- Anusha Uppaluri

ContentsProblem Problems ImportanceRelated WorkConclusionReferencesQuestions

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ProblemFood Webs – Complex networks formed by

one species of animals feeding on the other species

Target Groups – Species of animals which when extinct cause the near extinction/ extinction of a large number of species of animals

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A Very Simple Food Web

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Problems ImportanceFood webs make a very complex network and

its understanding would be interestingInterdependence of species in food web is

important to maintain balanceHelps in understanding the do’s and don'ts in

trying to preserve our environment and ecosystem

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Related WorkDunne et. al.

• 16 food webs are analyzed• Food webs either have the power law, partial

power law, exponential or uniform degree distribution

• Relationship exists between the degree distribution and the connectance

• Connectance – degree to which nodes of a system are connected to each other

• High connectance – uniform degree distribution• Middle connectance – exponential distribution• Low connectance - power law or partial power law

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Food webs with low or high connectance aren't usually found

Understanding of food web degree distribution and connectance helps estimating its reaction towards structural changes

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Dunne et. al.16 food webs were consideredPrimary species loss was simulatedRobustness of food webs in terms of

secondary extinctions is measuredFood webs are more robust in case of random

species removal than in case of removal of species with many links to other species

Posses thresholds beyond which food webs are quiet sensitive to highly connected species removal

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Food webs with high connectance display high sensitivity from the beginning

Food webs displaying power law degree distribution most vulnerable to attacks

Over fishing of coastal marine vertebrates have impacted highly connected species with associated cascading effects

Most relevant to the question being focused onTalks about how the food web is actually

affected by eliminating certain species

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Alison J. GilbertChanges in connectance cause changes in

robustness?If poorly connected species are lost =>

connectance increases. Does this => increase in robustness?

Does converse hold good?Robustness – likelihood to lose nodes secondarilyFragility - converse of robustnessConnectance – degree to which nodes of a system

are connected to each other10

Loss of poorly connected species – positive change in connectance

Loss of highly connected species – negative change in connectance

Intercept – number of connections of the species at which change in connectance becomes positive from negative

Intercept is always and integer and loss in species will lead to change in connectance

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Four removal protocols applied to 16 food webs used by Dunne et. al.

Four removal protocols – highly connected species according to connectance, species with least connectance, Switched for least connected to highly connected after certain percentage of species have been lost and species were removed randomly

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Steep decline in connectance are more likely to cause secondary species loss

Huge negative changes to connectance => large negative impact on robustness

Increase in connectance from removal of least connected nodes => lesser secondary losses but still reduces robustness

Loss of any species causes reduction in robustness irrespective of their connections

Connectance changes little with random species removal

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Loss of species => change in connectance => loss of robustness

Connection between robustness , small negative and huge positive changes to connectance cannot be ascertained

Highly robust food webs => smaller changes to connectance with species loss

In food webs with uniform degree distribution, species loss do not cause large changes to connectance

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Robustness is related to degree distributionRobustness doesn’t just depend on

connectanceHelps with my research as it answers the

question of how certain species removal might affect

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Conclusion• High connectance – uniform degree distribution• Middle connectance – exponential distribution• Low connectance - power law or partial power

law• Food webs are more robust in case of random

species removal than in case of removal of species with many links to other species

• Food webs with high connectance display high sensitivity from the beginning

• Loss of any species causes reduction in robustness irrespective of their connections

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Huge negative changes to connectance => large negative impact on robustness

Increase in connectance from removal of least connected nodes => lesser secondary losses but still reduces robustness

Robustness doesn’t just depend on connectance

Robustness is related to degree distribution

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ReferencesDunne J. A., Williams R. J., Martinez N. D., “

Food-web structure and network theory: The role of connectance and size”, Proceedings of National Academy of Sciences of United States of America, Vol. 99 No. 20, pp. 12917-12922, 2002

Dunne J. A., Williams R. J., Martinez N. D., “ Network structure and biodiversity loss in food webs: robustness increases with connectance”, Ecology Letters, Vol. 5, pp. 558-567, 2002

Gilbert A.J., “ Connectance indicates the robustness to food webs when subjected to species loss”, Ecological Indicators, Vol. 9, pp. 72-80, 2009

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Questions?

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