Ecological succession

Ecological succession is the gradual process by which ecosystems change and develop over time. Nothing remains the same and habitats are constantly changing.In order to understand succession, it is necessary to clearly understand the difference between these four terms:

  • Habitat
  • Population
  • Community
  • Ecosystem

 

Habitat

A place where organisms live e.g. a pond.

 

Pond

 

Population

A group of individuals of the same species in a particular location.

For example, all of the Great Diving Beetle larvae and adults in the pond.

See Great Diving Beetles up close.

 

Community

All of the populations of species in a given area.

For example, all of the numerous species of micro-organisms, plants and animals living in the pond.

 

Ecosystem

The community, together with the physical and chemical
environment of a particular area.

The pond ecosystem includes all of the non-living components – the water, the soil surrounding the pond, the mud on the bottom, the weather and microclimate, together with the living community of organisms.

 

Ecological succession is the gradual process by which ecosystems change and develop over time.

For example, a bare patch of ground will not stay bare. It will rapidly be colonized by a variety of plants.

A recently cleared patch of ground (in Britain). The same ground 2 years later,
now covered in grasses and low flowering plants.

In the process of succession, the species present in an area will gradually change.

Succession takes place because the environmental conditions in a particular place change over time. Each species is adapted to thrive and compete best against other species under a very specific set of environmental conditions. If these conditions change, then the existing species will be replaced by a new set of species which are better adapted to the new conditions.

As an example, the environmental conditions present on the bare patch of ground above would have been quite different 2 years later. Some of these differences are highlighted below.

Bare Ground Two Years Later
No plant competition for light, space, nutrients or water.Soil mobile and liable to erosion and loss.

A more extreme surface microclimate because the bare soil both absorbs and reflects heat more than soil covered in vegetation.
A drier environment because there is no plant cover to hold moisture above ground and little humus to hold it in the soil.

Lower nutrient levels in the soil.

Intense plant competition for space and other resources.Soil bound by roots and plant cover.

The plant cover provides a certain amount of ground insulation from extremes of temperature. There are now also a variety of microclimates within the vegetation.

Plant cover and increasing humus levels help to retain water.
The nutrient levels in the soil will have increased.

The bare ground conditions favourpioneer plant species. These are often species which grow best where there is little competition for space and resources.

Mosses are often pioneer species. Most moss species are low growing, carpeting the ground and with little height. As a result of this growth form, many mosses are unable to successfully compete for space amongst taller, dense ground cover.This makes bare ground ideal for the establishment of a number of different moss species. These mosses then provide a microhabitat equivalent to a miniature forest for a variety of invertebrates such as mites and spiders. The moss also acts like a sponge when wet, in some cases providing a semi-aquatic microhabitat.

 

Pioneer species are often also ‘opportunist’ species which are able to rapidly exploit a sudden new opening in ground plant cover. 

Left: Dandelion

 

Typical examples in the UK would be dandelion, Foxglove and willowherb.

 

 

Seeds arrive, germinate and grow quickly, rapidly reproducing themselves before other slower-colonizing species arrive to outcompete them.

Along with the plants will come the animals which feed on them or use them for shelter.

 

 

Left: Bee on Foxglove

Pioneer species are often characterized by having light seeds, which are easily transported by the wind. Many of them belong to the flowering plant family ‘Compositae’Such plants produce large numbers of seeds with parachutes of fine hairs which help to keep them afloat in air currents.

 

(Left: Ragwort seed)

 

 

By the time two years have passed, succession has already taken place on our bare patch of ground. A new community of plants, animals, fungi and micro-organisms has largely replaced the earliest pioneer species which first colonized the bare ground.

The environmental conditions on the same patch of ground are considerably different to the conditions present when the first pioneers arrived.

By now, a variety of longer-lived, slower colonizers have displaced many of the species in the early pioneer community. These include grasses and flowering herbs such as dock, clover and Bird’s-foot Trefoil. The ground is almost completely covered in plants.The plants are not the only species to have colonized the area. Along with the plants have come the herbivorous invertebrates which feed on those particular plant species. The presence of the herbivores then attracts a range of carnivorous invertebrates to feed on them.

Left: ‘Cuckoo Spit’ – the foamy home of the young stage of a herbivorous insect known as a Froghopper. On Birds’s-foot Trefoil.     See inside the ‘spit’.

Larger herbivores such as rabbits and deer will also graze such an area.Their dung provides another wonderful microhabitat for many different species of fungi and invertebrates. Nothing is wasted in nature and one animal’s waste is another’s food source!

The soil will be enriched as the dung is recycled by the dung-dwelling community. Interestingly, the dung community also goes through a succession of its own as the droppings age.

Left: Roe Deer droppings

One of the fungi occurring in various types of animal dung is called Pilobolus. The fruiting bodies are produced on the surface of the dung (left). It has a fascinating method of ensuring that it is always in the right place at the right time.The fruiting bodies are only 3mm high, with a little black   packet of spores on the top. The bulb beneath the spore packet explodes shooting the packet of spores out onto surrounding grass. Another animal will come along and eat the grass together with the spores. These then pass through the gut of the animal and out in the dung, where the fungus can then develop all over again.
A number of small mammals will also now be present, including shrews (left), voles and mice. These will attract carnivores such as owls and foxes to feed on them.

Because there are now many more species of plants and animals in the area, the number of possible interactions between species has greatly increased.

