Ecosystems undergo changes in their structure and function as time passes. Some of these changes are minor and only affect a small area, others are the cause of major changes in the species present and affect the ecosystem as a whole.
Major changes can be caused by changes in climate, external factors such as fire, trampling or pollution and development due to the system itself.
Succession is a result of these changes and is defined as a series of progressive changes in an area with one community replacing the other until a climax community is created.
A climax community consists of plants and animals, the animals present are dictated by the plants available. There will normally be a dominant species of plant and animal or a number of co-dominant species, these species are normally the most numerous and have the greatest biomass. The climax community is normally seen as the ultimate development of the ecosystem.
There are 2 types of succession; primary and secondary.
Primary succession is the colonisation of an area which has not supported an ecosystem before, eg sand dunes, volcanoes and new ponds. Secondary succession is ecological succession in an area which has supported an ecosystem whose development to a steady state has been prevented by inhibitory factors such as grazing or fire. When the inhibitory factor is removed secondary succession takes the ecosystem to maturity.
The sequence of communities during succession is known as a sere, the type of sere is determined by the environment being colonised, eg a hydrosere is a series of successions in an aquatic environment and a xerosere is a succession beginning in a dry area.
The individual communities of each succession are the seral stages, the seral stages are not normally distinct but tend to merge into one another as the succession progresses.
During succession energy flow in an ecosystem is fundamentally changed, the changes are demonstrated in the quantity of standing crop in the ecosystem.
During early seral stages the energy inputs to the system are larger than the energy outputs, this results in little change in the standing crop. The energy flow is at a maximum during the climax stage. When the ecosystem is disturbed by external factors, eg fire the energy loss is greater than energy inputs. This results in a decrease in the amount of standing crop.
The accumulation of energy as biomass is most marked in land based ecosystem. In these, trees usually represent the form of the climax community and the standing crop is at a maximum. In aquatic ecosystems, the climax community may be represented by phytoplankton making the standing crop small yet the high metabolic rates gives high productivity which maximises the energy flow in the system.
The productivity of an ecosystem undergoing succession increases as the succession progresses towards the climax community. The increase is proportional to the amount of standing crop. The percentage gross productivity is fixed as net productivity is not continuous with progression towards the climax community, this is because in early seral stages dominant plants are small and have short lives, to make up for this they have a high yield and individual plants require very little energy for maintenance. In later seral stages the dominant plants are large and have long lives, when fully grown they use high levels of their gross productivity for respiration to maintain their bodies. Most organisms have maximum growth rates when young and their net productivity decreases as they age, this results in large, long lived organisms spending most of their lives in unproductive states and this is shown by the productivity of the system as a whole.
The number of species present in all successions progresses rapidly as plants and animals colonise the area. At first the increase in diversity is very rapid, however, in later seres the rate of increase decreases. Typically there is a decrease in the diversity towards the end of the succession. This decrease is mostly due to increasing interspecial competition and means that it is the intermediate seres which contain the largest number of species present at any one time during the succession.
Species diversity may continue to increase as the climax community approaches if the structure of the community and the energy available to the system allow colonisation to continue.
In terms of the trophic structure of the system, the early seres are short, linear food chains which are easily upset if one element in the food chain is removed. As the succession progresses the ecosystem becomes more layered and species diversity increases creating a complex food web. The more complex food webs give the system greater stability which allows for alternative energy flows when one element of the food chain is disrupted. Early sere's trophic structures are characterised by grazing food chains, the detritus food chains are more important in later seres when the system has matured.
Succession is highly important, especially in agriculture where the demand for high yield is directly responsible for basic conflicts in the ecosystem.
The plants cultivated for Man's use are typical of the plants found in early seral stages, ie they are short lived, small plants which use most of their energy in reproduction producing seeds with high energy content. Crops such as fruit trees or trees for timber are characteristic of middle seres and could not survive in a climax community.
Since the crops grown are those of early seral stages they are highly unstable thus they must be artificially maintained. This involves the use of chemical herbicides, insecticides and fertilisers. As farmers use larger areas of land for crops, succession is reversed and removes complex ecosystems causing mass extinctions and biogeochemical cycles on a large scale. The removal of these ecosystems can lead to soil erosion and affect nutrient cycling.
An example of primary succession is the colonisation of bare rock. The pioneer community is lichens and as these die and decompose they provide enough nutrients to support a community of small plants such as mosses, as the mosses die out they are typically replaced by ferns. With erosion of rock and increasing amounts of organic material a large layer of soil is gradually built up. This soil allows plants such as grasses and small flowering plants to grow followed by shrubs and trees. The climax community in this case is a woodland and thus a stable community of plants and animals is achieved.
Secondary succession occurs after forest fire or clearance of agricultural land, spores, seeds and vegetative organs may remain viable in the soil and there will be an influx of animals and plants through dispersal and migration from the surrounding area, in this circumstance the succession does not begin with pioneer species but with species from intermediate seres.
Another example is in the Australian outback where forest fires regularly destroy the forested areas. After only a few days small plants take advantage of the absence of the forests to grow. When further time has passed they are replaced by shrubs which are replaced or are eventually joined by regrown or new trees to restore the climax community to its original state.