Enemy-victim dynamics II

Question 1

The idea of ‘time budget’ in ecology refers to the time spent doing an activity. In prey capture, what 2 factors affect the time budget of predators?

Answer 1

1. Handling time of prey, i.e. how long between catching it and consuming it
2. Satiation of appetite, how much of prey does it eat/how many prey animals does it eat

Question 2

Define a functional response.

Answer 2

The intake rate of a consumer as a function of food density.

Question 3

Define a numerical response.

Answer 3

The reproduction rate of a consumer as a function of food density. Basically how many prey animals does a predator need to eat to make babies.

Question 4

On a graph showing a functional response, what goes on the y axis?

Answer 4

Number of prey animals consumed.

Question 5

On a graph showing a functional response, what goes on the x axis?

Answer 5

Density of prey population.

Question 6

There are 3 types of functional response in predation. Explain Type I.

Answer 6

The relationship is linear: as prey density increases so does consumption rate.

Question 7

There are 3 types of functional response in predation. Explain Type II.

Answer 7

The graph begins as a steady slope then plateaus off. At first search rate is constant, but as the predators become satiated the graph begins to plateau. The predators can only eat so much food, so prey mortality rate declines with prey density.

Question 8

Holling (1959) created the disc equation. What does it a) assume and b) tell us about predator consumption?

Answer 8

a) Assumes that predators spend time handling prey in searching for, chasing, killing, eating and digesting it.
b) Holling’s disc equation states that predator consumption is limited: even when prey is so abundant that the predator does not need to waste time searching for it, it must still waste time handling it.

Question 9

There are 3 types of functional response in predation. Explain Type III.

Answer 9

The graph begins with a steeper slope than a Type II curve. This is because predators increase their search activity with increasing prey density, thus consumption is faster. After a while the graph plateaus off as the predators become satiated, as in Type II.

Question 10

Describe what happens to prey mortality rate in a) Type I, b) Type II and c) Type III curves.

Answer 10

a) Prey mortality rate is constant
b) Prey mortality rate declines with prey density
c) Prey mortality rate first increases then declines with density

Question 11

How do predators increase their search activity in Type III functional responses? Give 2 examples.

Answer 11

1. Predators become better at recognising prey, e.g. kairomones are chemicals emitted by prey that can help to find them.
2. Switching to different prey sources.

Question 12

Give examples of predators in nature that show a) Type I, b) Type II and c) Type III functional response.

Answer 12

a) Passive predators like spiders: the number of flies that get caught in their web is proportional to fly density
b) Small insectivorous mammals: for example when population size is small effects of predation are highly damaging. In larger populations the effects are negligible.
c) Polyphagous predators like birds: they can switch to the species that is the most abundant by learning to recognise it visually.

Question 13

If predator density is constant, then they can only regulate prey density under which kind of functional response? Why?

Answer 13

Type III: this is the only functional response whereby prey mortality rate increases with density. The regulatory effect of predation is limited to this interval only.

Question 14

What happens if prey density exceeds the upper limit for predator regulation in a Type III functional response?

Answer 14

Mortality due to predation begins to decline and prey numbers become uncontrollable. Then other factors begin to limit their reproduction, e.g. food shortage.

Question 15

What is the name of the phenomenon when prey density exceeds the upper limit for predator regulation in a Type III functional response? Who discovered it?

Answer 15

Escape from natural enemies, discovered by Takahashi.

Question 16

Define the paradox of enrichment.

Answer 16

Where increasing food availability for prey animals causes the predator population to destabilise.

Question 17

Explain how the paradox of enrichment works.

Answer 17

If there is excess food available to prey their population will grow unbounded, causing a large increases in the number of predators that proves unsustainable.

Question 18

There are 5 major conditions that prevent the paradox of enrichment being fulfilled. What are they?

Answer 18

1. Invulnerable prey, e.g. prey that is able to hide adequately and prevent consumption
2. Unpalatable prey that does not fulfil the nutritional requirements of a predator
3. Environmental heterogeneity that disadvantages predators in prey capture/gives prey an advantage
4. Predator-induced defence by prey
5. Prey toxicity, whereby the prey are inedible

Question 19

Give an example of host-parasite oscillations.

Answer 19

There are oscillations in Red Grouse populations every 4-8 years. They play host to parasitic nematodes. Their population growth is thus negatively correlated to that of nematode population growth.

