The network of feeding relationships in an ecosystem. It describes who eats whom, and because energy is lost at every transfer, it explains why large predators are rare and why ecosystems have the structure they do.
A food chain is a linear sequence: grass to rabbit to fox. It is a useful simplification and is almost never accurate, since most organisms eat several things and are eaten by several things.
A food web is the network of all such relationships in a community, and real webs contain hundreds or thousands of links.
Trophic level describes position in the sequence. Primary producers, generally plants and algae, capture energy from sunlight. Primary consumers eat producers. Secondary consumers eat primary consumers, and so on.
Decomposers occupy a position outside this ordering, since they consume dead material from every level and return nutrients to the producers. They are frequently omitted from diagrams and handle the great majority of the energy flow in most ecosystems.
Many organisms occupy several levels at once. An omnivore eating both plants and animals has a fractional trophic level, and the tidy integer scheme is an approximation.
Energy transfer between levels is inefficient. A widely used approximation is that around ten per cent of the energy at one level is incorporated into the next, with the figure varying between about five and twenty per cent by system.
The losses have identifiable causes. Most consumed energy is spent on respiration rather than growth. Much of what is eaten is not digestible. And not everything at one level is eaten at all.
Three consequences follow directly.
Food chains are short. Four or five levels is typical and rarely exceeded, because there is not enough energy left to support another.
Biomass declines with level, producing the pyramid of biomass. Large predators are necessarily rare, which is why their populations are small and why they are vulnerable to disturbance.
Eating lower on the chain feeds more people from the same land, which is the ecological basis of the argument about the land and energy cost of meat production.
Some substances behave in the opposite way to energy, concentrating rather than dissipating as they move up.
A persistent, fat-soluble compound that is not readily excreted accumulates in an organism over its life, and a predator consuming many such organisms accumulates the total.
DDT is the standard case. Concentrations in water were low, in fish higher, and in fish-eating birds high enough to impair eggshell formation, which caused population collapses in raptors and led to restriction of the pesticide.
Mercury in marine food chains follows the same pattern, which is why advice on fish consumption distinguishes long-lived predatory species from short-lived ones.
Removing or adding a species produces effects that propagate through the network, and they are frequently not confined to the immediate neighbours.
Trophic cascades occur when a change at one level alters levels further down. Removing a top predator can allow herbivore populations to rise and vegetation to be suppressed, and the reverse can occur on reintroduction. The trophic cascade capsule treats the evidence, which is stronger in some systems than others.
Keystone species have effects disproportionate to their abundance, a concept developed by Robert Paine from experiments removing a starfish from a rocky shore, after which the community was restructured within a short period.
Connectance, the proportion of possible links that are realised, and the pattern of link strengths determine how disturbance propagates, and this is an active area of theoretical ecology.
Introduced species disrupt webs because they arrive without the predators and competitors that constrained them elsewhere, which is why invasive species effects are often severe and difficult to predict.
Direct observation of feeding works for large conspicuous animals and fails for most organisms.
Gut content analysis identifies what an individual recently ate and misses soft material that digests quickly.
Stable isotope analysis has become standard. Nitrogen isotope ratios increase predictably with trophic level and carbon isotopes indicate the original energy source, so a tissue sample places an organism in the web without observing it feed.
DNA metabarcoding sequences material from gut contents or faeces, identifying prey from fragments, and has revealed feeding relationships that observation had missed entirely.
Food webs explain the basic structure of ecosystems from a single physical constraint: energy is lost at every transfer, so the shape of a community follows from thermodynamics rather than from anything specific to the species involved.
They are also the framework for predicting the consequences of removing a species, which is what conservation and fisheries management require, and the recurring finding is that those consequences reach further through the network than intuition suggests.
