Ecosystems

In Grade 6 you learned that a food chain shows what eats what, and that the arrows trace energy. This year we follow that energy more closely.

Energy enters nearly every ecosystem as sunlight. Producers (green plants) trap a small part of it in photosynthesis. When a consumer eats a producer, some of that energy passes on. When a predator eats that consumer, a little more passes on again.

But here is the important new idea: energy is lost at every step. A rabbit uses most of the energy in the grass it eats — for moving, keeping warm and respiration — and only a small part is stored in its body. So only that small part can pass to the fox.

Because so much energy is lost each time, a food chain can only have a few links. There is rarely enough energy left to support a fifth or sixth animal in the chain. The Sun keeps the whole system running because new energy arrives every day.

Because energy is lost at every step, there is always less energy higher up a food chain. We can show this with an energy pyramid.

Each bar in the pyramid stands for one feeding level. The bottom bar — the producers — is the widest, because they hold the most energy. Each bar above is narrower, because energy was lost moving up.

This shape explains a lot about ecosystems. It is why there are many more grass plants than rabbits, and many more rabbits than foxes. There simply is not enough energy at the top to support large numbers of big predators.

Energy flows through an ecosystem and is lost — but nutrients behave differently. Nutrients are the materials, such as those in dead leaves and animals, that living things are built from. These are recycled, and decomposers do that job.

You met decomposers in Grade 6. Now look at why they matter so much. Decomposers — mainly fungi and bacteria — feed on dead plants, dead animals and waste. As they break this material down, the nutrients locked inside are released back into the soil.

Producers absorb those nutrients through their roots and use them to grow. The nutrients then pass along the food chain again. Without decomposers, dead material would pile up and the supply of nutrients in the soil would run out — and plants could not grow.

An ecosystem has only so much food, water, light and space. When living things need the same limited resource, they are in competition.

• Plants compete for light, water, space and nutrients in the soil.
• Animals compete for food, water, space and shelter, and sometimes for mates.

Competition shapes the size of a population — the number of one kind of living thing in an area. If a population grows large, there is less food to go around, so some individuals do not survive and the population falls again.

When food is plentiful a population grows quickly. As it gets bigger, competition increases, and growth slows. The population usually settles around the largest size the ecosystem can support. Predators and disease can lower it again. In this way an ecosystem stays roughly in balance.

An ecosystem stays in balance only while conditions stay roughly the same. A change can disturb the whole web of life.

Changes can be natural — a drought, a fire, a very cold winter, or a new disease. They can also come from human activity, and these effects are often large and fast.

• Habitat loss — clearing forests or draining wetlands for farming and building removes the homes of many living things.
• Pollution — chemicals, waste and litter can poison water and soil. Polluted water can kill the producers a food web depends on.
• Overfishing and hunting — removing too many of one animal affects everything that eats it or that it ate.

Because a food web is connected, a change to one living thing spreads. If pollution kills the water plants in a pond, the small animals that eat them have no food, and the fish that eat those animals are affected too. Following the arrows of a food web lets you predict these knock-on effects — and helps people protect ecosystems before damage is done.

Pulling these ideas together, you can see that an ecosystem only works because all its parts depend on each other — what scientists call interdependence.

• Producers depend on the Sun for energy and on decomposers for recycled nutrients.
• Consumers depend, in the end, on producers for all their food.
• Decomposers depend on dead plants and animals as their food source.
• Predator and prey numbers rise and fall together, keeping each other in balance.

This is why ecosystems are described as being in balance. If one part changes too much, the balance is upset and many other living things feel the effect.

It is also why protecting ecosystems matters. Looking after one living thing — a bee, a tree, a fish — often means protecting a whole web of life that depends on it.

Understanding ecosystems now prepares you for many later topics:

• Photosynthesis and respiration — energy flow starts with photosynthesis and is lost through respiration.
• Adaptation and variation — you will study why living things differ and how they suit their habitats.
• Human impact and the environment — pollution, climate change and conservation all build on these ideas.
• The carbon cycle — recycling of nutrients leads on to how carbon moves around the planet.

Whenever a later topic feels tricky, picture an energy pyramid or a food web and ask where the energy and the nutrients are going. Take your time — really understanding how ecosystems work is worth far more than rushing through it.