Food Chains and Food Webs

A food chain is a straight-line sequence of organisms in which each organism feeds on, and obtains both energy and nutrients from, the organism immediately before it in the sequence, with energy being transferred step by step from one trophic level to the next. Two broad kinds of food chain are recognised, distinguished by where the chain begins.

A grazing food chain (GFC) begins with living green plants (producers) at its base, proceeds to herbivores that feed directly on those plants (primary consumers), and continues on to one or more levels of carnivores that feed on those herbivores and on each other (secondary and tertiary consumers) -- a typical example being grass, eaten by a grasshopper, eaten by a frog, eaten in turn by a snake, which is finally eaten by a hawk. The grazing food chain is the more familiar and, in most terrestrial and many aquatic ecosystems, the dominant conduit through which energy captured by producers is passed on to the rest of the community.

A detritus food chain (DFC), by contrast, begins not with living plant material but with dead organic matter -- detritus -- and proceeds through detritivores and decomposers (bacteria and fungi, sometimes called saprotrophs) that break this dead material down, and through any smaller organisms that in turn feed on those detritivores and decomposers. Although the grazing food chain is generally the more conspicuous of the two, the detritus food chain can be an equally or even more significant route for energy flow in certain ecosystems -- densely vegetated wetland and mangrove ecosystems, for example, where a very large proportion of plant production ultimately falls as litter and is processed through detritus-based pathways rather than being consumed directly while still alive.

In real ecosystems, however, isolated food chains of either kind are rarely found running in true isolation. Most consumers are not restricted to feeding on just one type of prey, and many organisms -- crows, foxes, bears and humans among them -- are deliberately omnivorous, feeding on more than one trophic source and being themselves preyed upon by more than one kind of predator. When multiple grazing and detritus food chains of this kind interconnect and overlap within a single community, sharing producers, consumers or predators between them, the resulting branching network of feeding relationships is called a food web. A food web is a considerably more realistic representation of how energy and nutrients actually move through a natural community than any single food chain drawn in isolation, since it captures the reality that most organisms have several potential food sources and several potential predators rather than just one of each. …