Ecological successionEdexcel A-Level Biology A: Revision notes
Section 1
What is succession?
Ecological succession is the process by which the species in a community change over time, as one community replaces another in the same area. It begins with colonisation of new or disturbed ground and ends with a climax community.
Each of the intermediate communities is a seral stage. At each stage the organisms change the environment, so that the abiotic conditions suit a different set of species.
There are two types:
- Primary succession begins on newly formed land with no soil, such as bare rock, lava, sand or ground exposed by a retreating glacier.
- Secondary succession begins on ground that already has soil, and often seeds, after a community has been destroyed, for example by fire or by abandoning farmland. It is faster than primary succession.
Do not say that secondary succession starts from nothing. It starts with soil that contains nutrients and seeds, which is why it is faster.
Section 2
Pioneer species and the first stages
The first species to colonise bare ground are pioneer species, such as lichens, algae, mosses and, on dunes, marram grass. They can tolerate extreme conditions (little water, wind, few nutrients) and spread by lightweight spores or seeds. Some, such as lichens, can also weather the rock.
When the pioneers die and decompose, they add organic matter (humus). Together with the weathered rock, this forms a thin soil, which holds more water and nutrients. Some species, such as alder, have nitrogen-fixing bacteria in their root nodules, which raise the nitrate content of the soil.
The abiotic conditions become less hostile, so new species with less tolerance can colonise.
Section 3
Intermediate stages: why the species change
As the soil deepens, the next species colonise: grasses and herbs, then shrubs, then trees. At each stage:
- The changed conditions (more nutrients and water, more shelter) allow new species to colonise.
- Taller, larger species shade and outcompete the earlier ones for light, water and nutrients, which are then replaced.
- The biomass and the number of species increase, and the food webs become more complex.
- The environment becomes less extreme: the temperature range, wind speed and water loss decrease.
The pioneer species are outcompeted because they have been successful: they changed the environment into one in which other species are better competitors.
In a succession answer, explain each change in two parts: how the organisms alter the abiotic conditions, and why the next species can then colonise and replace them.
Section 4
The climax community
The final stage is the climax community. It is stable: the species stay much the same over a long time, because the community is in balance with the climate and the soil. The biomass and the number of species no longer increase.
It is made up of a few dominant species. In a temperate area of the UK it is typically oak woodland, but the climax community depends on the climate and soil: in other regions it may be grassland or rainforest.
A climax community can be changed by a disturbance, such as fire or a storm, and succession then starts again.
Section 5
Studying succession
Succession can take hundreds of years, so ecologists do not follow one site from start to finish. Instead they:
- sample along a transect across a habitat where age changes with position (for example sand dunes that are older inland, or land exposed by a retreating glacier) and assume that each position shows an earlier stage of the sequence;
- measure abiotic factors, such as organic matter in the soil, pH, or the nitrogen content, and record the species present using quadrats.
Typical results on dunes are that organic matter, water retention and the number of species all increase with distance from the sea, from a few pioneer grasses to herbs, shrubs and then trees.
This method assumes that all the sites have had similar conditions, and chance events can mean that succession does not always follow the same path.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Ecological succession
- A new volcanic island formed when an undersea volcano erupted. At first it was bare lava rock and ash, with no soil. Within a few years the first mosses and lichens had appeared. Grasses, whose seeds were carried by wind and birds, came next, and later a colony of gulls settled on the island. Several decades later there were many species of flowering plants and some small shrubs, but no trees.Suggest why there were no trees on the island several decades after it formed.2 marks
- A field of wheat was abandoned in 1990 and was no longer ploughed, sown or grazed. In the first year annual weeds grew on the bare soil. Over the next few years perennial grasses and tall herbs replaced the weeds, and after about 10 years brambles and hawthorn shrubs were present. By 40 years birch and ash trees had grown up through the shrubs, and by about 150 years the field had become a woodland dominated by oak.Explain how the tall herbs in the field were replaced by shrubs.2 marks
- Ecologists took samples along a transect across a sand dune system, from the sea inland. The dunes further inland are older. At 10 m from the sea the sand contained 0.2% organic matter and the only plants were sand couch grass and marram grass. At 100 m the organic matter was 1.1%, at 300 m it was 3.5% and at 600 m it was 8.5%. At 600 m the plants included mosses, lichens, flowering herbs and some shrubs, such as gorse and birch saplings.Describe the trend in the organic matter of the sand along the transect and explain how the pioneer plants cause it.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).