Transcript Predation
Exploitation High Low Intimacy Parasitoids Parasite Predator Grazer Low Lethality High There are 4 general categories • “True” predators • Herbivores – – – – Grazers Browsers Granivores Frugivores • Parasites • Parasitoids “True” predators 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. Alfred Lotka (1880-1949) Vito Volterra (1860-1940) Predation start • Rate of increase of prey population dH/dt = rH population abundance 12000 10000 8000 6000 4000 2000 0 0 2 4 6 8 10 time 12 14 16 18 20 Predation • 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 90 population abundance dP/dt = -qP – If only predators exist, no prey, so predators die 100 80 70 60 50 40 30 20 10 0 0 10 20 30 40 50 time 60 70 80 90 100 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 Predation • 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 Predator Pop size dH/dt =0 r/a’ Prey pop size Predation dP/dt =0 • Graphical Equilibrium Predator Pop size q/fa’ Prey pop size – 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 • Predator-Prey interaction – The stable dynamic of predators and prey is a cycle dP/dt =0 Predator Pop size r/a’ q/fa’ Prey pop size 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. i) Prey iscoline Predator density N2 Prey increase ii) Predator iscoline Prey density K1 N1 N2 Predator density K2 Predator decreases Predator increases Prey density N1 Predator-Prey Models – Superimpose prey and predator isoclines • One stable point emerges: the intersection of the lines • Three general cases Predator isocline a) Population density Predator Density – Inefficient predators require high densities of prey Damped oscillations Prey isocline Prey Density Time Predator-Prey Models • Three general cases (cont.) Predator Density b) Predator equilibrium density Prey Density Population density – A moderately efficient predator leads to stable oscillations of predator and prey populations Stable oscillations Time Predator-Prey Models • Three general cases (cont.) Population density – A highly efficient predator can exploit a prey nearly down to its limiting rareness Prey Density Increasing oscillations Time 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 •Limited by handling time •The rate of capture by predator •Alters Behavior –Type I –Type II –Type III Eat all you want Eat all you can Eat all you can, if you can find it! C. S. Holling (1930–) Types of functional responses Slide 25 Not all predators are created equal Keystone predator • Bob Paine at University of Washington mussel is a competitive dominant in this system Keystone predator absent Keystone predator present Other examples of keystone predators Keystone predator absent Keystone predator present The effects of herbivory • Individual plants are affected in the following areas – plant defenses – plant compensation • plant growth • plant fecundity Chemicals Defenses • Quantitative Defenses: • Qualitative 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). • 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.