The Open Door Web Site
Part XXII: The Interdependence of Living Things : The Ecosystem : An Interacting Community Index
A rocky shore
THE ECOSYSTEM: AN INTER-ACTING COMMUNITY
Ecosystem Example: A Rocky Shore
A rocky shore can be divided into three sections. The upper section is called the splash zone. This section is not covered by sea water when the tide is in. It does, however, receive plenty of salt water spray from the waves. The animals and plants which live in the splash zone need to be able to tolerate high levels of salt concentration. Many salt resistant lichens can be seen covering the rocks.
The section of the rocky shore which is covered by sea water when the tide is in and uncovered when the tide is out is called the littoral zone. The plants and animals which live in the littoral zone are adapted to withstand the movement of the waves as the tide moves in and out. Many algae, called seaweeds, are attached to the rocks by special structures called hold fasts. They are covered in a slimy mucus to prevent them from losing too much water when the tide is out and the seaweeds are exposed to the sun.
The animals which live in the littoral zone are also adapted to keep their positions as the water rushes over them. Tube worms and barnacles literally cement themselves to the surface of the rocks. A limpet has a muscular foot with which it fixes itself to a firm surface. Both barnacles and limpets are conical in shape and this helps to reduce the effect of wave action. Other animals, such as shrimps and crabs, hide in the holes between the rocks, using their legs as anchors.
The third and lowest section of a rocky shore is called the sub-littoral zone. This zone is always covered by sea water. Here are found other kinds of algae, the giant kelps. These algae can reach up to 30 metres in length.
The upper and lower limits of the littoral zone are not always found in the same places. Once every two weeks the Earth, the Sun and the Moon are in line with each other. This means that there is a greater pull on the sea water, producing high tides.
Tides are caused by the pull of the moon
Tides are caused by the pull of the Moon and the Sun on the seas and oceans. There are normally two tides each day. Waves are caused by the action of the wind on the surface of the water. Waves can reach up to 15,25 metres in height in a violent storm.
As the Earth, Sun and Moon change their positions with respect to each other, so the limits of the littoral zone change. When the Earth, the Sun and the Moon are at right angles to each other the pull on the sea water is at its minimum. This also happens once every two weeks and produces low tides.
The highest tides are called spring tides and these occur twice each year, in March and in September. Spring tides occur because the sun is over the equator and has its greatest pull on the sea waters.
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