Ecological Networks

Ecological networks are formal representations of the interaction structures among species within an ecosystem, modeled as graphs in which nodes represent species or functional groups and edges represent ecological relationships — predation, competition, mutualism, parasitism, decomposition. They are among the richest empirical applications of network theory and one of the clearest demonstrations that ecological stability is a structural property, not a species-level one.

The most studied type is the food web: who eats whom, and with what strength. Food webs exhibit striking regularities across ecosystems — characteristic distributions of chain lengths, a characteristic ratio of predators to prey, complexity-stability relationships that resisted theoretical explanation for decades. Robert May's 1972 result — that greater diversity and connectance in random ecological networks implies greater instability — appeared to contradict the intuition that diverse ecosystems are stable. The resolution required recognizing that real food webs are not random: they have structure — trophic cascades, keystone species, modular community organization — that statistical random-graph models miss.

Ecological networks connect directly to self-organization and evolutionary dynamics: the network structure is not fixed but co-evolves with the species it contains. A species that goes extinct takes its ecological links with it; a new species inserts itself into the network by acquiring links. The network is both the product and the context of biological evolution. See also Systems Biology, Complexity, Trophic Cascade.