Who eats whom: food webs

Producers, consumers and decomposers

An ecosystem brings together a physical environment and all the living things inhabiting it, with all their interactions. The most visible of these interactions is feeding: who eats whom.

Trophic levels

   PRODUCERS         build their organic matter by photosynthesis
                     plants, algae, phytoplankton
        |
        v
   CONSUMERS I       herbivores: rabbit, caterpillar, zooplankton
        |
        v
   CONSUMERS II      carnivores: fox, tit, fish
        |
        v
   CONSUMERS III     top predators: eagle, orca
        |
        v
   DECOMPOSERS       bacteria, fungi, earthworms

Each tier is a trophic level. The first is the only one bringing energy into the system: without producers, no ecosystem exists.

Chain or web?

A food chain is a convenient simplification:

   grass  ->  rabbit  ->  fox  ->  eagle

But reality is a web: the fox also eats voles, birds and fruit; the rabbit is eaten by buzzards, lynx and humans.

        grass ────> rabbit ────> fox
          │           │    ╲       │
          ▼           ▼     ╲      ▼
       cricket ───> tit      ╲──> buzzard

This web structure confers robustness: if one prey disappears, a generalist predator switches to another. A specialist predator, however, disappears with its prey — which is why the most specialised species are also the most vulnerable.

Decomposers, the invisible link

They are systematically forgotten, yet they are indispensable. They turn dead organic matter — carcasses, leaves, droppings — into mineral matter reusable by plants.

   dead organic matter
            |
            |  decomposers
            v
   CO₂, water, mineral ions (nitrates, phosphates)
            |
            |  uptake by roots
            v
   new organic matter

Without them, two simultaneous disasters: dead matter would pile up indefinitely, and mineral elements, locked in carcasses, would quickly run short. An ecosystem without decomposers stops for want of raw material, buried under its own waste.

Other relationships

Feeding is not the only link between species:

COMPETITION    two species exploit the same resource
PREDATION      one consumes the other
PARASITISM     one lives at the other's expense without killing it at once
MUTUALISM      both gain   (bee and flower, lichen)
COMMENSALISM   one gains, the other is indifferent

Mutualism is more widespread than supposed: about 80 % of flowering plants depend on an animal pollinator, and nearly all land plants live in symbiosis with fungi at their roots — the mycorrhizae, which supply water and minerals in exchange for sugars.

Summary