Info on consumers can limit resource populations..
• Predators limit the size of prey populations
o Many studies have demonstrated that predators can reduce populations of prey
• E.g., Klamath weed has brought under control by the introduction of beetles (biological controls)
• E.g., examples from outline
What are exclosure experiments?
• Take area where predator lives and build barriers around area
• Compare results of area where predator cannot enter and where predator is feeding.
• E.g., vole (small native rodent)
Area where vole couldn't feed on plant were seen to have a greater biomass of food plants then of those areas where the voles could feed.
Shows that voles can control their resources; have an effect on the size of the prey population
What are characteristics of effective predators
• High reproductive capability relative to prey
Can reproduce in response to prey
• Excellent dispersal ability
Predator can follow prey
• Can switch prey if primary prey declines to low numbers
Regular population cycles are associated with species that live where?
in the northern part of the northern hemisphere
For populations of predators and prey that cycle together, as prey increases, what happens to the predator population?
It increases after a time lag
*This works vice versa, also. As prey decreases, predators will over time, too.
Cycles of pathogens can be caused by what?
The development of immunity in host populations.
Info on the classic work by Gause on the coexistence of predator and prey.
o Paramecium preyed upon by Didinium
o Efficient predators can eat all of their prey in a simple lab environment
• Results in extinction of predator, also.
o Adding complexity to environment (e.g., providing hiding spots)
• Predators & resources can co-exist indefinitely if prey can hide
Info on classic work by Huffacker
o Worked with mites
o Introduced 4 oranges to 4 positions on table with 40 positions total
• Rest of positions were filled with rubber balls
• Oranges protected on most surface
Makes it possible to count mites on top exposed area as opposed to entire orange
o Would move oranges randomly
o With relatively simple background, predators (mites) were able to exterminate the mites (prey species of mite)
• But, the distribution of oranges (whether they're close together or not) dictated the time it took to eliminate prey.
o Then made it more complex
• Introduced tray with 120 positions
• Induced cyclicity by providing a mosaic of habitats
If you have a complex habitat, the prey can hide from the predator → allows the two to coexist
o How did he prevent the prey mites from being extinct
• Increased positions from 40-120
• Put Vaseline on boundaries that surround cells
• Predatory mite disperses by walking, so they could not cross some areas due to the Vaseline.
Info on the Lotka-Volterra Model
o This model is based on differential equations
o One predator species & one prey species, so 2 equations are necessary
o V = # of prey ("victim"), P = # of predators
o r = exponential growth rate
o c = capture efficiency
o a = efficiency at which food is converted into population growth
What assumptions does the Lotka-Volterra model make?
• Predators cannot switch prey
• Death rate (d) is constant (of prey/resource)
• Environment doesn't favor either species (neutral environment)
• Genetic adaptation is slow enough that neither species gains an advantage over the other
What is the equation for growth rate of prey population?
• dV/dt = rV - cVP
• rate of change in prey population = intrinsic growth rate of prey population - removal of prey individuals by predators
What is the equation for growth rate of predator population?
• dP/dt = acVP - dP
• rate of change in predator population = a birth term a multiplied by the number of prey captured (cVP) - change sin the # of individual predators
What does the Lotka-Volterra model tell us?
o Prey populations decrease in size when they have a relatively large number of predators
o Prey populations increase in size when they have a relatively small number of predators
o Predators decline when there are few prey present
o Predators increase when there are many prey present
• If P > r/c, then V decreases
Combining isoclines results in what?
a joint population trajectory
Joint equilibrium is where?
• Joint equilibrium is the point where lines representing P & V lines cross
Prey and predator populations do what with each other?
follow each other in increase and decrease
Info on bottom quadrants on circle..
Decline in predators will lead to an increase in the number of prey
Info on top quadrants on circle..
An increase in predators leads to a decrease in number of prey
If r (rate of increase) increases or c (capture efficiency) decline, we see what?
then we see a decrease in the prey isocline (which means that the prey could then support a larger number of predators)
If death rate of predators increase, and either a or c decreases, we see what?
then the predator isoclines are going to increase
As productivity of resource increase (prey), what happens?
it affects the number of predators, not prey.
- Number of prey doesn't change because predators remove extra production