Every plant association has a history and passes through several stages in its history. This slow development to a more or less stable stage is called plant sucoession. The earliest stage in a sucoession is the colonization of a bare area, which in Great Britain may be rock or earth exposed by a landslide, a bare patch left after a fire, a mud-flat, a colliery mound or a disused quarry. In the case of the more rocky sites the first colonists are lichens and mosses, with liverworts and Alga; in the damper parts. Where there is a little soil, patches ofappear, the majority of which are with wind-borne that have fallen there and germinated. These include groundsel, dandelion and thistle. As the rock surface weathers and plant remains accumulate, more soil is formed, and herbaceous perennials invade this. Insects, slugs and snails arrive and provide food for a bird population. In their turn, birds bring such seeds as the blackberry and hawthorn, which have passed undigested through their bodies. As time passes the dominant species will be replaced by taller plants, the shading effects of which will cause changes in the ground flora. Bushes and shrubs will be supplanted by trees, and the plant community will reach the stable form, which is called the climax of the sucoession.
Another example of succession is seen in the colonization of sand dunes. At first these are occupied by the marram grass, which is able to withstand the dry conditions, and which, by the binding action of its rhizome andsystem, prevents the dune from being blown away. As the plants die, the dead matter accumulates and makes the soil moister and more fertile. This enables herbaceous plants to grow, and these may be finally followed by a vegetation of low shrubs. Another type of sucoession is taking place on many areas of swampy ground which are to be found round the edges of and shallow lakes, and along the sides of slow-running streams, such as are to be found in the ’Broadlands ’of Norfolk. These fringes are occupied chiefly by reeds and sedges, and the remains of these plants do not rot quickly, but collect in the water about the base of the living plants. In this way the ground is raised slowly, until it is permanently above water-level. Then conditions become dry enough for fen plants to grow, and as the ground becomes drier and firmer shrubs and bushes appear.
In recent years there has appeared in Southampton Water a grass known as the rice grass, which slowly spreads on the mud flats, converting them into solid ground covered by tall green grass. This grass is being used by the Dutch in the reclamation of land from the sea in the Zuider Zee. Slips of Spartina are planted in the mud 3 metres apart, where they multiply and spread, so that in six years they entirely cover the ground, which has become raised about 2 ft. above the level of the original mud flat. Then the reclaimed land may be ploughed and planted with crops.
Such plant sucoessions generally take place very slowly, and it may take hundreds of years for a bare patch of ground to develop to the climax stage of woodland. In a temperate climate the tendency is for dry places to become moister and wet places to become drier. Normally the climax of a sucoession in Great Britain is woodland, but frequently altitude or biotic factors, such as the activities of man or animals, prevent this stage being reached.
THE RELATION OF ANIMALS TO PLANT SUCCESSION
Since animals are directly or indirectly dependent on plants for food, they can only colonize an area devoid of life after it has been populated by plants, and the nature of the animal colonists will depend on the kind of food and cover provided by the vegetation. Among the first signs of animal life will be the microscopic animals living in pools of water, and which have developed from wind-borne spores. Flying insects will visit the area in search of food and for suitable plants on which to lay their eggs. Birdson such insects, as well as those on plant seeds, will begin to inhabit the locality. Creeping animals such as worms, slugs and snails will slowly make their way in from surrounding habitats, followed by small animals such as moles, hedgehogs and mice, together with their enemies, the weasels and stoats. Rabbits will appear as soon as there are enough plants to support them in both summer and winter, and in their turn will attract foxes to the once barren area.