Because of the twice daily rise and fall of the tides, conditions on a seashore vary widely between the different levels. At low water mark the rocks or sand are exposed to the air for relatively short periods at a time, while at the top of the beach, plants and animals are only covered by the sea for a few minutes around high water. Different communities of plants and animals can therefore be found at different levels on the beach. This arrangement of the communities into parallel bands or zones is known as zonation.

On rocky coasts, ribbons of brown oar-weed up to two metres (6 ft 6 in) in length are often found in the lowermost zone. A little higher, low water reveals slippery masses of a seaweed called serrated wrack, which has jagged edges to its fronds. Higher still grows the bladder-wrack, with rounded air- bladders acting as floats. In the splash zone, seldom entirely covered by salt water, black and yellow lichens encrust the rocks.