Enemy-victim dynamics I

Question 1

There are always winners and losers in enemy victim dynamics. If a) predators, b) parasites and c) parasitoids are always the winners, who are the losers in each case?

Answer 1

a) Prey

b and c) Hosts

Question 2

Which is the longest continuous study of predator-prey dynamics in the world?

Answer 2

Wolf-Moose, studied for more than 5 years. The populations were small with little/no human interaction. The wolf is the moose’s only predator and the moose is the wolf’s only prey.

Question 3

What pattern was shown in the Wolf-Moose study and why?

Answer 3

Both populations fluctuated unpredictably. The fluctuations were dependent on the moose’s food source.

Question 4

Why was there a large crash in the wolf population in 1980/81?

Answer 4

There was an outbreak of canine parvovirus.

Question 5

There are 3 possible outcomes in predator-prey dynamics. What are they?

Answer 5

1. The predator has little effect on the prey population
2. Predator and prey both oscillate
3. Predator oscillates but prey remains constant

Question 6

There are coupled Lokta-Volterra equations to describe predator-prey relationships. Give the prey equation and explain the terms.

Answer 6

dN/dt = rN – aCN

dN/dt = density change in prey
r = growth rate or births-deaths
aCN = losses through predation

Question 7

There are coupled Lokta-Volterra equations to describe predator-prey relationships. Give the predator equation and explain the terms.

Answer 7

dC/dt = -qC + aƐCN

dC/dt = density change in predator
-qC = death rate of predator
aƐCN = the births via predation

Question 8

What is Ɛ specifically?

Answer 8

A conversion factor denoting the number of prey organisms a predator needs to eat in order to produce one offspring.

Question 9

Do the Lokta-Volterra equations take into account intraspecific prey competition?

Answer 9

No.

Question 10

In predator-prey interactions there is a natural tendency for cycling. Explain why this happens.

Answer 10

If there is an increase in prey, there will be an increase in predators due to higher food abundance. If there is an increase in predation the numbers of prey will then fall. If there is less food then the number of predators will fall. If there are less predators the number of prey animals is left to increase, and it begins again.

Question 11

What is a zero growth isocline?

Answer 11

It represents a combination of abundances of 2 species, when the growth of one species is set to 0. These are used to look at the cycling in predator-prey interactions.

Question 12

If Ɛ=1, what kind of enemy-victim interaction are we looking at?

Answer 12

Parasite/parasitoid-host, as only 1 host is required for survival of the parasite/parasitoid.

Question 13

The Lokta-Volterra predator-prey equations are the same for parasite-host interaction but the terms mean something slightly different. Give the equations and explain the changes.

Answer 13

dN/dt = rN – aCN is for the host

dC/dt = -qC + aƐCN is for the parasite

N = disease-free individuals (not prey)
C = infected individuals (not predator)
a = transmission rate (not births via predation)

Question 14

There is generally a time lag between a peak in the prey, then a peak in the predator population. True or false?

Answer 14

True.

Question 15

Give an example of time lags in predator-prey interactions.

Answer 15

Hare and Lynx: the hares eat grass. When there are high numbers of hares the grass begins to produce toxins to make itself unpalatable. These toxins accumulate in the grass for approx. 3 years. Thus the hares spend more time out in the open looking for other food. The lynx is then more able to catch them and increases in number. By this time the hare population has begun to decrease due to predation and lack of food.

Question 16

Define self-limitation.

Answer 16

Where the growth of an organism is limited by its own actions.

Question 17

Give an example of self-limitation in a) prey and b) predators.

Answer 17

a) Resource consumption, e.g. not eating enough

b) Competition or territoriality

Question 18

What is meant by a) top-down and b) bottom-up control of a population?

Answer 18

a) By predation

b) By resource consumption

Question 19

Briefly outline the Carl Huffaker experiment.

Answer 19

1. Included 2 species of mite, a prey and predator species
2. Prey mites feed on oranges. Only the top 10% of the orange was exposed so mites had to climb to reach it
3. 2 environments were created: a homogenous environment, and a heterogeneous one. In the heterogeneous environment the base of the orange was coated in Vaseline and sticks were added for climbing.

Question 20

What did the Carl Huffaker experiment show?

Answer 20

In the homogenous environment, the predatory mites quickly consumed all the prey. In the heterogeneous environment, cycling occurred as the Vaseline slowed down the predatory mites, plus the prey mites who can jump could take refuge on the sticks, where the predator mites who cannot jump could not reach them.

Question 21

Environmental heterogeneity encourages predator-prey cycling. True or false?

Answer 21

True.

Question 22

List 3 factors that stabilise predator-prey populations and prevent them from crashing. Briefly explain each.

Answer 22

1. Environmental heterogeneity: predators are not always at an advantage and cannot consume all prey
2. Territoriality among predators: restricts predator consumption
3. Generalist predation: when one prey species goes too low they switch to another food source, allowing the first to recover

Question 23

Give an example of generalist predation.

Answer 23

Owls and foxes eat brown lemmings among other prey species. Stoats feed solely on brown lemmings and thus have a greater effect on their population levels.