Interactions among populations IV: Ecological networks - mutualists and antagonists Flashcards
Species do not exist in isolation but in
complex networks of antagonistic (predation, herbivory and parasitism), or mutualistic interactions (e.g. pollination)
Food webs describe
antagonistic networks
List some antagonistic networks
- food webs
- predator-prey
- parasitoid-host
- herbivore-plant
- pathogen-host
List some mutualistic networks
- pollinators and flowers
- seed dispersers and fruits
Describe linkage networks
showing which species interact with which other species
Describe energy flow networks
showing the quantity of energy flowing among the species
Describe functional / interaction strength networks
- showing the most important interactions structuring the community
- require expts
Describe the “small world” property
6 degrees of separation
Describe the small world properties of ecological networks
- species from a variety of aquatic and terrestrial ecosystems are on average two links apart
- > 95% within three links of each other
Why is the small world property of ecological networks important?
- 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
What are the advantages of networks
- 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)
List some network attributes
- 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
Give the equation for connectance
- Actual Links / Possible Links
- L/[S(S-1)/2]
How can food webs be studied?
- 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
Describe observational studies
analyse patterns in published networks
Describe artefacts in observational studies
– Taxonomic bias
– Lumping
– Omnivory (feeding on more than one trophic level) and ‘rare’ links underestimated
* Newer analyses use ‘purpose built’ webs
Give an example of a robust observation
most food chains are short (3-4 levels)
Describe the energy attenuation hypothesis
- 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
What are the predictions of the energy attenuation hypothesis
- 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)
Describe the trophodynamics explanation of short food chains
- 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
Removing species with few trophic connections generally has
little effect
Robustness increases with
connectance
complexity (more species and/or more
connections) appears to
decrease stability
Manipulations can identify
‘keystone species’ and ‘indirect effects’
Excluding the starfish Pisaster from rocky shores
shifted food web structure and reduced species diversity
Community structuring
determine which species occur and their relative abundances
List some factors that may structure communities
- indirect interactions
- keystone species
