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Unlock This CourseEvery living thing needs energy to survive. Plants get theirs from the sun. Animals get theirs by eating other things. A food chain shows this simple idea who eats who in a straight line. It's one of the most important ideas in Environmental Management and once you get it, everything else about ecosystems starts to make sense.
An ecosystem is a community of living organisms (plants, animals, bacteria) interacting with each other and their non-living environment (soil, water, sunlight). Food chains and food webs show us the feeding relationships inside that community.
Key Definitions:
A food chain always starts with a producer something that can make its own food. Arrows show the direction energy travels. So grass → rabbit → fox means the rabbit eats the grass and the fox eats the rabbit. The arrow means "is eaten by" or "energy flows to."
Simple, right? But nature is never quite that tidy which is why we also use food webs.
Almost all energy in an ecosystem comes from the Sun. Plants capture solar energy through photosynthesis and store it as chemical energy in their leaves, stems and roots. When an animal eats a plant, it gets some of that stored energy. When another animal eats that animal, it gets some energy too but less each time.
Think of trophic levels like floors in a building. Each floor represents a feeding level. The ground floor is always the producers. As you go up, you move through different types of consumers. Most ecosystems only have 4 or 5 floors because energy is lost at every level and there simply isn't enough left to support more.
Here's how the levels break down. Every organism in an ecosystem fits into one of these categories:
Plants, algae and phytoplankton. They use sunlight to make food via photosynthesis. Examples: grass, oak trees, seaweed, wheat.
They form the base of every food chain. No producers = no ecosystem.
Herbivores that eat producers directly. Examples: rabbits, caterpillars, zebras, aphids, cows.
They are also called first-order consumers. They get the most energy from the chain after the producers.
Carnivores or omnivores that eat primary consumers. Examples: foxes, frogs, lions, thrushes.
They are second-order consumers. Energy available to them is already much reduced.
Top predators that eat secondary consumers. Examples: eagles, sharks, killer whales, tigers. These are often called apex predators nothing eats them (except decomposers when they die).
Bacteria and fungi that break down dead organisms at every level. They release nutrients back into the soil, which producers then use. Without decomposers, nutrients would be locked in dead matter forever.
On average, only about 10% of energy is passed on from one trophic level to the next. The other 90% is lost mostly as heat during respiration and some in waste products. This is why food chains rarely have more than 5 levels. By the time you reach the top predator, there's very little energy left to go around. It also explains why there are always far fewer top predators than there are plants!
A food chain is a useful starting point, but it's a massive simplification. In reality, most animals eat more than one thing and most organisms are eaten by more than one predator. A food web shows all these connections at once, giving us a much more accurate picture of how energy flows through an ecosystem.
Food webs are important because they show us stability. If one species disappears from a food web, other species can often compensate. But if a key species (called a keystone species) is removed, the whole web can collapse.
Predators and prey are linked in the Serengeti food web
The Serengeti in East Africa is one of the most studied ecosystems on Earth. It has an incredibly complex food web involving dozens of species across multiple trophic levels.
Savanna grasses (e.g. Themeda triandra), acacia trees, shrubs. These support the enormous herds of herbivores the Serengeti is famous for.
Wildebeest (over 1.5 million!), zebras, gazelles, elephants, buffalo. All herbivores feeding directly on the grasses and plants.
Lions, cheetahs, leopards, hyenas, wild dogs, crocodiles. These predators keep herbivore populations in check, preventing overgrazing.
The Serengeti shows perfectly why food webs matter. When rinderpest disease wiped out most wildebeest in the early 20th century, grass grew unchecked, leading to more frequent fires. When wildebeest numbers recovered after vaccination programmes in the 1960s, the ecosystem bounced back. One species the wildebeest had a massive knock-on effect on the entire web.
A typical British oak woodland has a rich food web. Oak trees (producers) support caterpillars and aphids (primary consumers). Blue tits and great tits eat the caterpillars (secondary consumers). Sparrowhawks eat the blue tits (tertiary consumers). Foxes and tawny owls may also act as top predators. Earthworms, fungi and bacteria decompose fallen leaves and dead animals, returning nutrients to the soil for the oak trees to use again. Remove the oak tree and the whole web falls apart.
To understand how energy moves through a food web, scientists use ecological pyramids. These diagrams show how the amount of organisms, energy, or biomass changes at each trophic level.
Shows the number of organisms at each trophic level. Usually widest at the bottom (lots of plants) and narrowest at the top (few top predators). However, it can be inverted one oak tree supports thousands of caterpillars, making the producer level appear smaller.
Shows the total mass of living material at each level. This is almost always a true pyramid shape there is always more biomass at the producer level than at any consumer level. This is because energy is lost at each stage, so less biomass can be supported higher up.
Human activity can seriously disrupt food chains and webs. When we remove one part of the web through hunting, habitat destruction, or pollution the effects ripple outwards in ways that are hard to predict.
Bioaccumulation (also called biomagnification) happens when a toxic substance like a pesticide or heavy metal is present in small amounts at the producer level but becomes more and more concentrated as it moves up the food chain. A tiny amount in plankton becomes a huge amount in a shark or eagle. This is why top predators are often most at risk from pollution, even if the pollution source seems far away from them.