5Herbivores Attack many prey items in a lifetime Consume only a bit of the victimDo not usually kill prey in the short term (but may do so in the long term)GrazerBrowserGranivoreFrugivore
6Parasites Parasitoids Consume part of their prey Do not usually kill their preyAttack one or very few prey items in their lifetimeParasitoids
7Parasites Parasitoids which straddle the parasite and true predator categories - they lay eggs inside their host which they eventually kill
8Predation is important because: It may restrict the distribution of, or reduce the abundance of the prey species.Predation, along with competition, is a major type of interaction that can influence the organization of communities.Predation is a major selective force, and many adaptations of organisms have their explanation in predator-prey coevolution.Evolutionary arms racePredation drives the movement of energy and nutrients in ecosystems.
10PredationstartRate of increase of prey populationdH/dt = rH
11Predation Rate of increase of prey population a' = capture coefficient dH/dt = rHPredators eat preydH/dt = rH-a'HPa' = capture coefficientH = Prey pop sizeP = Predator pop size
12Predation Rate of increase of predator populations dP/dt = -qP If only predators exist, no prey, so predators die
13Predation Rate of increase of predator populations dP/dt = -qPIf only predators exist, no prey, so predators diedP/dt = fa’HP-qPf = is a predation constantPredator’s efficiency at turning food into predator offspring.a' = capture coefficientq = mortality rate
15Predation Graphical Equilibrium Prey (H) equilibrium (dH/dt=0) is determined by predator population size.If the predator population size is large the prey population will go extinctIf the predator population is small the prey population size increasesPredatorPop sizedH/dt =0r/a’Prey pop size
16Predation Graphical Equilibrium q/fa’ dP/dt =0 Predator (P) equilibrium (dP/dt=0) is determined by prey population size.If the prey population size is large the predator population will increaseIf the prey population is small the predator population goes extinctPredatorPop sizeq/fa’Prey pop size
17Predation Predator-Prey interaction q/fa’ dP/dt =0 The stable dynamic of predators and prey is a cyclePredatorPop sizer/a’q/fa’Prey pop size
18Do these models apply to natural populations? Lynx/Snowshoe Hare - Arctic system where there is one predator and one prey
19Lynx/Snowshoe Hare - Arctic system where there is one predator and one prey Assumptions?
20Rosenzweig & MacArthur (1963) Three possible outcomes of interactionsThe oscillations are stable (classical oscillations of Lotka-Volterra equations).The oscillations are damped (convergent oscillation).The oscillations are divergent and can lead to extinction.
21i) Prey iscoline N Predator density Prey increase Prey density K N 2Predator densityPrey increasePrey densityKNii) Predator iscoline11N2K2Predator decreasesPredator increasesPredator densityPrey densityN1
22Predator-Prey Models Superimpose prey and predator isoclines One stable point emerges: the intersection of the linesThree general casesInefficient predators require high densities of preyDamped oscillationsPreyisoclinePredatora)Predator DensityPrey DensityTimePopulation density
23Predator-Prey Models Three general cases (cont.) A moderately efficient predator leads to stable oscillations of predator and prey populationsStable oscillationsPopulation densityPredatorequilibriumdensityb)Prey DensityPredator DensityTime
24Predator-Prey Models Three general cases (cont.) A highly efficient predator can exploit a prey nearly down to its limiting rarenessIncreasing oscillationsPredator densityPrey DensityTimePopulation density
25All these models make a series of simplifying assumptions A homogenous world in which there are no refuges for the prey or different habitats.There is one predator species eating one prey species and there are no other species involved in the dynamics of these two populationsRelaxing these assumptions leads to more complex, but more realistic models.All predators respond to prey in the same fashion regardless of densityFunctional Response
26Conclusions form field studies There is not a clear relationship between predator abundance and prey population size.In some, but not all cases, the abundance of predators does influence the abundance of their prey in field populations.
27What makes predators effective in controlling their prey? Foraging efficienyWithin a patch, the searching efficiency of a predator becomes crucial to its success.But searching efficiency varies with abiotic factors and can also decrease at high predator densities because of interference of other predators.
28Predation Response of predator to prey density Numerical Aggregative Functional
29Types of functional responses Limited by handling timeThe rate of capture by predatorAlters BehaviorType IType IIType IIIEat all you wantEat all you canEat all you can, if you can find it!C. S. Holling (1930–)
36Keystone predator absent Figure: rightCaption:When sea otters are not present, herbivory by sea urchins causes drastic reductions in the size and frequency of kelp forests.
37Keystone predator present Figure: leftCaption:When sea otters are present, kelp forests are common in the nearshore habitats of western North America.
38The effects of herbivory Individual plants are affected in the following areasplant defensesplant compensationplant growthplant fecundity
39Chemicals Defenses Qualitative Defenses: Quantitative Defenses: Prevent digestion as they accumulate in the gut.Usually found in large quantities in the plant parts that are eaten.Most of these compounds are “Carbon Rich”Common defense of plants growing in nutrient poor soils (conifers).Qualitative Defenses:Usually toxic in small quantities.Found in relatively small amounts in the portion of plants that is eaten (leaves).These compounds are “Nitrogen Rich” and therefor expensive to produce by the plant.More common in plants growing on nutrient rich soils.