Enemy-victim dynamics II Flashcards
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?
- Handling time of prey, i.e. how long between catching it and consuming it
- Satiation of appetite, how much of prey does it eat/how many prey animals does it eat
Define a functional response.
The intake rate of a consumer as a function of food density.
Define a numerical response.
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.
On a graph showing a functional response, what goes on the y axis?
Number of prey animals consumed.
On a graph showing a functional response, what goes on the x axis?
Density of prey population.
There are 3 types of functional response in predation. Explain Type I.
The relationship is linear: as prey density increases so does consumption rate.
There are 3 types of functional response in predation. Explain Type II.
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.
Holling (1959) created the disc equation. What does it a) assume and b) tell us about predator consumption?
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.
There are 3 types of functional response in predation. Explain Type III.
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.
Describe what happens to prey mortality rate in a) Type I, b) Type II and c) Type III curves.
a) Prey mortality rate is constant
b) Prey mortality rate declines with prey density
c) Prey mortality rate first increases then declines with density
How do predators increase their search activity in Type III functional responses? Give 2 examples.
- Predators become better at recognising prey, e.g. kairomones are chemicals emitted by prey that can help to find them.
- Switching to different prey sources.
Give examples of predators in nature that show a) Type I, b) Type II and c) Type III functional response.
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.
If predator density is constant, then they can only regulate prey density under which kind of functional response? Why?
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.
What happens if prey density exceeds the upper limit for predator regulation in a Type III functional response?
Mortality due to predation begins to decline and prey numbers become uncontrollable. Then other factors begin to limit their reproduction, e.g. food shortage.
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?
Escape from natural enemies, discovered by Takahashi.
Define the paradox of enrichment.
Where increasing food availability for prey animals causes the predator population to destabilise.
Explain how the paradox of enrichment works.
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.
There are 5 major conditions that prevent the paradox of enrichment being fulfilled. What are they?
- Invulnerable prey, e.g. prey that is able to hide adequately and prevent consumption
- Unpalatable prey that does not fulfil the nutritional requirements of a predator
- Environmental heterogeneity that disadvantages predators in prey capture/gives prey an advantage
- Predator-induced defence by prey
- Prey toxicity, whereby the prey are inedible
Give an example of host-parasite oscillations.
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.
Give 5 features of microparasites.
- Completes a full life cycle within the same host.
- Too small to be seen with the naked eye.
- There is direct reproduction within the host
- Large volumes of parasite involved
- Transmission is conspecific, i.e. to members of the same species.
Hosts always die from infection with microparasites. True or false?
False - the recovered host acquires immunity to the microparasite.
Give 3 examples of microparasites.
- Viruses
- Bacteria
- Protists
Give 5 features of macroparasites.
- They do not complete a full lifecycle within one host
- Large enough to be seen with the naked eye
- Reproduction occurs outside of the host
- Immune response depends on past/present parasite numbers and is short-lived. Infections tend to persist.
- Often have complex life cycles involving numerous hosts
Give examples of macroparasites.
- Nematodes
- Flukes
- Tapeworms
We must also consider parasite evolution within the host. True or false?
True, with both micro and macro parasites.
Give an example of a disease caused by a macroparasite.
Schistosomiasis caused by blood flukes.
There are multiple phases of infection. Define the latent period.
The time between the host being infected and becoming infectious.
There are multiple phases of infection. Define the infectious period.
The time during which parasite transmission can occur.
There are multiple phases of infection. Define the incubation period.
The time from infection to the appearance of symptoms.
There are multiple phases of infection. Define the recovery period.
The time from which hosts are immune. This is often lifelong.
When looking at microparasite infection, the host population is typically divided into 4 compartments. These are?…
- Susceptible
- Exposed
- Infectious
- Recovered
Thus what is the model called when studying microparasite infection?
SEIR.
The equation for the susceptible population is:
dS/dt = mN - bSI -dS.
Explain each term.
dS/dt = change in density of susceptible pop. mN = birth rate bSI = contact rate between susceptible and infectious individuals dS = death rate of S
The equation for the exposed population is:
dE/dt = bSI - aE - dE
Explain each term.
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
The equation for the infectious population is:
dI/dt = aE - gI - dI
Explain each term.
dI/dt = change in density of infectious pop. aE = rate of exposed becoming infectious gI = recovery rate of I dI = death rate of I
The equation for the recovering population is:
dR/dt = gI - dR
Explain each term.
dR/dt = density change in recovering pop gI = recovery rate of I dR = death rate of recovering pop.
What is R0?
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.
R0 = abS/(a + d)(g + d). Explain the terms.
abS = infectious period x birth rate of susceptible pop. d = death rate g = recovery rate
What is a threshold density in terms of infection?
The minimum population size required for disease to spread.
What must R0 be for disease to invade a population of susceptibles?
R0 > 1
How do we calculate threshold density for disease? Give the equation.
NT = g/b
g = recovery rate b = contact rate
Will disease spread if the population is below threshold density?
No.
If proportion p is vaccinated against a disease, what is remaining susceptible proportion?
1 - p
What is the effective reproductive rate of a disease if proportion p has been vaccinated?
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
What does it mean if R < 1?
The infection will not maintain itself and be eradicated.
Thus what is the critical proportion that must be immunised?
pc = 1 – (1/R0)
