Primary succession
- Starts from bare rock or water.
- Occurs in an environment that is devoid of vegetation, organisms and soil (such as a lava flow or area left from retreated glacier).
- Basically, it’s the gradual growth of an ecosystem over time.
- Quite unusual in the world today.
- Sand dunes are one of the few places where it takes place.
- Volcanic islands (e.g Surtsey, Iceland)
•In primary succession the first organisms to settle on bare rock are pioneer species that are usually lichens.
•Lichens are made of a fungus and an alga living together. The fungus anchors the lichen to the rock and absorbs water from rain or dew. The alga carries out photosynthesis and supplies the food needed for growth.
•As lichens grow they slowly break down the rock surface with acids released by the fungus. Small fragments of rock mix with dead lichen material to create the first thin layer of soil. This early soil holds moisture and nutrients, which makes the environment more suitable for mosses and small plants.
•Over time these new species add more organic matter, deepen the soil and allow larger plants to establish. Lichens therefore start the entire process by transforming bare rock into a habitat that other species can use.
•Lichens slowly penetrate and break up the rock surface, helping to form the first loose material.
•When they die and decompose they add humus, which increases moisture and nutrient levels.
•This early soil allows simple plants such as mosses and ferns to establish and continue the process of succession.
•Mosses and ferns cast shade to stop further growth of lichens
•Their roots are longer and so further break up the rock.
•As these larger plants die they contribute more organic matter, which improves water retention.
•This creates deeper, richer soil that supports grasses, small flowering plants and shrubs.
•As shrubs and early trees become established the environment becomes more stable. Soil depth increases and moisture levels rise.
•Fast-growing trees dominate at this point because they can take advantage of the improved light and nutrient supply.
•Their leaf litter enriches the soil and their canopy creates more shade, which favours species adapted to lower light. This community is still changing as new species continue to arrive and replace earlier ones.
•The climax community forms once conditions have stabilised and species turnover slows.
•The soil is now deep and rich enough to support long-lived trees that grow more slowly. These trees form a stable canopy and create a consistent microclimate beneath them.
•The community becomes self-sustaining with balanced rates of growth, death and nutrient cycling.
•At this stage the ecosystem remains relatively unchanged unless disturbed.
The plants present at each seral stage modify the environment making it more favourable for other plant species to colonise and out compete the current plants (e.g. the growth, death and decay of lichens produces organic matter which allows the growth of mosses which could not grow before the lichens).
↓Bare rock
↓Pioneers
↓Grasses/herbaceous plants
↓Perennial herbs
↓Shrubs/scrub
↓Small trees
Deciduous woodland – the climax community
Pioneer species are effective colonisers because of some specific features:
•Tolerate extreme conditions such as high salt, low nutrients or intense sunlight
•Grow quickly and reproduce fast, often with short life cycles
•Produce many seeds that disperse easily by wind or animals
•Germinate in exposed soil and do not require complex soil structure
•Fix nitrogen or improve soil quality, allowing other species to establish later
•Have broad ecological niches, so they can survive in variable conditions
•Resist desiccation and other environmental stresses
•Establish roots rapidly to stabilise loose or disturbed ground