Succession is “A sequence of ecological changes in an area whereby one group of plant or animal species successively gives way to another, culminating in a climax community.” This succession occurs in a number of sereal stages. A sere is a stage in a sequence of events by which succession occurs. The complete chain of seres is called a prisere. It begins with a pioneer community and ends with a climax community. This climax community occurs when the ultimate vegetation has become in equilibrium with the local environment.
There are four main areas where succession occurs rocky environment called a lithosere, sandy environment called a psammosere, a fresh water environment called a hydrosere and a salt-water environment called a halosere.
Two examples of a litosere are Sutsey a newly, formed volcanic island and Krakatoa an island striped of all its vegetation by a series of volcanic eruptions. Sutsey is located to the south east of Iceland and is slowly becoming colonised. The first colonisers of the bare rock on a lithosere environment are blue-green bacteria and single cell photosynthesisers, both of which have no need of a root system. Blue-green bacteria are autotrophs producing their own food through photosynthesis. The presence of blue-green bacteria along with weathering processes help to break up soil creating a very thin soil layer. More advanced plant life can then be supported in this thin soil. Once this plant life dies it rots down to create a humus layer to the soil. This humus layer is then capable of supporting grass seeds. As time progresses the grass species will give way to more dominant species such as small shrubs which in turn will replaced by relatively fast growing trees such as rowan. Eventually these trees will face competition from slow growing trees such as ash and oak. Sutsey however is unlikely to ever reach this climax community due to its frequent salty spray, storms and low temperatures for much of the year. Krakatoa however has reached its climax community with eight hundred different species being recorded in 1983 one hundred years after the clearance of the island.
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Psammoseres occur in a sandy environment. An example of this is Camber Sands, which has developed over time from a small beach to a complete dune system. The pioneer community in a psammosere is usually lyme grass, sea twitch and marram grass that begin to initiate dune development. These plants hold the sands structure together and create embryo dunes on the high water mark. Marram grass is usually the only inhabitant of the next two stages, which are fore-dunes located just above the highest watermark and the on the main ridge. This is because these two areas are extremely arid and very windy. Marram grass has successfully adapted to these conditions and is key in holding the dunes structure together. The dunes behind the main ridge have lost their supply of sand but have gained an increased humus content hence them being grey. This humus is created by the decomposition of marram grass. These grey dunes can support a much wider range of plants due to the increased water level, the reduction in wind and the increased humus level. Some of these plants include red fescue, gorse, bracken and heather. On the land ward side of the dunes about four hundred metres away from the sea are the climax of the succession the ash and oak trees.
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Hydroseres such as those created around the Talybont reservoir in Wales develop from fresh water lakes and pools. Initially these bodies of water hold little nutrients but sediment carried into the water will enrich the water with nutrients. The pioneer community will use these nutrients, which in this case are algae and mosses. These grow forming vegetation rafts whose surface supports bacteria and insects. Next there will be water loving plants such as lilies which us the nutrients recycled by bacteria from the pioneer community. Marsh plants such as bulrushes, sedges and reeds begin to encroach into the body of water as deposition increases. As marsh plants grow further out into the body of water there is increased deposition of sediment at the expense of the water and small trees may begin to take root forming a marshy thicket. Over time the lake is likely to contract and become deoxygenated and then finally to be replaced by an ash and oak climax.
Halosere mainly occur in river estuaries such as the Severn Estuary where large amounts of silt and mud are deposited by the ebbing tide and in flowing rivers. The pioneer communities are blue green algae and eel grass. These survive because they are able to tolerate being submerged for most of the twelve-hour tide cycle. Both of these plants trap mud causing it to be deposited thus allowing other species to become developed. Two of the next sets of colonisers are Salicorna and Spartina Townsendii. Both of these are able to tolerate saline conditions but only for a four-hour period. Two major zones develop in the river’s estuary the slob zone, which receives new sediment every day therefore making it water logged and lacking in oxygen. The other zone is the sward zone, which is inhabited by plants that can only tolerate four hours of submergence. Further inland non-halophytic grasses and shrubs enter the succession further in still these give way to small trees which finally climax in ash and oak trees.
Although many environments reach a climatic climax there are also some which never do. These environments are said to be at a sub-climax. A sub-climax occurs in an environment where arresting factors have cause the natural succession to be halted. This has occurred in the Ashdowne Forest where in order to preserve the heather population the growth of other plants is kept in check. It has also occurred on the South Downs where the heavy grazing of rabbits and sheep prevents the growth of any plants, normally found in the later stages of succession.
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There is also a difference in the plants ant the different stages of succession the early plants are said to be -r selection. These plants are opportunists with a rapid life cycle. An example of a plant in -r selection is the dandelion, which produces thousands of seeds. In contrast to this the plants later in the process of succession -K selected. Examples of this include the oak and ash trees. They are characterised by long lives and a high amount of investment into their seeds. This high investment therefore means that the seeds are better protected from the environment and therefore less are produced.
“Zonation is the distribution of plants or animals into specific zones according to altitude, depth, etc., each characterised by its dominant species.” Examples of zonation occur at a beach. The species of plants will vary in zones with an increase in distance from the high water mark. The factors affecting this zonation will be varied but may include things like strength of the tide and the amount of time they are underwater. The also occur on a mountain side where as the climatic conditions change the plants which can best survive in those conditions flourish thus developing clear zones.
