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Food chains, Interdependence and Adaptation
All creatures need a source of energy to stay alive. Plants get their energy from the sun and animals get energy from eating plants or other animals.
A food chain shows what eats what in a habitat. This is a simple food chain for a garden.
A simple food chain in a garden
There may be lots of food chains in a habitat, because lots of animals eat grass, other animals eat snails and birds get eaten too!
A food web shows how all the living things in a habitat can be connected.
Here are a few more living things that you might find in a garden, connected together as a food web.
A simple food web
Every habitat will have its own food chains and webs. Most habitats have lots of plants, and fewer animals. Do you know why? Look on the habitat pages for examples of food chains and food webs, or you can download some here.
Plants use the energy from sunlight to make food in their leaves by a process called photosynthesis. They are called producers because they produce their own food.
Most animals cannot make their own food. They must eat something else in order to get the energy to live and grow. They are called consumers.
Some animals eat plants. They are called herbivores. They need to eat a lot of plants, because plants contain relatively little energy.
Animals that eat meat are called carnivores. They get their meat by eating other animals, usually smaller than themselves.
Omnivores (like humans) eat both plants and animals.
Some animals, like earthworms, eat bits of plants and animals that are dead, rotten or are other animal’s droppings! They are called decomposers. They are important for recycling nutrients. Fungi and micro organisms are also decomposers. The nutrients go back into the soil where they can be used again by plants to help them grow.
Pyramid of numbers
Plants are the starting point for most food chains as they are the only organisms that can bring energy into the system. Of course not every bit of plant gets eaten so there are many more plants than there are animals. In the same way, not every animal (prey) is eaten by a carnivore (predator), so there are more herbivores than there are carnivores. There are relatively few animals at the very top of the food chain like tigers or golden eagles.
If you counted the numbers of living things in the layers of a food web you would get a ‘pyramid’ of numbers. If you weighed the living things then you get a similar pyramid of ‘biomass’.
Pyramid of numbers
Decomposers are important in a habitat. They feed on the dead and dying bits of plants and animals and help the nutrients and the remaining energy to be recycled.
The organisms in a habitat depend on each other.
Animals need plants for food, for shelter from the weather and as hiding places from predators. Without plants there would be no animals for the carnivores to eat.
Plants need animals to pollinate their flowers, to spread (disperse) their seeds and to help keep the soil fertile. Squirrels like to eat acorns, but they are also good at taking them away and burying them. Often the young or larvae of a species will eat different things to the adults. Caterpillars usually eat leaves whereas the butterflies feed on nectar (pollinating the flowers by accident at the same time).
Orange tip butterfly
Decomposers also play an important part. They get energy by breaking down the dead plant and animal material and at the same time they release nutrients necessary for plant growth. Without decomposers, such as worms, there would be a shortage of in minerals in the soil. The plants would not grow well and then there would be less food for all.
Some plants and animals live in more than one habitat. Some plants are good at growing anywhere (we call many of these weeds!). Animals are able to move around so they can graze or hunt in more than one habitat. The fox will hunt in woods and fields. Dragonflies spend their larval stage in the stream eating water animals. Adult dragonflies leave the water and fly, catching and eating other flying insects in the meadow. Both the stream and the meadow habitat are necessary for their survival.
Habitats are different. Some have too much water and others are very dry. Some soils have chalk or salt in them, others are very hard or soft.
South facing hills are very sunny, hot and dry. North facing slopes are cooler, damper and often shaded.
The plants and animals that live in a habitat are adapted to the conditions there. Plants growing on sand dunes need to be able to cope with very dry and exposed conditions. Saltmarsh plants have to tolerate salt. Plants in streams have to cope with living in water and the risk of being washed away.
Sea anemone | Photo: Emmy Kelly
Herbivores are adapted to live on plants by having teeth that can grind leaves up into small pieces so that they can be more easily digested. Some animals like cows have several stomachs because it takes so long to digest grass.
Some carnivores actively hunt their food so they have legs to run, wings to fly or are able to swim. They also tend to have sharp teeth and jaws that are able to deal with eating meat. Other animals stay in one place and eat what comes past them. For example sea anemones can stick fast to a rock and put out tentacles to filter the water and catch anything edible.