseli Elephants

Elephants not excluded

Elephants Excluded

Baobab

Baobab, Elephant Damage

Functional Response Change in predator’s attack behavior as prey density increases Basic 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 # attacked/pred/time Prey density I II III

Toxorhynchites

Toxorhynchites brevipalpus

Toxorhynchites Functional Response, sympatric & allopatric prey: NC (sympatric) IL (allopatric)

Fraction killed per predator/time Type IType IIType III Prey Density Type II and III: satiable predators become less effective at controlling prey as prey become more abundant.

Lotka-Volterra Predator-Prey Model: Assume: 1)Random search, producing encounters between prey and predators (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 equation Prey: Without predator, dN/dt=rN If predator searches with attack rate a’, and there are C Predators, 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 = q Birth rate: #prey consumed x conversion constant, f = (#prey consumed)x f # prey consumed = a’CN (see prey equation) births = a’CNf birth rate = a’Nf dC/dt = (a’Nf - q)C

Equilibrium Conditions, Prey Prey: dN/dt = rN - a’CN = 0 r-a’C = 0 C = r/a’ C N Too many predators Not enough predators

Equilibrium conditions, predators dC/dt = (a’Nf - q)C = 0 a’Nf - q = 0 N = q/a’f C N More than enough prey Not enough prey

Changes in both species: C N

The prey curve has a hump

Humped Prey curves Rotifer density 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) Neutral stability C N

Effects of a humped prey curve: Increasing oscillation (unstable) Damped oscillation (stable point) Neutral stability C N time