Ecological network

An ecological network is a representation of the biotic interactions in an ecosystem, in which species (nodes) are connected by pairwise interactions (links). These interactions can be trophic or mutualistic. Ecological networks are used to describe and compare the structures of real ecosystems, while network models are used to investigate the effects of network structure on properties such as ecosystem stability.

Properties of ecological networks

Historically, research into ecological networks developed from descriptions of trophic relationships in aquatic food webs; however, recent work has expanded to look at other food webs as well as webs of mutualists. Results of this work have identified several important properties of ecological networks.

Complexity (linkage density): the average number of links per species. Explaining the observed high levels of complexity in ecosystems [Williams R.J., Berlow, E.L., Dunne, J.A., Barabasi, A.L., and N.D. Martinez. (2002) Two degrees of separation in complex food webs. "Proceedings of the National Academy of Sciences", 99: 12913-12916.] has been one of the main challenges and motivations for ecological network analysis, since early theory predicted that complexity should lead to instability [Pimm, S.L. (1984) The complexity and stability of ecosystems. "Nature", 307: 321-326.] .

Connectance: the proportion of possible links between species that are realized (links/species²). In food webs, the level of connectance is related to the statistical distribution of the links per species. The distribution of links changes from (partial) power-law to exponential to uniform as the level of connectance increases [Dunne, J.A., Williams, R.J., and N.D. Martinez. (2002) Food-web structure and network theory: The role of connectance and size. "Proceedings of the National Academy of Sciences", 99: 12917-12922.] .

Clustering: the proportion of species that are directly linked to a focal species. A focal species in the middle of a cluster may be a keystone species, and its loss could have large effects on the network.

Compartmentalization: the division of the network into relatively independent sub-networks. Some ecological networks have been observed to be compartmentalized by body size [Schmid-Araya, J.M., Schmid, P.E., Robertson, A., Winterbottom, J., Gjerlov, C., and A.G. Hildrew. (2002) Connectance in stream food webs. "Journal of Animal Ecology", 71: 1056-1062.] [Reuman, D.C. and J.E. Cohen. (2004) Trophic links’ length and slope in the Tuesday Lake food web with species’ body mass and numerical abundance. "Journal of Animal Ecology", 73: 852–866.] and by spatial location [Krause, A.E., Frank, K.A., Mason, D.M., Ulanowicz, R.E., and W.W. Taylor. (2003) Compartments revealed in food-web structure. "Nature", 426: 282-285.] .

Nestedness: the degree to which species with few links have a sub-set of the links of other species, rather than a different set of links. In highly nested networks, guilds of species that share an ecological niche contain both generalists (species with many links) and specialists (species with few links, all shared with the generalists). In mutualistic networks, nestedness is often asymmetrical, with specialists of one guild linked to the generalists of the partner guild [Bascompte, J., Jordano, P., Melian, C.J., and J.M. Olesen. (2003) The nested assembly of plant-animal mutualistic networks. "Proceedings of the National Academy of Sciences", 100: 9383-9387.] .

Stability of ecological networks

The relationship between ecosystem complexity and stability is a major topic of interest in ecology. Use of ecological networks makes it possible to analyze the effects of the network properties described above on the stability of an ecosystem. Ecosystem complexity was once thought to reduce stability by enabling the effects of disturbances, such as species loss or species invasion, to spread and amplify through the network. However, other characteristics of network structure have been identified that reduce the spread of indirect effects and thus enhance ecosystem stability. Interaction strength may decrease with the number of links between species, damping the effects of any disturbance [Vazquez, D.P., Melian, C.J., Williams, N.M., Bluthgen, N., Krasnov, B.R., and R. Poulin. (2007) "Oikos", 116; 1120-1127. ] and cascading extinctions are less likely in compartmentalized networks, as effects of species losses are limited to the original compartment [Krause, A.E., Frank, K.A., Mason, D.M., Ulanowicz, R.E., and W.W. Taylor. (2003) Compartments revealed in food-web structure. "Nature", 426: 282-285.] . Furthermore, as long as the most connected species are unlikely to go extinct, stability increases with connectance [Sole, R.V. and J.M. Montoya. (2001) Complexity and fragility in ecological networks. "Proceedings of the Royal Society of London B", 268: 2039-2045.] [Dunne, J.A., Williams, R.J., and N.D. Martinez. (2002) Network structure and biodiversity loss in food webs: robustness increases with connectance. "Ecology Letters", 5: 558-567.] [Okuyama, T., and J.N. Holland. (2008) Network structural properties mediate the stability of mutualistic communities. "Ecology Letters", 11: 208-216.] and nestedness [Memmot, J., Waser, N.M., and M.V. Price. (2004) Tolerance of pollination networks to species extinctions. "Proceedings of the Royal Society of London B", 271: 2605-2611.] [Burgos, E., Ceva, H., Perazzo, R.P.J., Devoto, M., Medan, D., Zimmermann, M., and A.M. Delbue. (2007) Why nestedness in mutualistic networks? "Journal of Theoretical Biology", 249: 307-313.] [Okuyama, T., and J.N. Holland. (2008) Network structural properties mediate the stability of mutualistic communities. "Ecology Letters", 11: 208-216.] .

