Interactions among populations IV: Ecological networks - mutualists and antagonists

Question 1

Species do not exist in isolation but in

Answer 1

complex networks of antagonistic (predation, herbivory and parasitism), or mutualistic interactions (e.g. pollination)

Question 2

Food webs describe

Answer 2

antagonistic networks

Question 3

List some antagonistic networks

Answer 3

• food webs
• predator-prey
• parasitoid-host
• herbivore-plant
• pathogen-host

Question 4

List some mutualistic networks

Answer 4

• pollinators and flowers
• seed dispersers and fruits

Question 5

Describe linkage networks

Answer 5

showing which species interact with which other species

Question 6

Describe energy flow networks

Answer 6

showing the quantity of energy flowing among the species

Question 7

Describe functional / interaction strength networks

Answer 7

• showing the most important interactions structuring the community
• require expts

Question 8

Describe the “small world” property

Answer 8

6 degrees of separation

Question 9

Describe the small world properties of ecological networks

Answer 9

• species from a variety of aquatic and terrestrial ecosystems are on average two links apart
• > 95% within three links of each other

Question 10

Why is the small world property of ecological networks important?

Answer 10

• changes to abundance of one species will propagate rapidly
• dynamics within ecosystems can be highly interconnected
• biodiversity loss, over-harvesting and species invasions may affect more species than previously thought

Question 11

What are the advantages of networks

Answer 11

• summarise complexity of community interactions (more realistic than models of a few interacting species)
• understand community structure (which species occur in a community; why some are rare and some are common)
• understand community dynamics (what happens if we interfere with ecological communities)

Question 12

List some network attributes

Answer 12

• number of species in the web, S
• number of links or connections, L
• connectance, C: the fraction of possible links in the web that actually occur

Question 13

Give the equation for connectance

Answer 13

• Actual Links / Possible Links
• L/[S(S-1)/2]

Question 14

How can food webs be studied?

Answer 14

• observation: what patterns can be seen in ‘real’ webs?
• modelling: look at the properties of empirical or computer- generated webs
• experiment: test food web theory in the laboratory or in the field

Question 15

Describe observational studies

Answer 15

analyse patterns in published networks

Question 16

Describe artefacts in observational studies

Answer 16

– Taxonomic bias
– Lumping
– Omnivory (feeding on more than one trophic level) and ‘rare’ links underestimated
* Newer analyses use ‘purpose built’ webs

Question 17

Give an example of a robust observation

Answer 17

most food chains are short (3-4 levels)

Question 18

Describe the energy attenuation hypothesis

Answer 18

• energy is lost at each tropic level (heat, respiration, etc.) due to the 2nd law of thermodynamics
• eventually there is not enough energy for a further trophic level

Question 19

What are the predictions of the energy attenuation hypothesis

Answer 19

• communities with more efficient energy transfer should have longer food chains (e.g. endotherms v. ectotherms)
• communities with higher productivity should have longer food chains (at very low productivity they do, but few communities have > 3 or 4 levels)

Question 20

Describe the trophodynamics explanation of short food chains

Answer 20

• chance variations in population size are amplified up the food chain leading to unpredictable dynamics for the top predator
• food chains are indeed shorter in unpredictable environments

Question 21

Removing species with few trophic connections generally has

Answer 21

little effect

Question 22

Robustness increases with

Answer 22

connectance

Question 23

complexity (more species and/or more
connections) appears to

Answer 23

decrease stability

Question 24

Manipulations can identify

Answer 24

‘keystone species’ and ‘indirect effects’

Question 25

Excluding the starfish Pisaster from rocky shores

Answer 25

shifted food web structure and reduced species diversity

Question 26

Community structuring

Answer 26

determine which species occur and their relative abundances

Question 27

List some factors that may structure communities

Answer 27

• indirect interactions
• keystone species