Ecological network
Encyclopedia
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 symbiotic
. 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.
Complexity (linkage density): the average number of links per species. Explaining the observed high levels of complexity in ecosystems has been one of the main challenges and motivations for ecological network analysis, since early theory predicted that complexity should lead to instability.
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.The observed values of connectance in empirical food webs appear to be accountable for by constraints on an organisms diet breadth driven by optimal foraging behaviour.This links the structure of these ecological networks to the behaviour of individual organisms..
Degree distribution
: the degree distribution of an ecological network is the cumulative distribution for the number of links each species has. The degree distributions of food webs have been found to display the same universal functional form. The degree distribution can be split into its two component parts, links to a species' prey (aka. in degree) and links to a species' predators(aka- out degree). Both the in degree and out degree distributions display their own universal functional forms. As there is a faster decay of the out-degree distribution than the in degree distribution we can expect that on average in a food web a species will have more in links than out links..
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 and by spatial location. Evidence also exists which suggests that compartmentilization in food webs appears to result from patterns of species' diet contignuity
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.
Network motif
: Motifs are unique sub-graphs composed of n-nodes found embedded in a network. For instance there exist thirteen unique motif structures containing three species, some of these correspond to familiar interaction modules studied by population ecologists such as food chains, apparent competition, or intraguild predation
. Studies investigating motif structures of ecological networks, by examining patterns of under/over representation of certain motifs compared to a random graph, have found that food webs have particular motif structures
. 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 and cascading extinctions are less likely in compartmentalized networks, as effects of species losses are limited to the original compartment. Furthermore, as long as the most connected species are unlikely to go extinct, stability increases with connectance and nestedness.
s, 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.
Biological interaction
Biological interactions are the effects organisms in a community have on one another. In the natural world no organism exists in absolute isolation, and thus every organism must interact with the environment and other organisms...
in an ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
, in which species (nodes) are connected by pairwise interactions (links). These interactions can be trophic or symbiotic
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
. 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 has been one of the main challenges and motivations for ecological network analysis, since early theory predicted that complexity should lead to instability.
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.The observed values of connectance in empirical food webs appear to be accountable for by constraints on an organisms diet breadth driven by optimal foraging behaviour.This links the structure of these ecological networks to the behaviour of individual organisms..
Degree distribution
Degree distribution
In the study of graphs and networks, the degree of a node in a network is the number of connections it has to other nodes and the degree distribution is the probability distribution of these degrees over the whole network.-Definition:...
: the degree distribution of an ecological network is the cumulative distribution for the number of links each species has. The degree distributions of food webs have been found to display the same universal functional form. The degree distribution can be split into its two component parts, links to a species' prey (aka. in degree) and links to a species' predators(aka- out degree). Both the in degree and out degree distributions display their own universal functional forms. As there is a faster decay of the out-degree distribution than the in degree distribution we can expect that on average in a food web a species will have more in links than out links..
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
Keystone species
A keystone species is a species that has a disproportionately large effect on its environment relative to its abundance. Such species play a critical role in maintaining the structure of an ecological community, affecting many other organisms in an ecosystem and helping to determine the types and...
, 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 and by spatial location. Evidence also exists which suggests that compartmentilization in food webs appears to result from patterns of species' diet contignuity
Nestedness
Nestedness
Nestedness is a measure of order in an ecological system, referring to the order in which the number of species is related to area or other factors. The more a system is "nested" the more it is organized....
: 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
Guild (ecology)
A guild is any group of species that exploit the same resources, often in related ways. As can be seen from the list of examples below, it does not follow that the species within a guild occupy the same, or even similar, ecological niches...
of species that share an ecological niche
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
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.
Network motif
Network motif
Network motifs are connectivity-patterns that occur much more often than they do in random networks. Most networks studied in biology, ecology and other fields have been found to show a small set of network motifs; surprisingly, in most cases the networks seem to be largely composed of these...
: Motifs are unique sub-graphs composed of n-nodes found embedded in a network. For instance there exist thirteen unique motif structures containing three species, some of these correspond to familiar interaction modules studied by population ecologists such as food chains, apparent competition, or intraguild predation
Intraguild predation
Intraguild predation, or IGP, is the killing and eating of potential competitors. This interaction represents a combination of predation and competition, because both species utilize the same prey resources and also benefit from preying upon one another...
. Studies investigating motif structures of ecological networks, by examining patterns of under/over representation of certain motifs compared to a random graph, have found that food webs have particular motif structures
Stability of ecological networks
The relationship between ecosystem complexity and stability is a major topic of interest in ecologyEcology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
. 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 and cascading extinctions are less likely in compartmentalized networks, as effects of species losses are limited to the original compartment. Furthermore, as long as the most connected species are unlikely to go extinct, stability increases with connectance and nestedness.
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 metapopulationMetapopulation
A metapopulation consists of a group of spatially separated populations of the same species which interact at some level. The term metapopulation was coined by Richard Levins in 1970 to describe a model of population dynamics of insect pests in agricultural fields, but the idea has been most...
s, epidemiology
Epidemiology
Epidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health research, and helps inform policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive...
, and the evolution of cooperation
Co-operation (evolution)
Co-operation or co-operative behaviours are terms used to describe behaviours by organisms which are beneficial to other organisms, and are selected for on that basis. Under this definition, altruism is a form of co-operation in which there is no direct benefit to the actor...
. In these cases, networks of habitat patches (metapopulations) or individuals (epidemiology, social behavior), make it possible to explore the effects of spatial heterogeneity.
Specific
- 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.
- Stouffer, D.B. (2010) Scaling from individuals to networks in food webs Functioonal Ecology, 24: 44–51
- 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.