Other applications

Additional applications of ecological networks include exploration of how the community context affects pairwise interactions. The community of species in an ecosystem is expected to affect both the ecological interaction and coevolution of pairs of species. Related, spatial applications are being developed for studying metapopulations, epidemiology, and the evolution of cooperation. In these cases, networks of habitat patches (metapopulations) or individuals (epidemiology, social behavior), make it possible to explore the effects of spatial heterogeneity.

Notes

References

pecific

* Bascompte, J., Jordano, P., Melian, C.J., and J.M. Olesen. (2003) The nested assembly of plant-animal mutualistic networks. "Proceedings of the National Academy of Sciences", 100: 9383-9387.

* Burgos, E., Ceva, H., Perazzo, R.P.J., Devoto, M., Medan, D., Zimmermann, M., and A.M. Delbue. (2007) Why nestedness in mutualistic networks? "Journal of Theoretical Biology", 249: 307-313.

* Dunne, J.A., Williams, R.J., and N.D. Martinez. (2002) Network structure and biodiversity loss in food webs: robustness increases with connectance. "Ecology Letters", 5: 558-567.

* Dunne, J.A., Williams, R.J., and N.D. Martinez. (2002) Food-web structure and network theory: The role of connectance and size. "Proceedings of the National Academy of Sciences", 99: 12917-12922.

* Krause, A.E., Frank, K.A., Mason, D.M., Ulanowicz, R.E., and W.W. Taylor. (2003) Compartments revealed in food-web structure. "Nature", 426: 282-285.

* Memmot, J., Waser, N.M., and M.V. Price. (2004) Tolerance of pollination networks to species extinctions. "Proceedings of the Royal Society of London B", 271: 2605-2611.

* Okuyama, T., and J.N. Holland. (2008) Network structure properties mediate the stability of mutualistic communities. "Ecology Letters", 11: 208-216.

* Pimm, S.L. (1984) The complexity and stability of ecosystems. "Nature", 307: 321-326.

* Reuman, D.C. and J.E. Cohen. (2004) Trophic links’ length and slope in the Tuesday Lake food web with species’ body mass and numerical abundance. "Journal of Animal Ecology", 73: 852–866.

* Schmid-Araya, J.M., Schmid, P.E., Robertson, A., Winterbottom, J., Gjerlov, C., and A.G. Hildrew. (2002) Connectance in stream food webs. "Journal of Animal Ecology", 71: 1056-1062.

* Sole, R.V. and J.M. Montoya. (2001) Complexity and fragility in ecological networks. "Proceedings of the Royal Society of London B", 268: 2039-2045.

* Vazquez, D.P., Melian, C.J., Williams, N.M., Bluthgen, N., Krasnov, B.R., and R. Poulin. (2007) "Oikos", 116; 1120-1127.

* Williams R.J., Berlow, E.L., Dunne, J.A., Barabasi, A.L., and N.D. Martinez. (2002) Two degrees of separation in complex food webs. "Proceedings of the National Academy of Science", 99: 12913-12916.

General

* Bascompte, J. (2007) Networks in ecology. "Basic and Applied Ecology", 8:485-490.

* Montoya, J.M., Pimm, S.L., and R.V. Sole. (2006) Ecological networks and their fragility. "Nature", 442: 259-264.