Ecosystems are dynamic (constantly changing), whether it be through changes in the abiotic conditions or the species that live within them. The time scale of change varies depending on the environment and its inhabitants.
Definition of ecological succession
A barren land will start off with a pioneer species and gain more complexity over time until it becomes a complex and stable ecosystem.
The general process of succession is described below and will be broken down in more detail later on:
In order for organisms to colonise an area, they must be adapted to severe abiotic conditions. These are pioneer species.
An example of a pioneer species is lichen which survives on bare rock and breaks it down to produce soil on its surface over time. Lichen are able to do this because they have the ability to break down minerals from which the rock is made of.
Pioneer species modify the habitat over time and make the conditions less severe which allows other species to colonise and potentially out-compete the pioneer species which slowly die out.
As new species colonise the area, they continue to change conditions and allow more species to colonise.
As changing abiotic factors become less extreme, biotic factors become more prevalent and adaptations to these are more required for survival.
Abiotic means non-living and some abiotic factors include temperature, water availability and in this case soil quality. Biotic factors are those that are living and in this case refer to predation and competition.
These steps continue until a stable community (or climax community) develops. This remains dominant until the climate changes again.
Ecological succession: The process of change in species structure of an ecological community over time.
Pioneer species: species which are adapted to colonise inhospitable environments and start an ecological community.
Climax community: A stable community in its final stage of ecological succession.
Abiotic factors: The non-living aspects of an ecosystem for example such as climate, temperature, water, and soil type.
Biotic factors: The living aspects of an ecosystem, for example plants and animals.
Different stages of ecological succession
Primary succession has multiple stages that happen over hundreds of years.
Primary succession
Primary succession occurs when a barren and mainly inhospitable terrain is colonised slowly over time by living organisms. The land involved is newly formed or exposed, with no species present.
An example of primary succession is when retreating glaciers expose bare rock or following volcanic eruption or a landslide and pioneer species colonise it.
An example of primary succession
Primary succession occurs in a series of stages. Let’s take a look at an area of exposed rock.
Seeds and spores are carried by the wind from areas of existing communities and begin to grow on the exposed rock. These species are often moss and lichens, also known as pioneer species.
There are also some animal pioneer species. Earthworms and ants are able to change soil characteristics. Burrows made by worms aerate the soil and ant hills alter sediment and particle size.
Pioneer species are important because they help to slowly break apart the top of the rock by releasing chemicals that release minerals and cause the rock to disintegrate. They make the abiotic conditions less harsh for the next colonising species. Broken down rock and humus (the fertile constituent of soil) forms soil that makes the environment more suitable for other species, such as mosses, which are also a type of pioneer species. These mosses grow and a thin layer of soil forms on the rock surface. Lichens are not adapted to grow on soil so they die off thus contributing organic matter to the soil so that other plants can grow there.
Seeds, carried in the wind, land on the soil and grow. Later on in the process, they may be carried and transported by animals such as birds and rodents. They grow roots that prevent the new soil from being washed away. These include shrubs which are used for food and shelter by animals.
These plants then die and decompose as more and more biotic competition arrives that prevents them from surviving.
The soil gains more and more nutrients and becomes deeper and deeper as decomposers break down organic matter and add to the humus. Larger plants that are better adapted to grow in deeper soil can store more water and therefore grow faster. The first trees to grow are tolerant of wind and extreme temperatures as they are not protected by larger trees and are not tolerant to shade.
Large trees can begin to grow and become the dominant species. They outcompete other species which can no longer grow due to the shade caused by the tree canopy.
Each of these phases is known as a sere (think ‘series’).
The climax community is formed when the community becomes stable and many species are supported. The community is very biodiverse.
Over time both the depth and nutrient content of the soil as well as the diversity of species increases. Each new colonising species changes the conditions and makes them less hospitable for the previous species.
Lichen, moss → Grass, small plants → Shrubs, large plants, small trees → large trees
Secondary succession
Secondary succession occurs when the starting point is bare soil that already exists.
It is similar to primary succession, however, the pioneer species are often fast-growing plants and often come from pre-existing groups of organisms in the area. Some examples of secondary succession include:
Forest regrowth after logging.
Forest regrowth after a fire.
Crop growth after harvest.
Tree growth after a hurricane.
Plant growth after flooding.
Plant regrowth after pests or disease.
For example:
- Bare soil is colonised by pioneer plants.
- Over time, grasses begin to dominate.
- Shrubs replace grasses.
