The most essential relationship among the elements of the ecosystem is realized through nutrition, and the food chain and food web constitute the nutritional relationship between species. The nutritional structure of an ecosystem refers to an organizational form formed by the transfer of nutrition or food between inorganic environment and biological communities and between producers, consumers and decomposers in the ecosystem, which is the most essential structural feature of the ecosystem.

Food chain and food web are the basis of nutritional structure of ecosystem;

(1) food chain: The fixed energy of producers is continuously transmitted among the organisms in the ecosystem due to the food relationship, which is a one-way nutritional relationship. Every link in the food chain has become a nutritional level.

A predatory food chain: starting from living animals and plants (mainly marine ecosystems)

B scavenger food chain: starting from dead organisms or organic matter (mainly terrestrial ecosystems)

C parasitic chain: a food chain formed in a parasitic way.

(2) Food web: In an ecosystem, the complex nutritional relationship of many food chains is called food web.