Lecture 15: Suggested Exam Questions

1. Name and describe the three types of consumer-resource interactions. What is an example of each?
Answer: Consumer-resource interactions are forms of predation, and include carnivory, herbivory, and parasitism. Carnivory, like an alligator eating a water buffalo, is an animal eating another animal in a way that is lethal to the prey. Herbivory, like a water buffalo grazing on grassland, is an animal eating a whole plant or parts of plants and is usually non-lethal. Parasitism, like tapeworms in humans, consume part of a living host which increases the probability of the host dying, and are increasingly lethal at increasing densities. Some parasites, like certain wasps in caterpillars, are parasitoids which consumer their hosts from within and then cause its death.

2. What is the difference between a functional and a numerical response?
Answer: A functional response describes the number of prey killed per predator as prey density changes, which is a proportional demonstration of an individual response. Functional responses might occur because of prey-switching, where a predator targets the most abundant prey in an area. A numerical response describes the number of predators as prey density changes, which is a population-level response. Predator density can increase as a result of reproduction or immigration.

3. Keystone species
a. What is a keystone species? How do scientists determine if a species is a keystone?
A keystone species is one that has such a strong effect on an ecosystem that its presence or absence (or change in abundance) strongly affects other species in the community. They produce effects that are out of proportion to their abundance. Scientists can test if a species is a keystone or not through addition or removal experiments and evaluating the impact on the broader ecosystem.
b. Give an example of a keystone species and explain its impact on an ecosystem
A starfish is a keystone species in rocky intertidal zones, like the coast of Washington. A removal experiment showed that, absent starfish, mussels will crowd out other species. With starfish (who prey on mussels) present, barnacles, echinoderms, and other marine invertebrates are able to maintain a presence in the ecosystem.

Question 3: List several plants that employ defensive techniques and give a short explanation of each.
Tobacco plants produce a toxic secondary compound known as nicotine. Cacti have sharp spines that may pierce predators and cause an allergic reaction. Magnolia leaves have a tough, waxy covering that make it difficult for predators to eat and digest. The pitcher plant leaves secret nectar to attract insects to eat herbivores.

Question 4: Describe aposomatic coloration and what some species have developed as a result of aposomatic success.
Aposomatic coloration is basically a warning to predators to indicate toxicity or foul taste. The toxic prey offers bright colors or bold patterns to serve as the warning. Some species have developed mimicry as a defense to escape predation by resembling toxic prey.

Question 5: What are the two types of mimicry? Give examples of each type.
The first type of mimicry is Batesian where the model is toxic, the mimic is palatable. This requires a level of learning by the predator. Batesian mimicry is destabilized by a high abundance of mimics. This is the more common form of mimicry. It requires a predator that is smart enough to learn—there's no point in looking dangerous if a predator doesn't care and comes along and eats you anyway! The second type of mimicry is Mullerian where multiple species converge on the same warning coloration. Many rain forest butterflies exhibit this type of mimicry.

Question 7: Consumer-Resource Interactions Multiple Choice
1) This model is relatively complicated but still is built upon some rather simple assumptions. Which of the following is not an assumption of the predator-prey model? Hint: think about the equations:
$\frac{dV}{dt}=rV-cVP$ and $\frac{dP}{dt}=acVP-dP$
A. the prey population increases exponentially in absence of the predator population.
B. the predator population increases exponentially in absence of the prey population.
C. The predator population decreases at a constant, per-capita rate.
D. The only way prey die is by being killed and consumed by predators.

Answer: B

2) What happens to the prey population when predators are absent? When you set P = 0 then the prey population

$\frac{dV}{dt}=rV-cVP$ and $\frac{dP}{dt}=acVP-dP$
A. goes extinct.
B. grows exponentially.
C. grows logistically.
D. remains in equilibrium.
E. none of the above.

Answer: B

3) Assuming the predator population is not extinct what happens to the predators when the prey are extinct? The predator population
$\frac{dV}{dt}=rV-cVP$ and $\frac{dP}{dt}=acVP-dP$

A. goes extinct.
B. increases exponentially.
C. remains in equilibrium.
D. none of the above.

Answer: A

What is the Lotka-Volterra model? What do each of the letters stand for? When is the system at equilibrium?

dN/dt = rN - cNP, and dP/dt = acNP - dp

d=predator death rate, N (or V)=prey density, P=predator density, c=predation capture efficiency, and ac=predator growth rate (capture efficiency * efficiency of converting prey biomass into new predators)

The model resulting from this equation changes in a cycle, with more prey appearing when rN>cNP, r>cP, and P<r/c, and more predators appearing when acNP>dp, acN>d, and N>d/ac (Prey increases when predators are low, predators then increase when prey are high, high predation causes fewer prey, fewer prey causes fewer predators, and then repeat). The model shows neutral stability, oscillating around a certain point. The sizes of the oscillations are based on the starting density.

Describe the hare-lynx consumer-resource interaction. How was the interaction tested in the large-scale experiment discussed in the lecture? What were the results of the study?

The people in charge set aside plots in the Yukon at 1 km2 each, and studied them over eight years. They fenced out the lynx with electric fences, and added nutrients to grow more plants, and both in some plots. With lynxes excluded alone, the population of hares doubled, with food added alone, the population of hares tripled, and with both together, the population of hares increased tenfold!
Also, in Newfoundland, there were some observations that after snowshoe hares were first introduced, they provided more food for the lynx than just arctic hare alone, and made the lynx even more able to kill caribou calves.

What are the natural stabilizers of predator-prey systems? How can human actions disrupt these systems?

Stabilizers: environmental spatial heterogeneity/complexity, prey defenses, switching to alternate prey when one is rare, density-dependent mechanisms in predators or prey (i.e. Type III functional response)
Humans may alter the cycles by introducing other predators or prey, killing off local predators or prey, and others.

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