- Fast-growing trees begin to appear.
- Slow growing trees form the climax community.
An example of secondary succession is on Mount St Helen in the US. On May the 18th, 1980, Mount St. Helens volcano erupted, in what was the worst volcanic disaster in the history of the United States.
During the eruption, the surrounding forest was flattened and an 80000 feet high cloud of ash circled the globe. 230 square miles of forests, lakes, meadows, and streams were basted. Despite this, some plants and animals did survive. Animals in underground burrows, ice-covered lakes, and those hibernating in the mud were protected from the hot, stone-filled wind of the blast. Plants such as willow, vine maple, and black cottonwood re-sprouted from roots protected in moist soil. Many of these species didn’t survive in the harsh abiotic conditions (such as decreased sunlight due to ash) of the new environment but some did survive and made the conditions more favourable for new colonisers. The wind brought seeds and insects to the area, enabling them to establish themselves. It is predicted that it will take hundreds of years for the land to return to how it looked before the eruption. Three years after the eruption, the average plant coverage was 1 percent. Eleven years later, plant coverage was up to 38 percent and then six years after that, plant coverage was approximately 66 percent.
Fig. 3 — Mount St Helen eruption. Source: pixabay.com
Primary vs secondary succession
Primary succession | Secondary succession |
Starts on bare rock | Soil is already present |
The soil must be formed before plants can grow. | Grasses, birches and alders are the first plants to grow. |
Pioneer species include lichens and moss. | Occurs where organisms have lived previously. |
No previous life. | Could occur after forest fires, flooding, harvest, disease etc. |
Could occur after lava cools and hardens into rock. | |
Could occur after a glacier retreats, exposing rock. | |
Both occur over a long period of time and result in a climax community.
Characteristics of ecological succession
When many climax communities exist within the same climate, it is referred to as a polyclimax (many-climax). Different types of "new surfaces" give rise to different types of succession.
Different types of sere
Different seres show different characteristics which are described below.
Halosere
Halosere is a succession in saline water bodies such as lagoons, saltmarshes or tidal mudflats.
Marine algae and sea lettuce are examples of early colonisers of mudflats where tidal water is clear enough to let appropriate levels of light through. Sea lettuce is adapted to grow very fast as it has a large surface area to volume ratio meaning it can photosynthesise more and grow faster. These early colonisers stabilise the estuary mud and pioneer species trap sediment and therefore calm tidal waters. Plants like glasswort and cordgrass cause the bank to rise above the water surface - species not adapted to living underwater can now colonise. Grass species that can tolerate high saline and pH conditions colonise followed by low growing flowering plants. Eventually, non-halophytic (non-salt tolerant) species take over.
Halophyte: Salt tolerant plants that grow in places of high salinity, for example in swamps, marshes and seashores.
Fig. 4 – A salt marsh. Source: pixabay.com
Lithosere
When new rock surfaces are colonised (eg. after a volcanic eruption or on a raised beach formed by tectonic uplift).
Succession in the lithosere:
Weathering of exposed rock.
Growth of lichen.
Moss outcompetes lichen.
Grasses, ferns and small herbs colonise.
Small shrubs such as gorse and bramble colonise the surface.
Pioneer trees such as birch outcompete shrubs and make the environment shady.
Legumes make nitrogen which increases soil fertility.
Larger trees such as ash and deciduous oak colonise and cast shade on smaller species that die off.
Climax community forms.
Hydrosere
A wet environment that becomes drier as plants grow, die and decay, raising the level of the bed, thus trapping more sediment.
Succession in the hydrosere:
Rooted plants can't survive in the water.
Over time, sediments build up and the water gets shallower.
Emergent species like yellow iris and red mace grow partly in the water.
Rooted, submerged plants like pondweed and starwort establish on the surface.
A swamp forms.
The death and decay of plants lead to rising sediment and nutrient levels.
Species adapted to very wet conditions die out, and the swamp becomes a marsh due to more and more life growing there and taking up the water.
The transpiration of plants such as willows dries the soil meaning rushes and ferns thrive.
Small flowering herbs adapted to wet soil and low light levels established.
Decay and aerobic decomposition add nutrients to the soil.
Oak and ash may develop along with flora such as wild garlic.
Psammosere
Occurs in sand dunes where plants colonise the sand surface and change the ecosystem.
