- Anusha Uppaluri

Contents Problem Problems Importance Related Work Conclusion References Questions 2

Problem Food 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 3

A Very Simple Food Web 4

Problems Importance Food webs make a very complex network and its understanding would be interesting Interdependence of species in food web is important to maintain balance Helps in understanding the do’s and don'ts in trying to preserve our environment and ecosystem 5

Related Work Dunne 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 6

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 7

Dunne et. al. 16 food webs were considered Primary species loss was simulated Robustness of food webs in terms of secondary extinctions is measured Food 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 8

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 on Talks about how the food web is actually affected by eliminating certain species 9

Alison J. Gilbert Changes 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 secondarily Fragility - converse of robustness Connectance – degree to which nodes of a system are connected to each other 10

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 11

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 12

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 13

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 14

Robustness is related to degree distribution Robustness doesn’t just depend on connectance Helps with my research as it answers the question of how certain species removal might affect 15

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 16

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 17

References Dunne 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 , 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 , 2002 Gilbert A.J., “ Connectance indicates the robustness to food webs when subjected to species loss”, Ecological Indicators, Vol. 9, pp ,

Questions? 19