H4 Lewis- Interactions among populations IV: Ecological networks - mutualists and antagonists

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Last updated 4:03 PM on 2/12/26
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6 Terms

1
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what is the small world property of ecological networks? what are the implications of this?

  • the nodes of a network are typically very closely connected (by edges/links)

  • on average every species is two links apart, and 95% are within three links of each other

this is important because:

  • changes in the abundance of one species will propagate rapidly through the network

  • biodiversity loss, over-harvesting and species invasions may affect more species than previously thought

2
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how can we quantify the connectedness of networks?

  • the connectance, C, of a network is equal to the number of links that occur, L, divided by the number of possible links, S

  • C = L/(S(S-1)/2)

  • S(S-1)/2 on the bottom, not S², because we aren’t considering interactions within species (same as S!, S factorial)

<ul><li><p>the <strong>connectance</strong>, C, of a network is equal to the number of links that occur, L, divided by the number of possible links, S</p></li><li><p><strong>C = L/(S(S-1)/2)</strong></p></li><li><p>S(S-1)/2 on the bottom, not S², because we aren’t considering interactions within species (same as S!, S factorial)</p></li></ul><p></p>
3
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what is an issue with relying on observation for ecological network analysis?

studying patterns observed in previously published networks is somewhat problematic, because:

  • taxonomic bias (too little priority given to smaller organisms eg. bacteria/insects)

  • lumping (many individuals of one species being considered as one eg. plankton)

  • omnivory (feeding on more than one trophic unit)

this is why newer analyses use purpose built webs that don’t have these biases

<p>studying patterns observed in previously published networks is somewhat problematic, because:</p><ul><li><p><strong>taxonomic bias</strong> (too little priority given to smaller organisms eg. bacteria/insects)</p></li><li><p><strong>lumping </strong>(many individuals of one species being considered as one eg. plankton)</p></li><li><p><strong>omnivory </strong>(feeding on more than one trophic unit)</p></li></ul><p>this is why newer analyses use purpose built webs that don’t have these biases</p><p></p>
4
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why are food chains short?

productivity:

  • energy is lost/wasted (eg. by respiration, incomplete digestion) at every trophic level, so there isn’t enough energy near the top for further levels to exist

  • communities which are more efficient should have longer food chains eg. ectotherm chains longer than endotherms

  • communities with more efficient primary production should have longer food chains

trophodynamics (instability of long chains):

  • lower trophic levels are dependent on the stochasticity of food availability, so the populations are variable

  • chance variations in population size are amplified up the food chain, so they become even more unpredictable for the top levels

5
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how does network complexity impact stability?

  • according to may’s model, randomly-generated webs were stable as long as B(SC)1/2 < 1, where:

  • B (beta) is the strength of interactions

  • S is the number of species

  • C is the connectedness

this predicts that increased network complexity reduces stability- this is because:

  • many components have to be balanced very precisely

  • there is more potential for trophic cascades

  • this goes against previous predictions (eg. elton) that increasing complexity would increase stability because there is more buffering against fluctuations

<ul><li><p>according to may’s model, randomly-generated webs were stable as long as <strong>B(SC)<sup>1/2</sup> &lt; 1</strong>, where:</p></li><li><p>B (beta) is the strength of interactions</p></li><li><p>S is the number of species</p></li><li><p>C is the connectedness</p></li></ul><p></p><p>this predicts that <strong>increased network complexity reduces stability</strong>- this is because:</p><ul><li><p>many components have to be balanced very precisely</p></li><li><p>there is more potential for trophic cascades</p></li></ul><p></p><ul><li><p>this goes <strong>against </strong>previous predictions (eg. elton) that increasing complexity would increase stability because there is more buffering against fluctuations</p></li></ul><p></p>
6
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what factors are integral in defining the structure of ecological networks?

indirect interactions, eg:

  • between two species on one trophic level which have apparent competition for not being preyed upon, etc- eg. red vs grey squirrels with the squirrel pox virus

  • tropic cascades

keystone species:

  • species with much larger impacts on the ecosystem than might be expected from its abundance

  • if these were to go extinct, the effects will be widespread, often but not always predators

<p>indirect interactions, eg: </p><ul><li><p>between two species on one trophic level which have apparent competition for not being preyed upon, etc- eg. red vs grey squirrels with the squirrel pox virus</p></li><li><p>tropic cascades </p></li></ul><p></p><p>keystone species:</p><ul><li><p>species with much larger impacts on the ecosystem than might be expected from its abundance</p></li><li><p>if these were to go extinct, the effects will be widespread, often but not always predators</p></li></ul><p></p>