In the last lecture we looked at the different types of veld, and we noted their characteristics. An important concept to understand is that veld occurs naturally; the grasses, shrubs and trees that are found growing in the veld have not been planted by man. In the same way, land that has been cropped by a farmer and then left alone will produce a cover of weeds and grasses in one season. As time passes, the species will change until that land once again becomes normal veld. To understand how this happens, it is necessary to look at the ecology of the veld, because ecology is the study of organisms, plants or animals, and their environment or surroundings. An area of veld is an ecosystem which is a complex collection of plants and animals, each one affecting another. Under natural conditions, the animals will vary from small micro -organisms to herds of game, but when a farmer fences off an area of veld and stocks it with cattle, he is introducing a new species and an unnatural element into the ecosystem.
2. PLANT SUCCESSION
Plant communities begin their development on bare areas, whether such areas are primary bare areas (e.g. new lava flow, new water area) or secondary bare areas (i.e. areas which retain at least some influence of previous occupation by plants, such as old lands). T he factors which are responsible for the formation of such areas are classed as Initiating factors. These are briefly as follows:
- Topographic or physiographic factors e.g. erosion and deposition, flooding and drainage, elevation and subsidence. Such factors normally produce primary bare areas.
- Climatic factors. Climatic extremes may produce secondary bare areas by destroying the vegetation without having much influence on the soil e.g. drought, wind, snow, hail, frost and lightning.
- Biotic factors which largely produce secondary bare areas e.g. cultivation, overgrazing, insect and fungus outbreaks. They do, however, produce primary bare areas where the soil is badly disturbed following the destruction of the vegetation e.g. severe soil erosion, the formation of new banks of parent material, new water areas etc.
Once vegetation begins to develop on bare areas or on areas whose vegetation has been disturbed so that it no longer measures up to the potential of the area, it follows a sequence of changes which are determined by various conditions imposed on the plants which grow in the area. The initial group of pioneers appear and occupy the area, but they are usually replaced by other communities in turn until the area comes under the control of a plant community which is capable of perpetuating itself indefinitely. Such a community, whose further development is restricted by some limiting factor in the environment, is termed a climax community. All temporary communities in the sequence leading up to the climax are collectively referred to as a sere (adj. seral). Each floristically and structurally distinctive segment of a sere is termed a stage.
PROCESSES INVOLVED IN PLANT SUCCESSION
Succession is basically controlled by two processes viz invasion and reaction. Invasion is the former being responsible for the supply of suitable plant types to the sere while reaction is responsible for conditioning the environment to enable the area to support the new invaders.
The overall process of invasion involves a number of closely related but distinct processes. These are migration, ecesis, aggregation and competition.
Any bare areas which do not contain any seed or other items of plant propagation must receive their first inhabitants (pioneers) through the process of migration. It includes all the possible ways that plants can be transported from their original habitats in the form of seeds, spores and vegetable reproductive organs. Migration starts when the disseminules (plant parts capable of reproduction) leave the parent area and ends only when they reach their final resting place. The process may consist of a single or several separate movements. The three factors which enter into migration are
- mobility of disseminules, (ii) agent of transport and (iii) presence of barriers. The value of mobility to the plant is dependant on the presence of proper agents for bringing about movement, and the successful operation of these two factors is much affected by distance and topography. Plants differ greatly in their ability to migrate, this ability being dependent chiefly on the size, weight and the nature of the surface of the disseminules.
Ecesis is the adjustment of a plant to a new home after the migratory stage is complete. It consists of three essential processes – germination, growth and reproduction. A plant may be able to complete one or two of these processes in a new environment without being able to complete all three. In this case ecesis has failed. It is the decisive factor in invasion. Migration without ecesis is in most cases ineffective. It can only play a part in an area if it is repeated each year so that new disseminules are constantly being made available. If, however, the migrating species is able to complete its development and produce seed in the new area, the permanent invasion of the species into this area is more likely to succeed. Furthermore, migration followed by ecesis provides a nucleus for further migrations.
The first critical process in ecesis is germination. Its success depends on the suitability of–conditions for germination for only a short period of time, so that this process is seldom the limiting process to ecesis. This is followed by growth, which includes establishment of the seedlings and general growth and development of the plant to maturity. The rate at which the plant grows is also important since this will in part determine its ability to compete (particularly for light) with other plants. Early seedling establishment is generally critical here. Once this process is completed, growth and development to vegetative maturity generally follows.
Reproduction is the final step to ecesis. This is often a critical factor, particularly when plants migrate long distance’s and so possibly into areas with different photoperiods. Temperatures also play an important role in flowering and seed production and so may prevent successful ecesis even while allowing for establishment and growth.
The pioneers in a bare area or the invaders in a vegetated area arrive there through migration and after successful ecesis of the invaders develop into families or colonies through the process of aggregation. The most simple form of aggregation results from the grouping of new plants around the parent plant, as occurs when the seeds of the invading species are immobile. Here a family consisting of a large number of individuals of the same species develops on bare areas. Simple aggregation therefore favours dominance.
When seeds or plant material capable of reproduction are carried away from the family group by migration, or strange individuals are brought into it by the same means, then aggregation is mixed.
Mobility of seeds is usually a prerequisite of this type of aggregation, but mixed aggregation can also owe its origin to the over-lapping of two or more simple aggregations composed of plants having immobile seeds. A family may be converted into a mixed community through the invasion of mobile seeds. The change occurs when one or more individuals of a second species become established in the family group.
