Communities are often made up of a series of layers. For example, in a woodland one can see the canopy, made up of the boughs of trees such as oak, beech or elm, 5-10 metres (16 ft - 32 ft) from the ground, a shrub-layer made up of hawthorn, elder or hazel shrubs 2- 3 metres (6 ft 6 in- 9 ft 9 in) high, a herb- layer a metre (3 ft 3 in) or so from the ground as well as the topsoil and subsoil.

Different animals tend to live within each layer. Rooks may nest in the topmost branches, wood pigeons in the tangle of hawthorn beneath, and pheasants may lay their eggs in a hollow on the woodland floor.

Nevertheless important links exist between the layers. Winter- moth caterpillars feed on the tree- leaves in oak woods but crawl down the trunks to pupate (turn into chrysalids, the half-way stage between caterpillar and moth) in the soil.