Food webs

Question 1

Define a food web

Answer 1

A representation of feeding relationships in a community that includes all of the links revealed by dietary analysis.

Question 2

Give the 2 main ways a food web can be formed.

Answer 2

By observation

By molecular analysis of stomach content

Question 3

Are there consistencies between completely disparate communities ?

Answer 3

Yes

Question 4

Pisaster and Sea otters are examples of what ?

Answer 4

Keystone species

Question 5

How do Pisaster engineer their ecosystems ?

Answer 5

Pisaster preys on starfish
This creates a habitat for competitively inferior species
Mussels are competitively dominant, exclude all organisms
Pisaster mediates co-existence

Question 6

What happened when Pisaster was removed from its ecosystem for an experiment ?

Answer 6

Diversity dropped from an average of 15 to 8 species.

Question 7

Destabilisations at the highest trophic level induce….

Answer 7

Structural changes in the ecosystem

Question 8

When the removal of one species occurs, does the whole ecosystem usually collapse ?

Answer 8

No, very rare that this happens. Will only occur in non-robust ecosystems.

Question 9

What affect did the fur trade on the otter ecosystem ?

Answer 9

Otters were removed in large numbers
Consume sea urchins which were no longer controlled
Allowed to feed unchecked causing damage to ecosystem

Question 10

What entices ants to participate in seed dispersal ?

Answer 10

The seed has a fatty deposit, encouraging the ant to remove the seed away from the parent plant

Question 11

Give an example of a robust ecosystem.

Answer 11

Common seed dispersal ant, (65%) was removed
Self healing network, other species stepped in to fulfil role
Maintaining dispersal.

Question 12

What is Connectance ?

Answer 12

A measure of complexity
actual no. of interaction/
Possibl no. of interactions

Question 13

Give the 6 measures of an ecosystem

Answer 13

Robustness
Quantitative connectance
Vulnerability
Nestedness
Evenness

Question 14

How would you make a web of a paristoid network ?

Answer 14

Choose a plot
Quantify plant abundance,
Sample herbivore with transect
Collect all herbivores on trans.
Taxonomy
Analyse Data.

Question 15

How many trophic levels do communities usually have ?

Answer 15

3 to 4

Question 16

Give the 3 reasons trophic levels are limited to 3-4 per community.

Answer 16

Energy flow hypothesis
Dynamic fragility
Constraints on predator design and behaviour

Question 17

What is the energy flow hypothesis ?

Answer 17

That a maximum of 30% of energy per trophic level can be transferred as food to the next level.

Question 18

What is dynamic fragility ?

Answer 18

Food webs with long chains are much more prone to severe population fluctuations.
Extinction of the top predator is also more.

Question 19

Can dynamic fragility be measured in an ecosystem ?

Answer 19

No, requires a computer model of the species and population dynamics

Question 20

What are the constraint on predator design and behaviour ?

Answer 20

No organism can evolve to outcompete the current top predator in a community (become too big)

Question 21

Do top predators have predators ?

Answer 21

Not on a trophic interaction

Do have parasites.

Question 22

Give some of the uses of food webs

Answer 22

Monitor effect of alien species
Organic farming pest control
Pollination 
Landscape scale conservation
Effectiveness of restoration

Question 23

Give an example of the use of food webs in the restoration of a landscape

Answer 23

Removal of the Hornwort
ant pollinated alien species in Avon gorge
Food web showed they are deeply embedded in a network of pollinated species
Currently weeding them out.