1. Exploitative Interactions
Predation / Herbivory / Parasitism

2. 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.

3. Consumer-Resource Interactions
Predators: kill and eat other organisms

4. Consumer-Resource Interactions
Parasite: lives on the tissue of its host, but does not usually kill it

5. Consumer-Resource Interactions
Parasites can alter the behavior of their hosts and sometimes kill their host…

6. Consumer-Resource Interactions
Herbivores: consume plants but do not usually kill them

7. Consumer-Resource Interactions
Consumer species can influence the distribution and abundance of their resource species

8. 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

9. Consumer-Resource Interactions
St. John’s wort / Chrysolina beetles

10. Consumer-Resource Interactions
California sea otters, purple urchins, and kelp

11. 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

12. 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

13. Consumer-Resource Interactions

14. 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

15. 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

16. Consumer-Resource Interactions: Predator/Prey Cycles
Climate change – warmer winters in Northern Scandinavia

17. 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

18. 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

19. 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

20. 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

21. Consumer-Resource Interactions: Host/Pathogen Cycles
..but, in some places, it takes up to 9 years for the virus to stop the infestation…

22. 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]

23. 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

24. 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

25. Lotka-Volterra model for cyclic predator-prey interactions

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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