L11 Tosh - Predators, parasites, herbivores and the impacts on popualtions

Question 1

Describe the occurrence of the Myxoma virus in Australia

Answer 1

• Myxoma originates in S.America where it has little effect on native rabbits
• Introduced to the UK in 1950s and within 2 years, 95% of the rabbit population were dead
• Virus lethality declined - resistant rabbits lived and reproduced, or less lethal strains of the virus survived, or virus changed over time
• rabbit population is now recovered

Question 2

Describe what a Parasitoid is

Answer 2

• mainly hymenoptera
• host is killed
• host can be an egg, larvae, pupa or adult
• Hyperparasitiods exist

Question 3

Describe 4 defence types plants have against herbivores

Answer 3

1. Chemical - noxious or poisonous
2. Mechanical - structures such as thorns
3. Nutritional - less nutritious for grazers (less N and P)
4. Tolerance - adapting to regrow quickly

Question 4

Indirectly what do mechanical plant defences such as spines do?

Answer 4

The attracts relatively high insect herbivore species richness and population

Question 5

Give an example of delayed density dependent coupled oscillations

Answer 5

Lynx and Snowshoe hare

• Flux in a 10 year cycle
• hare reproductive rates peak several years before hare densities reach a maximum
• they then drop for 2-3 years after lynx peaks

Question 6

How might good supply cause the lynx rabbit oscillations?

Answer 6

• at peak hare densities vegetation is short, and is a problem for the hairs

BUT

• some hare populations didn’t have a lack of food, and some didn’t recover when given food

Question 7

How might predation cause the lynx rabbit oscillations?

Answer 7

Up to 95% of hares are killed by predation

• birth-rates drop as the hare population drops
• they only slowly recover after the number of predators falls
• poor physical condition of hairs at low population density

Question 8

What did the manipulative experiments done by Krebs in the lynx and rabbits show?

Answer 8

That both predation and food supply individually had small effects, but combined they had a major effect on the hare population

Question 9

Sometimes there seems to be no link between predator and prey numbers, why is this?

Answer 9

• Predators and prey don’t live in a 2 species environment

- Even if there were only 2 species, the interaction is still very complex - behaviour, coevolution and specialisation

Question 10

Define each kind of predator:

a) monophagous
b) Oligophagous
c) Polyphagous

Answer 10

a) monophagous - single prey type (most likely to have coupled oscillations)
b) Oligophagous - a few types of prey
c) Polyphagous - many prey

Question 11

Why is there pressure on predators to have a specialised choice of prey?

Answer 11

Prey adapt to get away from predators, different prey with different methods; speed, mechanical, poison etc.

Predators cannot specialise/coevolve to all prey adaptations so there is pressure to specialise its hunting to one prey

Question 12

What is the problem with predators specialising to hunting one or very few kinds of prey?

Answer 12

They become reliant on that prey

Question 13

What is the amount of time spent on searching and handling for each type of predator:

a) specialist predators
b) generalist predators

Answer 13

a) specialist predators: search time is short compared to handling time
b) generalist predators: search time is long compared to handling time

Question 14

What is predator switching?

Answer 14

Polyphagous predators select which prey they want.

Preferred prey may change in abundance over a predators lifetime too

Question 15

How do parasites effect population dynamics?

Answer 15

They have a destabilising effect on population dynamics. They inverse density dependence.

Question 16

How do parasites effect hosts?

Answer 16

They divert energy away from the host to themselves, reducing survival, reproduction and growth of the host

Host behaviour may change due to parasite

Host defences will have been lowered

Question 17

What are the ecological effects of parasites and disease?

Answer 17

• may cause near extinction of the host
• may affect hope population dynamics
• wider landscape affects - Asian cryphonectria fungus on American chestnut trees

Question 18

What effects do herbivores have on plants?

Answer 18

• seed herbivores directly effect the next generation of plants
• grazers are less likely to kill plants
• moderate levels of grazing can alter vegetation structure or soil chemistry - sometimes benefiting the net primary production, but excess grazing is damaging