L12 Tosh - Population interactions and community structure

Question 1

Give an example of both indirect and direct interactions between species

Answer 1

Indirect: Bee pollination is affected by cats. Cats predate field mice, mick attack combs and kill bees therefore more cats = more bees

Direct: a cat predates a field mouse

Question 2

What is a trophic cascade?

Answer 2

where consumption at 1 trophic level causes a change in abundance/composition at a lower trophic level

Question 3

Give an example of a trophic cascade

Answer 3

Sea otters predating sea urchins in the USA

Question 4

What is trophic facilitation

Answer 4

where the consumer is indirectly facilitated by positive interaction between its prey and another species

Question 5

Give an example of trophic facilitation

Answer 5

Commensal interaction between marsh rush and shrub. Indirect positive impact on aphids feeding on shrub in new England marshes. Mechanism:

1. removal of rushes led to a decline in shrub growth
2. but removal of shrub doesn’t effect the marshes
3. Rushes create a decrease in soil salinity - increase in soil O2
4. Shade from the rushes reduce evaporation therefore decrease salinity - and roots increase O2
5. aphids growth at a higher rate when rushes are present

Question 6

What is a competitive network?

Answer 6

where every species has a negative effect on another species in the community; circular rather than linear

Question 7

What is a hierarchy?

Answer 7

A dominant species always dominates - linear not circular

Question 8

What is disturbance?

Answer 8

It is a normal event in most communities
It can vary both in intensity and frequency
It can be abiotic:
- forest fires, severe storms
It can be biotic:
- grazing, predators, trampling

Question 9

What effect does disturbance have on the community?

Answer 9

It opens up gaps in the community, which is important for sedentary or immobile species.

Real communities are made up of a mosaic of patches . Species interact with the patches and move between them

Question 10

How does different stages of succession effect species richness

Answer 10

Species richness is highest in mid succession

Species richness falls at climax community due to the success of competitive species

Question 11

Give an example of disturbance

Answer 11

1. Slowly the glacier in Alaska retreat
2. Different stages of succession as the glacier retreats revealing land
3. show stages of succession

Question 12

What is intermediate disturbance hypothesis theory?

Answer 12

It was originally proposed to explain high species richness in tropical rain forests and coral reefs

Intermediate sized boulders have the highest species richness

Question 13

How does the level of disturbance relate to the size of boulders?

Answer 13

Large boulders: don’t move and don’t get cleared - high succession

Small rocks: move too much, low succession

Medium rocks: move sometimes - mid stage succession = highest species richness