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, if including cannibalism). 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 constrained by the variability of the physical environment, by habitat type, which will reflect on an organism's diet breadth driven by
optimal foraging behaviour. This ultimately 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 compartmentalization in food webs appears to result from patterns of species' diet contiguity and adaptive foraging
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. The level of nestedness is determined not by species features but overall network depictors (e.g. network size and connectance) and can be predicted by a dynamic adaptive model with species rewiring to maximize individual fitness or the fitness of the whole community.
In-block nestedness: Also called compound structures, some ecological networks combine compartmentalization at large network scales with nestedness within compartments.
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
Trophic coherence: The tendency of species to specialise on particular trophic levels leads to food webs displaying a significant degree of order in their trophic structure, known as
trophic coherence, which in turn has important effects on properties such as
stability and prevalence of
cycles. == Stability and Optimisation ==