The simple food chains of the eariest pioneer stage, when few species were present, have developed into more complex foodwebs.

Many, if not all of the environmental changes which have taken place on our original bare area have actually been brought about by the communities living there. This is because in the processes of living, growing and reproducing, species interact with and modify their habitat.

Using our previous example to illustrate this, the grasses and small flowering herbs have by now bound the bare soil with their network of roots, preventing erosion and soil loss. The soil has been somewhat enriched through the addition of dung and the decomposition of dead plants. Legumes such as the clover and trefoil will also have helped to add nutrients to the soil through nitrogen fixation in their root nodules.

The cover provided by the vegetation has affected the microclimate of the ground surface, while also providing a great variety of microhabitats for invertebrates.

Left: Shield Bug on buttercup

The environmental changes brought about by organisms often result in their subsequent elimination from the area.For example, pioneering mosses may grow into thick cushiony carpets with great water-holding capacity, which act like sponges. These provide an ideal new environment for the germination and establishment of the seeds of other competing plants, which otherwise might die from lack of water.

These rhododendron seedlings (left) will eventually grow into tall, tangled, dark thickets which will shade out and displace the moss which enabled them to survive in the first place. (Note :Rhododendron ponticum is not native to Britain and causes great ecological damage.)

In the space of two years, the biodiversity (variety of life) on our bare patch of ground has soared, as it has been colonized by fungi, plants and animals. The ecosystem has developed from a very simple one with few interactions, to a much more complex system with a staggering number of interactions going on between individuals, species and the habitat itself.

If left undisturbed, the area will pass through a number of further different successional stages, each with its own characteristic mix of species. All of these different successional stages are known collectively as a sere.

Each new community will be better adapted to the changed environment which has been provided by the previous community.

Eventually, a climax or ‘final’ community is reached. At this point, the succession will not go any further. However, this does not imply that there will be no further change.

The climax community for our original bare patch of earth would be oak woodland.As part of the natural sequence of life, trees mature and eventually die. When they fall to the ground, an opening is provided in the woodland and the process of succession will start all over again on this new opening.

The species which colonize the opening will be different to the original bare ground pioneers because the environmental conditions have been altered.

Because there is a different starting point, this cycle of succession would be known as a secondary succession.

It differs from the succession which first started on our original bare patch of ground, because this had never been colonized before. This first succession is therefore known as a primary succession.

Ecological Succession – Summary

An example of succession. Open water such as a lake or pond eventually becomes climax woodland.
Hold your mouse pointer over each image for more information or for a full explanation click here.

 

Ecological succession is the gradual process by which ecosystems change and develop over time. Nothing remains the same and habitats are constantly changing.There are two main types of succession, primary and secondary.

Primarysuccession is the series of community changes which occur on an entirely new habitat which has never been colonized before. For example, a newly quarried rock face or sand dunes.

Secondary succession is the series of community changes which take place on a previously colonized, but disturbed or damaged habitat. For example, after felling trees in a woodland, land clearance or a fire.

 

The Major Points:

  • The species living in a particular place gradually change over time as does the physical and chemical environment within that area.
  • Succession takes place because through the processes of living, growing and reproducing, organisms interact with and affect the environment within an area, gradually changing it.
  • Each species is adapted to thrive and compete best against other species under a very specific set of environmental conditions. If these conditions change, then the existing species will be outcompeted by a different set of species which are better adapted to the new conditions.
  • The most often quoted examples of succession deal with plant succession. It is worth remembering that as plant communities change, so will the associated micro-organism, fungus and animal species. Succession involves the whole community, not just the plants.
  • Change in the plant species present in an area is one of the driving forces behind changes in animal species. This is because each plant species will have associated animal species which feed on it. The presence of these herbivore species will then dictate which particular carnivores are present.
  • The structure or ‘architecture’ of the plant communities will also influence the animal species which can live in the microhabitats provided by the plants.
  • Changes in plant species also alter the fungal species present because many fungi are associated with particular plants. more
  • Succession is directional. Different stages in a particular habitat succession can usually be accurately predicted.
  • These stages, characterised by the presence of different communities, are known as ‘seres’.
  • Communities change gradually from one sere to another. The seres are not totally distinct from each other and one will tend to merge gradually into another, finally ending up with a ‘climax’ community.
  • Succession will not go any further than the climax community. This is the final stage.This does not however, imply that there will be no further change. When large organisms in the climax community, such as trees, die and fall down, then new openings are created in which secondary succession will occur.
  • Many thousands of different species might be involved in the community changes taking place over the course of a succession. For example, in the succession from freshwater to climax woodland.
  • The actual species involved in a succession in a particular area are controlled by such factors as the geology and history of the area, the climate, microclimate, weather, soil type and other environmental factors.For example, the species involved in a succession from open freshwater to climax woodland in Central Africa, would be quite different to those which have been quoted in these pages as occurring in Britain. However, the processes involved would be the same.
  • Succession occurs on many different timescales, ranging from a few days to hundreds of years.It may take hundreds of years for a climax woodland to develop, while the succession of invertebrates and fungi within a single cow pat (cow dung), may be over within as little as 3 months.

    By this time, the dung has been transformed into humus and nutrients and has been recycled back into the soil. The holes clearly visible in the cow pat (right) have been made by the animals which have colonized it.

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