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Exploitative Interactions Predation / Herbivory / Parasitism Consumer-Resource Interactions • Occur along a continuum of harmfulness to the resource…reduced fitness—death. • Common thread: one species exploits another, one benefits and one suffers. Consumer-Resource Interactions • Predators: kill and eat other organisms Consumer-Resource Interactions • Parasite: lives on the tissue of its host, but does not usually kill it Consumer-Resource Interactions • Parasites can alter the behavior of their hosts and sometimes kill their host… Consumer-Resource Interactions • Herbivores: consume plants but do not usually kill them Consumer-Resource Interactions • Consumer species can influence the distribution and abundance of their resource species Consumer-Resource Interactions • Consumer species can influence the distribution and abundance of their resource species • Introduced invasive species often successful due to release from consumer species present in their native range • Biological control / integrated pest management often utilize a specific consumer species to control pests Consumer-Resource Interactions • St. John’s wort / Chrysolina beetles Consumer-Resource Interactions • California sea otters, purple urchins, and kelp Consumer-Resource Interactions • Predatory / prey & other consumer-resource interactions are dynamic • Consumers influence the abundance of resource populations… • The abundance of resource populations affects the size of consumer populations • These dynamics only make sense if all trophic levels are considered Consumer-Resource Interactions: Predator/Prey Cycles • Increase in primary producers (plants) provides more food for primary consumers (herbivores) • Herbivores show a numerical response to increased food supply and increase in numbers • Secondary consumers (predators) also show a numerical response to increased availability of prey (herbivores) • Eventually, depletion of their food supply and predation pressure cause the herbivore populations to decline in number • Reduced availability of prey also causes the numbers of predators to decline …fewer herbivores means more plants can grow…more food available…replay cycle Consumer-Resource Interactions Consumer-Resource Interactions: Predator/Prey Cycles • Predator-prey cycles have a built in time delay because of the time required for both populations to produce offspring • Ongoing occurrence of cycles is generally stable, but the periodicity and intensity of cycles can vary due to environmental conditions Consumer-Resource Interactions: Predator/Prey Cycles • Owl/vole populations in Scandinavia: • Northern Scandinavia – 4 year cycle • Snow cover in winter protects voles from owl predation, lessens effects of predation and extends cycle • Southern Scandinavia – 1 year cycle • Less snow cover, owls can hunt through much of the winter, quicker cycle Consumer-Resource Interactions: Predator/Prey Cycles • Climate change – warmer winters in Northern Scandinavia Consumer-Resource Interactions: Host/Pathogen Cycles • Host immunity slows spread of pathogen, causes time delays in epidemics • Measles – highly contagious disease, but stimulates life-long immunity • Produces epidemics at 2-year intervals in unvaccinated human populations • Two years required for population to accumulate a high enough density of susceptible infants to sustain a measles outbreak Consumer-Resource Interactions: Host/Pathogen Cycles • Host density also an important factor: • as host population density increases, rate of transmission between hosts also increases • But…if pathogens increase host mortality or impair host reproduction, host population densities will drop low enough to break the chain of transmission, and host densities will again begin to rise Consumer-Resource Interactions: Host/Pathogen Cycles • Forest tent caterpillars – periodic population booms can defoliate stands of trees over thousands of square kilometers • Usually brought under control by a virulent pathogen that causes high mortality of caterpillars at high host densities • Most places, infestation lasts 2 years before virus can bring the host population under control Consumer-Resource Interactions: Host/Pathogen Cycles • Forest tent caterpillars – periodic population booms can defoliate stands of trees over thousands of square kilometers • Usually brought under control by a virulent pathogen that causes high mortality of caterpillars at high host densities • Most places, infestation lasts 2 years before virus can bring the host population under control Consumer-Resource Interactions: Host/Pathogen Cycles • ..but, in some places, it takes up to 9 years for the virus to stop the infestation… Lotka-Volterra model for cyclic predator-prey interactions Assumes exponential growth of the resource population, limited only by consumers: • Growth of prey population = [the intrinsic growth rate of the prey population] – [the removal of prey individuals by predators] Lotka-Volterra model for cyclic predator-prey interactions dNh/dt = rhNh – pNhNp rhNh = Exponential growth by host population Opposed by: p = rate of parasitism / predation Nh = Number of hosts Np = Number of parasites / predators Lotka-Volterra model for cyclic predator-prey interactions Consumer population growth rate = [rate of conversion of food into offspring] – [mortality rate of consumer population] dNp/dt = cpNhNp - dpNp cpNhNp = Rate at which consumer species converts food (resource species) into offspring (c = conversion factor) dpNp= Death rate of consumer species Lotka-Volterra model for cyclic predator-prey interactions Lotka-Volterra model for cyclic predator-prey interactions • Elements of reality that the model does not incorporate: • Time lags in predator/prey response • Carrying capacities for predator and prey populations • No functional response in predator Lotka-Volterra model for cyclic predator-prey interactions • Experiments that seek to replicate the situation in the Lotka-Volterra model generally fail • In order for cycles to occur, some destabilizing factor must be present to drive the system (e.g., time delay in the response of population to a change in its food supply) • To extend predator prey cycles, stabilizing factors (generally more than one) have to be in place to balance these destabilizing factors Lotka-Volterra model for cyclic predator-prey interactions • Stabilizing factors include: • Predator inefficiency (or enhanced prey escape/defense) • Density-dependent limitation on either the predator or the prey population by external factors • Alternative food source for the predator • Refuges for the prey at low prey densities • Reduced time delays in predator responses to changes in prey abundance • Influx of new replacement individuals from source population Consumer-resource systems can have more than one stable state • Alternative stable states can arise when different factors limit populations at low and high densities • At low densities, individuals of the resource population may be so difficult to locate that the consumer species will switch to another resource Consumer-resource systems can have more than one stable state • At low densities, individuals of the resource population may be so difficult to locate that the consumer species will switch to another resource • At low densities, the resource species will tend to grow faster than the consumer species can remove it, because resources are not limiting • As the resource population grows, however, the consumer species will focus on the increasingly abundant food supply and eventually bring the population under control at a low stable equilibrium well below its carrying capacity – Consumer-imposed equilibrium Consumer-resource systems can have more than one stable state • If a resource population is at a size well above its consumerimposed equilibrium, consumer efficiency should go up as the population density increases • At some point, however, consumers themselves become satiated (type II or III functional response) or the consumer population becomes limited by external factors, such as suitable nest sites or their own predators. • If this point is reached, the resource population can escape consumer control and continue to increase until it reaches the size limit imposed by its own carrying capacity – a resource-imposed equilibrium