Food Chains and Food Webs

Food chains

A food chain shows the transfer of energy from one organism to the next, beginning with a producer. For example:

grass → grasshopper → frog → snake

The arrows show the direction of energy transfer. So grass → grasshopper means that chemical energy stored in the grass is transferred to the grasshopper when it eats the grass — the arrow is not simply shorthand for “is eaten by”; its biological meaning is specifically the direction energy moves. Getting the arrow backwards is one of the most common ways marks are lost on an otherwise correct answer, so it’s worth checking the direction every time before you commit to a chain.

Every food chain begins with a producer, because producers are what introduce chemical energy into the feeding system in the first place — nothing further along the chain can supply that energy itself.

Constructing and interpreting a food chain

Suppose a habitat contains grass, rabbits that eat the grass, and foxes that eat the rabbits. The chain is:

grass → rabbit → fox

To construct a food chain, begin with the producer, identify what feeds on it, then continue through each successive consumer, drawing every arrow in the direction energy is transferred. To interpret an unfamiliar chain, follow each arrow in turn and ask a single question at each step: which organism is supplying energy to the next one? If an organism’s numbers fall, organisms that depend heavily on it for food may also decrease, while organisms it normally eats may increase, since fewer of them are now being consumed. That reasoning becomes more powerful — and more heavily tested — once chains are joined into food webs.

Producers

A producer is an organism that makes its own organic nutrients, usually using energy from sunlight through photosynthesis. Green plants are the producers you’ll meet most often. Rather than obtaining organic nutrients by eating another organism, a producer manufactures them directly, which is exactly why every food chain has to start with one.

Consumers

A consumer is an organism that gets its energy by feeding on other organisms. Consumers occupy different positions along a chain:

producer → primary consumer → secondary consumer → tertiary consumer → quaternary consumer

A primary consumer feeds on the producer. A secondary consumer feeds on a primary consumer. A tertiary consumer feeds on a secondary consumer. A quaternary consumer feeds on a tertiary consumer. Not every food chain reaches all four consumer levels — plenty of real chains stop after a secondary or tertiary consumer, and that’s expected rather than incomplete.

Herbivores and carnivores

A herbivore is an animal that gets its energy by eating plants. A carnivore is an animal that gets its energy by eating other animals. A primary consumer feeding directly on a plant is therefore a herbivore, while any consumer feeding on another animal is a carnivore — the two classifications describe diet, and the trophic-level numbers describe position, so a single organism can be described accurately by both at once.

Decomposers

A decomposer is an organism that gets its energy from dead or waste organic material. Dead organisms and waste still contain organic substances, and decomposers obtain energy by breaking this material down. In doing so, they also contribute to the recycling of materials such as carbon and nitrogen — the exact pathways this feeds into are covered on the carbon cycle and nitrogen cycle pages.

Trophic levels

A trophic level is the position of an organism in a food chain, food web or ecological pyramid.

For example, in grass → caterpillar → bird → hawk, the grass is the producer at trophic level 1, the caterpillar is the primary consumer at trophic level 2, the bird is the secondary consumer at trophic level 3, and the hawk is the tertiary consumer at trophic level 4. A trophic level describes an organism’s feeding position, not where it physically lives in the habitat — a question that mentions where an animal nests or burrows is not asking about its trophic level at all. Trophic levels are also what the pyramids of numbers, biomass and energy are built from, one bar per level.

Food webs

A food web is a network of interconnected food chains. Real organisms usually have more than one food source, and predators often eat more than one species, so a food web gives a far more complete picture of feeding relationships than any single chain can on its own. Imagine that grass is eaten by both rabbits and grasshoppers, grasshoppers are eaten by frogs, and rabbits and frogs are both eaten by hawks — treating these as separate, unrelated chains would miss the point. Joined together, they form a food web, and food webs reveal interdependence: a population can be affected by another population it never directly feeds on, purely because the two are connected through a third.

For example, if a predator population falls, one or more of its prey populations may increase; those prey may then consume more of their own food; and populations elsewhere in the web may fall as a result — even though none of them ever interacted with the original predator directly. When interpreting a food web, the reliable method is to trace the arrows one relationship at a time, rather than trying to predict every knock-on change at once.

Human impact through overharvesting

Overharvesting occurs when humans remove organisms from a population faster than it can be maintained, for example by harvesting a food species faster than it can reproduce. Cambridge specifically wants you to use the food chain or food web to describe the effects, not simply to state that “the ecosystem becomes unbalanced.” If the harvested species is prey, its predators have less food available, so predator populations may decrease, while organisms the harvested species normally ate may increase because less of them is now being eaten. If the harvested species is itself a predator, its prey experiences less predation, so prey populations may increase, and that larger prey population may then put more pressure on its own food supply further down the web.

Introducing foreign species

A foreign species is a species introduced into a habitat in which it did not previously occur. Once introduced, it enters the existing feeding relationships: it may feed on native organisms, provide a new food source, compete with native organisms for food, or otherwise reduce particular populations and indirectly affect others further along the web. If an introduced predator consumes a native prey species rapidly, for instance, the prey population may fall, and native predators that also depend on that same prey may then have less food available to them. As with overharvesting, the required skill is using the actual food web to trace the consequences, rather than assuming every introduced species produces the same effect — the outcome genuinely depends on which feeding links the new species joins.