Species interactions lead to the flow of material within an ecosystem. For animals the most common processes are eating, respiration, excretion, and egestion. For plants, they are root uptake of water and nutrients, respiration, and photosynthesis. The outputs of material from one organism often become inputs to other. This focus on 'what eats what' led Elton to identify several patterns, notably the food chain and the food web, the food cycle, the ecological niche, and the pyramid of numbers.
Self-organization in ecosystems is evident in the structure in food webs, networks that describe trophic interactions among species. Within food webs, specific patterns of interaction may be prevalent. These patterns, termed ecological motifs, are thought to represent especially stable interactions. The concept of keystone species supposes that certain species play a crucial role in maintaining the integrity and stability of an ecosystem.
Analysis of food webs suggests that a small-world structure is common. That is, most species interact with only a small number of other species, but the connectivity of the web as a whole is maintained by a few species that interact with a large number of others. This observation provides a theoretical basis for the idea of keystone species. Functionally, small world networks are thought to be robust to random loss of nodes (e.g., species), but vulnerable to attacks that target their highly connected nodes (e.g., keystone species).
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