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 consumer-imposed 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