Herbivores –Attack many prey items in a lifetime –Consume only a bit of the victim –Do not usually kill prey in the short term (but may do so in the long term) Grazer Browser Granivore Frugivore
Parasites Consume part of their prey Do not usually kill their prey Attack one or very few prey items in their lifetime Parasitoids
Parasites Parasitoids –which straddle the parasite and true predator categories - they lay eggs inside their host which they eventually kill
Predation is important because: 1.It may restrict the distribution of, or reduce the abundance of the prey species. 2.Predation, along with competition, is a major type of interaction that can influence the organization of communities. 3.Predation is a major selective force, and many adaptations of organisms have their explanation in predator-prey coevolution. 1.Evolutionary arms race 4.Predation drives the movement of energy and nutrients in ecosystems.
Vito Volterra (1860-1940) Alfred Lotka (1880-1949)
Rate of increase of prey population dH/dt = rH Predation start
Rate of increase of prey population –dH/dt = rH –Predators eat prey dH/dt = rH- a 'HP –a ' = capture coefficient –H = Prey pop size –P = Predator pop size Predation
Rate of increase of predator populations dP/dt = -qP –If only predators exist, no prey, so predators die
Predation Rate of increase of predator populations dP/dt = -qP –If only predators exist, no prey, so predators die dP/dt = fa’HP-qP –f = is a predation constant »Predator’s efficiency at turning food into predator offspring. –a ' = capture coefficient –q = mortality rate
Equilibrium population sizes –Predator dP/dt = fa’HP -qP 0= fa’HP -qP fa’HP = qP fa’H= q H= q/fa’ –Prey dH/dt = rH-a’HP 0= rH-a’HP rH= a’HP r = a’P P = r/ a’ Predation
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 extinct –If the predator population is small the prey population size increases Prey pop size Predator Pop size r/a’ dH/dt =0
Graphical Equilibrium –Predator (P) equilibrium (dP/dt=0) is determined by prey population size. –If the prey population size is large the predator population will increase –If the prey population is small the predator population goes extinct Predation Prey pop size Predator Pop size q/fa’ dP/dt =0
Predation Predator-Prey interaction –The stable dynamic of predators and prey is a cycle Prey pop size Predator Pop size q/fa’ dP/dt =0 r/a’
Do these models apply to natural populations? Lynx/Snowshoe Hare - Arctic system where there is one predator and one prey
Lynx/Snowshoe Hare - Arctic system where there is one predator and one prey Assumptions?
Rosenzweig & MacArthur (1963) Three possible outcomes of interactions The oscillations are stable (classical oscillations of Lotka-Volterra equations). The oscillations are damped (convergent oscillation). The oscillations are divergent and can lead to extinction.
Prey increase i) Prey iscoline K N N N N K 1 1 1 2 2 2 ii) Predator iscoline Prey density Predator increases Predator decreases Predator density Prey density
Predator-Prey Models –Superimpose prey and predator isoclines One stable point emerges: the intersection of the lines Three general cases –Inefficient predators require high densities of prey Damped oscillations Prey isocline Predator isocline a) Predator Density Prey DensityTime Population density
Predator-Prey Models Three general cases (cont.) –A moderately efficient predator leads to stable oscillations of predator and prey populations Stable oscillations Population density Predator equilibrium density b) Prey Density Predator Density Time
Predator-Prey Models Three general cases (cont.) –A highly efficient predator can exploit a prey nearly down to its limiting rareness Increasing oscillations Predator density Prey Density Time Population density
All 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 populations Relaxing these assumptions leads to more complex, but more realistic models. All predators respond to prey in the same fashion regardless of density –Functional Response
Conclusions 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.
What makes predators effective in controlling their prey? Foraging efficieny –Within 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.
Predation Response of predator to prey density –Numerical –Aggregative –Functional
Types of functional responses Eat all you want Eat all you can Eat all you can, if you can find it! Limited by handling time The rate of capture by predator Alters Behavior –Type I –Type II –Type III C. S. Holling (1930–)
The e ffects of herbivory Individual plants are affected in the following areas –plant defenses –plant compensation plant growth plant fecundity
Chemicals 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.