Fig. 5 – A psammosere habitat in the UK. Source: pixabay.com
Mobile dunes in an upper beach area accumulate sand. Hardy species tolerant of salt, wind and exposure such as marram grass colonise the area. A vegetation mat develops and the dune system stabilises. Accumulating humus changes the soil to allow plantains, wild thyme and buckthorn to accumulate. Moisture-loving plants such as willow colonise followed by heath and woodland.
Marram grass is a xerophyte that is adapted to live in harsh environments. Like other xerophytes, marram grass is well adapted to its surroundings. Its rolled leaf traps water vapour within the leaf, and helps to prevent water loss in the windy conditions of a sand dune.
Significance of ecological succession
Ecological succession is very important for the growth and development of ecosystems.
Succession and human activities
Human activities often prevent succession. Deflected succession occurs when a community remains stable due to human activity preventing further succession. Human activities often prevent climax communities from forming. For example, regular mowing or grazing livestock.
For example, grazing animals can be introduced to a plot of land and eat the shoots of shrubbery and trees meaning they cannot grow. This prevents succession occurring.
Managing Succession
Many wildlife communities have developed in plagioclimax habitats maintained by long-term human activities.
Plagioclimax community: An area or habitat in which the human influence has prevented the ecosystem from expanding any further. This can be done for conservation reasons.
Sometimes it is important to prevent an ecosystem from expanding any further. This is because often there is a lot more diversity at earlier stages of succession. An ecosystem may support species that are important to conserve (e.g. species that are threatened or have important ecosystem functions) and that would die off if the climax community was reached.
The main methods of maintaining a plagioclimax community are:
- Grazing: in agriculture, grazing is a method of allowing domestic livestock to roam around outdoors and consume wild vegetation that is unsuitable for farming.
- Mowing
- Burning
- Coppicing: coppicing involves cutting young tree stems to near ground level to form a stool. New growth emerges and after a number of years, the coppiced tree is harvested, and the cycle begins again.
- Pollarding: pollarding is a pruning system that is carried out once trees and shrubs reach a certain height. It involves the removal of the upper branches of a tree, which promotes the growth of dense foliage and branches.
An example of preventing succession is in the Scottish moorland:
Moorlands and upland heathlands exist in Britain above 250m. Heathlands are dominated by heather. This is because:
They are mainly on impervious rocks e.g. Millstone Grits which provide acidic, low nutrient soils.
The higher altitudes give strong winds and low temperatures.
There is also increased precipitation and greater cloud cover.
As a result of these factors, diversity is low. Heather moorland is a plagioclimax community maintained by grazing and burning. Heather is a food source of deer and a winter food source for some breeds of sheep. Burning has been used for over 200 years to maintain the heather as rough grazing forage. Burning keeps the ratio of edible green shoots to woody tissue at an optimum. Moorland is burnt in small areas to prevent uncontrolled fires and every 12-15 years.
How is succession measured?
Ecologists study and monitor sites where volcanic eruptions, glacier retreats, or wildfires have taken place over long periods of time. They collect data about the types of species that colonise the area at different stages. Additionally, samples of peat bogs can be taken to compare how the peat has changed over many years.
Ecological Succession - Key takeaways
Ecosystems are dynamic meaning they are constantly changing.
Succession is a progressive series of changes in an ecosystem and its communities over time.
In primary succession, bare rock is colonised.
Secondary succession occurs in an area that has previously been occupied by living organisms and then is recolonised after disruption.
Human activities often prevent climax communities from forming.
A plagioclimax community is an area or habitat in which the human's influence has prevented the ecosystem from expanding any further.
References
- Fig. 3 - Mount St Helens Volcanic Eruption (https://pixabay.com/photos/mount-st-helens-volcanic-eruption-164847/) by WikiImages (https://pixabay.com/users/wikiimages-1897/) is licensed by CC0 1.0 Universal (https://creativecommons.org/publicdomain/zero/1.0/)
- Fig. 4 - Salt Marsh (https://pixabay.com/photos/salt-marsh-damp-wetland-coastal-5549537/) by Pete (https://pixabay.com/users/emphyrio-10920769/) is licensed by CC0 1.0 Universal (https://creativecommons.org/publicdomain/zero/1.0/)
- Fig. 5 - Penhale Sands (https://pixabay.com/photos/penhale-sands-perranporth-cornwall-1599270/) by InspiredImages (https://pixabay.com/users/inspiredimages-57296/?utm_source=link-attribution&utm_medium=referral&utm_campaign=image&utm_content=1599270) is licensed by CC0 1.0 Universal (https://creativecommons.org/publicdomain/zero/1.0/)
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