Habitats also change over a longer timescale, with the seasons. Away from the equator there can be drastic changes in temperature and the number of hours of daylight. This influences the organisms which make up the community in a given habitat. In European woodlands, for example, the deciduous trees lose their leaves in autumn. Birds migrate to warmer places as the insects on which they feed during the summer become less plentiful. Some animals hibernate as their food supplies diminish. Many plants die back to ground level to survive the winter, with seeds in the soil remaining dormant until the weather begins to warm up again.

Deciduous woodland shows an obvious seasonal change as autumn approaches.

In spring, the reverse occurs, the warmer temperatures stimulating new growth. Insects hatch as grubs or caterpillars just as the summer migratory birds arrive and begin breeding. As the days lengthen the plants flower, are pollinated by the large numbers of active insects, and set seed. The animals and plants are well-adapted to the changing seasons after thousands of years of evolution.

In spring in Europe, migratory birds like these swallows return from their wintering grounds in southern Africa.

Food chains

Plants convert energy from sunlight into energy stored in chemicals such as sugars or starch stored inside them. Animals are not able to get energy from sunlight in this way, and so they eat plant material, or other animals which have eaten the plant material. The plants are called 'producers' and the animals which feed on them 'consumers'. This dependence of one organism on another working back to a 'producer' is called a 'food chain'.

One example of a food chain is:

• plants -
• aphids -
• blue tits -
• sparrowhawk

It is noticeable that for consumers, further along the chain the organisms get larger, and there are fewer of them. In other words, we have a pyramid of numbers of the organisms. In the case of the food chain above we would have thousands of aphids, and a few blue tits to one sparrowhawk.

An example of a simple food chain - aphids feed on plants, blue tits eat the aphids, sparrowhawks will eat blue tits.