The appearance of each successive stage in succession is brought about by the interaction of migration and aggregation. Migration brings the species of each new stage and aggregation, following ecesis, results in the dominance of these species if they are better adapted to the conditions than are the plants which occupy the area.
When plants are carried into a group of other plants or a plant is surrounded by its growing offspring the struggle which develops between the individuals for the various requirements for growth is termed competition. Competition always occurs where two or more plants make demands for such factors as lights nutrient or water in excess of what is available. If there are enough of these factors, then clearly competition does not exist. This condition is usual during the early stages of succession when plants are widely spaced. Competition increases however, with the increasing density of the population, as succession proceeds and it remains a .powerful force even in climax communities.
Competition is greatest between individuals or species which make similar demands on the same supply at the same time. In general it is less severe between species which make their demands upon the same supply either at different times or at different levels. Competition for light varies somewhat from competition for most other factors of the environment in that competition is still severe between plants growing at different levels. Since plants demanding high light intensities cannot survive in the shade cast by dense dominant layers of vegetation, such environments are characteristically occupied by plants which are tolerant of shade and intolerant of high light intensities. In this way competition for light is reduced. The different plants assume a relation of dominance and subordination, the lower layers depending on the upper layer for their survival.
The overall process of invasion is therefore a complex one. It is a process which is going on at all times and in all directions. It may occur into bare areas, where it initiates succession, or into areas which are already vegetated, where it continues the sere by providing disseminules for the succeeding stages of the sere. Even when the climax community has developed invasions do not cease but they are either ineffective or the invader is adopted into the climax community.
There are two main types of invasion: (i) continuous or recurrent invasion is of a local nature, e.g. where willows invade new deposits of silt on the banks of a river, only to be destroyed each time the river floods. By invading after each flood the willows may eventually build up sufficient deposits to escape the effects of the floods. In the same way trees may invade grassland, only to be burned by grass fires. Repeated invasions may, however, reduce the density of the grass to such an extent that grass fires are no longer able to destroy the trees.
Recurrent invasion is characteristic of ecotonal (transitional areas between two or more communities) areas. Any change in the conditions of the environment may swing the balance in favour of one of the communities which may then successfully invade the adjacent community. This brings about a shifting of the position of the ecotonal area.
- Intermittent invasion commonly arises through intermittent or periodic movement of plants into distant regions. Such invasions are relatively infrequent but they are often quite striking, since plants often move large distances into areas in which they would not be expected to survive.
Invasion is said to be complete when the movement of invaders into a community is so great that the invaders dominate the community. Where the number of individuals is sufficiently small to enable them to be adopted into the community without changing it materially, invasion is said to be partial.
Invasion after the first pioneer stage may occur at three different levels. A community may be invaded at its level i.e. by a species of the same general height and form as the species in occupation e.g. trees into a forest. In this case the level of the invading species has no significance so that either species may dominate. The sequence of development in such a case would be determined by such factors as the ability of the seedlings of the invading and invaded species to withstand shade, etc. If the invaders are above the level of the occupants e.g. trees invading into grassland, the newcomers have a competitive advantage for light and so will become dominant provided they are well adapted to the environment. However, where invasion occurs at a level below that of the existing community it is unlikely to exert much influence on the community unless the plants of the existing community are handicapped in some way e.g. by overgrazing.
A plant or community produces various effects upon the habitat. All such effects are termed reactions. These are not to be confused with the adjustments and adaptations which .plants make to environmental conditions. Plants react on environmental factors (water, light nutrients etc.), changing one or more of these to varying degrees. In this way a seral community slowly makes conditions more suitable for the growth of plants higher in succession. These changes do not, however, continue indefinitely. There comes a time when the plants and the environment are in balance further changes to the environment being prevented by limits of some environmental factor, such as climate. It is at this stage: that the climax community develops.
Plant succession which depends on biotic reaction for its progress is known as autogenic *succession. The progress of autogenic succession therefore depends on the changes which the plants bring about in the environment through their reaction on such factors as soil fertility (addition of organic matter), soil and air temperature, light conditions at the soil surface, etc. In practice, however, changes can also be brought about in plant communities by continuously acting outside factors which by continually altering the habitat, make conditions less suitable for the occupants of a particular community and more suitable for other plant types. Examples of outside acting factors which commonly influence the environment include changes in climate, in the degree of leaching, topography (erosion and deposition), frequency and severity of fires, and changes in the degree and season of grazing. Plant adaptations could also be classed as such a factor. All these factors, which are independent of the plants in the community, gradually alter the habitat and thus bring about either progressive or retrogressive changes in the vegetation. Succession caused by these factors is known as allogenic succession.
Autogenic succession is generally more ordered and more predictable than most forms of allogenic succession. It continues until some limiting factor such as climate or soil depth restricts further development. If climate is the limiting factor, further advance is not possible unless the climate changes. However, if some factor such as soil depth restricts the normal progress of autogenic succession (as would be expected under the climatic conditions of the area), development can still continue since biotic reaction may slowly clear soil depth. At-this stage, however, succession is extremely slow and so the vegetation occupying such an area is often regarded as being climax.
Autogenic and allogenic succession generally occur simultaneously and often inopposite directions. Autogenic succession is always progressive, leading towards the climax; allogenic succession is often retrogressive. The balance between these two forces determines the changes which occur in vegetation.
- Autogenic : Due to the activities of the organisms (plants) themselves.
• Allogenic : Due to factors outside the control of the organisms (plants)
Such factors are fire, graziong etc.