Material Cycling and the Overall
Functioning of Ecosystems
So far I have moved gradually from simpler to more complex systems, starting with single populations (chapter 1), then continuing with competitive systems that have multiple species but a single trophic level (chapters 2, 3, and 5), and finally expanding the scope to food webs and interaction webs with multiple species and multiple trophic levels (chapters 4 and 5). Now has come the time to consider the ecosystem as a whole, and the specif c constraints that arise from its overall functioning.
An ecosystem represents the entire system of biotic and abiotic components that interact in a given location. As such, it includes a wide range of biological, physical, and chemical processes that connect organisms and their environment. Ecosystems have been approached from a variety of perspectives. Some approaches have focused on biotic interactions, in particular, consumer–resource interactions. Ecosystems are then looked at from the point of view of food webs or interaction webs. I have already considered this point of view in chapter 4. Ecosystem ecology, however, has generally focused on the overall functioning of ecosystems as distinct entities, in particular, on patterns of energy and material flows. Energy flows within ecosystems have traditionally received the most attention (Lindeman 1942; E. P. Odum 1953; H. T. Odum 1983) because energy is a universal requirement of all biological processes and is relatively easy to measure. From a theoretical perspective, however, energy flows within ecosystems do not offer major new questions and challenges compared with the food-web perspective. Energy is transferred between organisms via trophic interactions and is gradually dissipated through respiration along the food chain. As a consequence, energy flows through the ecosystem from its fixation by photosynthesis to its dissipation by heterotroph respiration, with virtually no energy recycled within the ecosystem (figure 6.1, left).
By contrast, material elements are heavily recycled within ecosystems. Material cycling is an inevitable consequence of energy flow in any physical