Question 20

Give 5 features of microparasites.

Answer 20

1. Completes a full life cycle within the same host.
2. Too small to be seen with the naked eye.
3. There is direct reproduction within the host
4. Large volumes of parasite involved
5. Transmission is conspecific, i.e. to members of the same species.

Question 21

Hosts always die from infection with microparasites. True or false?

Answer 21

False - the recovered host acquires immunity to the microparasite.

Question 22

Give 3 examples of microparasites.

Answer 22

1. Viruses
2. Bacteria
3. Protists

Question 23

Give 5 features of macroparasites.

Answer 23

1. They do not complete a full lifecycle within one host
2. Large enough to be seen with the naked eye
3. Reproduction occurs outside of the host
4. Immune response depends on past/present parasite numbers and is short-lived. Infections tend to persist.
5. Often have complex life cycles involving numerous hosts

Question 24

Give examples of macroparasites.

Answer 24

1. Nematodes
2. Flukes
3. Tapeworms

Question 25

We must also consider parasite evolution within the host. True or false?

Answer 25

True, with both micro and macro parasites.

Question 26

Give an example of a disease caused by a macroparasite.

Answer 26

Schistosomiasis caused by blood flukes.

Question 27

There are multiple phases of infection. Define the latent period.

Answer 27

The time between the host being infected and becoming infectious.

Question 28

There are multiple phases of infection. Define the infectious period.

Answer 28

The time during which parasite transmission can occur.

Question 29

There are multiple phases of infection. Define the incubation period.

Answer 29

The time from infection to the appearance of symptoms.

Question 30

There are multiple phases of infection. Define the recovery period.

Answer 30

The time from which hosts are immune. This is often lifelong.

Question 31

When looking at microparasite infection, the host population is typically divided into 4 compartments. These are?…

Answer 31

1. Susceptible
2. Exposed
3. Infectious
4. Recovered

Question 32

Thus what is the model called when studying microparasite infection?

Answer 32

SEIR.

Question 33

The equation for the susceptible population is:

dS/dt = mN - bSI -dS.

Explain each term.

Answer 33

dS/dt = change in density of susceptible pop.
mN = birth rate 
bSI = contact rate between susceptible and infectious individuals
dS = death rate of S

Question 34

The equation for the exposed population is:

dE/dt = bSI - aE - dE

Explain each term.

Answer 34

dE/dt = change in density of exposed pop.
bSI = contact rate between susceptible and infectious individuals
aE = rate of exposed becoming infectious
dE = death rate of E

Question 35

The equation for the infectious population is:

dI/dt = aE - gI - dI

Explain each term.

Answer 35

dI/dt = change in density of infectious pop.
aE = rate of exposed becoming infectious
gI = recovery rate of I
dI = death rate of I

Question 36

The equation for the recovering population is:

dR/dt = gI - dR

Explain each term.

Answer 36

dR/dt = density change in recovering pop
gI = recovery rate of I
dR = death rate of recovering pop.

Question 37

What is R0?

Answer 37

The average number of secondary infections caused by the introduction of an infected host into a completely susceptible host population.

N.B. is essentially the infectious period x no. of new infections per unit time.

Question 38

R0 = abS/(a + d)(g + d). Explain the terms.

Answer 38

abS = infectious period x birth rate of susceptible pop.
d = death rate
g = recovery rate

Question 39

What is a threshold density in terms of infection?

Answer 39

The minimum population size required for disease to spread.

Question 40

What must R0 be for disease to invade a population of susceptibles?

Answer 40

R0 > 1

Question 41

How do we calculate threshold density for disease? Give the equation.

Answer 41

NT = g/b

g = recovery rate
b = contact rate

Question 42

Will disease spread if the population is below threshold density?

Answer 42

No.

Question 43

If proportion p is vaccinated against a disease, what is remaining susceptible proportion?

Answer 43

1 - p

Question 44

What is the effective reproductive rate of a disease if proportion p has been vaccinated?

Answer 44

R = R0(1-p)

R0 = average number of secondary infections caused by introduction of an infected into a pop. of susceptibles
(1-p) = proportion that is still susceptible

Question 45

What does it mean if R < 1?

Answer 45

The infection will not maintain itself and be eradicated.

Question 46

Thus what is the critical proportion that must be immunised?

Answer 46

pc = 1 – (1/R0)