Food chains
A food chain shows how energy passes from one organism to another as each is eaten: for example, grass → rabbit → fox. The arrows point in the direction energy flows, from the food to the eater. Every food chain starts with a producer, an organism such as a plant or alga that makes its own food from sunlight by photosynthesis.
Trophic levels
- Producers make food from sunlight (grass, oak, berry bushes).
- Primary consumers are herbivores that eat producers (grasshoppers, rabbits, deer).
- Secondary consumers eat herbivores (frogs, snakes); omnivores such as foxes and songbirds eat both plants and animals.
- Tertiary consumers or top predators are not usually eaten by anything else (hawks, wolves).
Food webs
Real animals rarely eat just one thing, so food chains link together into a food web. A fox eats rabbits, mice, birds and berries; a mouse is eaten by snakes, foxes and hawks. Generalists with many food sources can switch when one runs short, which makes them resilient. Specialists, such as a frog that eats only insects, are much more vulnerable.
Energy flow and the 10 % rule
At each step along a food chain, most energy is used for movement, growth, keeping warm and other life processes, or is lost as heat and waste. Typically only about 10 % is passed on to the next level. That is why there is much more plant material than herbivores, and many more herbivores than predators, and why food chains rarely have more than four or five links. Drawn as a chart, this makes an energy pyramid.
Knock-on effects and trophic cascades
Because species are linked, a change to one affects many others. Removing a predator lets its prey increase, which can then overeat their own food: a trophic cascade. When wolves were reintroduced to Yellowstone National Park in 1995, elk numbers fell and changed their behaviour, letting willows and aspens regrow along rivers, which in turn helped beavers and songbirds. Species with an unusually large effect for their numbers are calledkeystone species.
Decomposers
Every food web relies on decomposers, mainly fungi and bacteria, along with detritivores such as earthworms. They break down dead organisms and waste, releasing nutrients such as nitrogen and phosphorus back into the soil for producers to reuse. Without them, nutrients would stay locked in dead matter.
Humans and food webs
Overfishing, hunting, habitat loss, pesticides and invasive species all disrupt food webs. Some pollutants, such as the pesticide DDT or mercury, are not broken down and become more concentrated at each level of a food chain (biomagnification). In the 1960s this thinned the eggshells of eagles and falcons, almost wiping them out until DDT was banned.
Key terms
- Trophic level
- A step in a food chain, such as producer or primary consumer.
- Keystone species
- A species whose effect on its ecosystem is much larger than its numbers suggest.
- Trophic cascade
- A chain of effects through several levels, caused by a change at one level.
Common misconceptions
- “Arrows show who eats whom.” They show the direction energy flows: from the food to the eater.
- “Removing a predator helps everything it ate.” Prey may boom and then crash after overeating their food.
- “Top predators are the most important.” Every level matters; without producers and decomposers nothing survives.
Key takeaways
- Food chains link into food webs; arrows show the flow of energy.
- Only about 10 % of energy passes between levels, so higher levels support fewer organisms.
- Changing one population causes knock-on effects throughout the web.
- Specialists are more vulnerable than generalists; decomposers recycle nutrients for everyone.
Model note: populations are shown relative to normal levels and respond to the average amount of food and predators. Real food webs involve many more species, seasons and behaviours.