Given the finite amount of chemical compounds in the biosphere, it is inevitable that the same material will be utilized repeatedly by different organisms. This phenomenon is addressed as 'recycling' or simply 'cycling' of energy and matter. Familiar examples of recycling of nutrients involve the so-called 'detritus chain', which decomposes organic matter that is unusable for some organism to its basic compounds that can be recycled into the grazing chain.
This article provides an overview of cycles and cycling indices in ecosystems ecology. Depending on the way of modeling ecosystems, cycles assume different meanings. In what follows, a general definition of cycles, taken from graph theory will be introduced. Then the concept of cycling is applied to (1) food webs (description of who eats whom in the ecosystem) and (2) ecological networks (weighted, mass-balanced versions of food webs). Simple ways of computing cycling indices and removing cycles will be provided.
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