Density dependence

Question 1

Define density dependence.

Answer 1

Population size is limited by density.

Question 2

List 3 factors that affect population density.

Answer 2

1. Resource availability
2. Competition
3. Space

Question 3

What are the 2 types of intraspecific competition?

Answer 3

1. Exploitation

2. Interference

Question 4

Define exploitation competition.

Answer 4

There is direct competition between individuals. Both animals gain some resource.

Question 5

Is exploitation competition symmetrical or asymmetrical? Why?

Answer 5

Symmetrical as both parties gain some resource.

Question 6

Define interference competition.

Answer 6

Indirect competition whereby the behaviour of one organism prevents another from gaining the resource.

Question 7

Is interference competition symmetrical or asymmetrical? Why?

Answer 7

Asymmetrical as one party is prevented from gaining any resource because of the other’s behaviour.

Question 8

Give an example of exploitation competition.

Answer 8

2 animals, for example hyenas, squabbling for food at a carcass.

Question 9

Give an example of interference competition.

Answer 9

Plants growing taller and blocking out the light for smaller species.

Question 10

Mortality rate decreases as density increases. True or false?

Answer 10

False - mortality rate is elevated when there are more individuals.

Question 11

Give an example of the effect of density on mortality rate.

Answer 11

Gobies hide in rock crevices from predators. If there are too many gobies, there are not enough rock crevices and individuals will die from predation.

Question 12

Density has been shown to have effects on fecundity, shown by experiments on Musculium secures (the pond fingernail clam). How?

Answer 12

An increased parental stock leads to a lowering of litter size.

Question 13

An experiment on sparrows in Canada concluded that increased density had 3 major negative effects on the population. What were they? Why did this happen?

Answer 13

1. Birds laid less eggs
2. More nests failed
3. Less young birds survived to adulthood

There was not enough food.

Question 14

Give 3 main reasons why animals disperse.

Answer 14

1. To avoid inbreeding
2. To avoid competition
3. To find more resources

Question 15

Density dependence causes behavioural changes. Give an example.

Answer 15

Increased density can cause animals to disperse.

Question 16

Increased density often leads to increased exploitation competition. Why might this be detrimental?

Answer 16

Arguing for food may alert predators.

Question 17

In experiments with black fly larvae, what happened in slower water currents in high densities and why?

Answer 17

Larvae detached themselves (dispersed) in order to find more food.

Question 18

In experiments with black fly larvae, what happened in faster water currents in high densities and why?

Answer 18

Larvae were able to stay in the same area as the high currents bought more food, thus a larger population could be sustained.

Question 19

What are the 2 types of dispersal?

Answer 19

1. Positive density dependence

2. Negative density dependence

Question 20

Define positive density dependence dispersal.

Answer 20

Positive density dependence contributes to over-dispersion of organisms. There are too many of them so they must disperse NOW.

Question 21

Define negative density dependence dispersal.

Answer 21

Negative density dependence contributes to under-dispersion of organisms. There are few organisms so they will not disperse.

Question 22

Mark and recapture techniques are used to measure dispersal. True or false.

Answer 22

True.

Question 23

Do plants disperse? If so, when?

Answer 23

Yes, once as seeds.

Question 24

Experiments have found that, in plants, ‘germination rate is inversely correlated to seed production’. What does this mean and why?

Answer 24

It means that less seeds germinate the more that are produced. This is to prevent competition. If there were many seeds that all germinated at once they would compete for resources.

Question 25

Do plants tend to produce more seeds in wet or dry years?

Answer 25

Wet, these are favourable conditions.

Question 26

Plants cannot disperse as adults. How do they cope with high population density?

Answer 26

They adapt their size. If there are many individuals they become smaller. If there are fewer they become larger.

Question 27

Explain the concept of ‘self-thinning’.

Answer 27

Plants regulate their size in response to population density.

Question 28

Explain the law of constant final yield.

Answer 28

In plants, as population density changes biomass stays the same. This is because of self-thinning.

Question 29

How was the law of constant final yield discovered?

Answer 29

In an experiment on horseweed in 1963 by Yoda et al.

Question 30

On a graph, plant density often has a gradient of -3/2. Explain this gradient.

Answer 30

1. Plants spread their roots/leaves as they grow, eventually overlapping. Area is represented by x(2).
2. When they begin to overlap they try to outgrow each other in height. Volume is represented by x(3).
3. Generally volume increase is greater than area increase, giving a gradient of 3/2.
4. Eventually upwards growth becomes limited by resources and individuals are lost, making the gradient -3/2.

Question 31

Does plant density always have a gradient of -3/2 on a graph?

Answer 31

No - the gradient is an ideal, but in the field the law of self-thinning and constant final yield come into play.