Ecological Succession

Ecosystems are not fixed, but constantly change with time. This change is called succession.

• Different species of plants naturally colonise a habitat in a predictable order, until finally a stable community is reached, called the climax community.
• Each plant species in turn changes its environment (e.g. by creating  deeper soil, or providing shade), making the environment more suitable for new species to colonise.
• New species are usually bigger plants with a larger photosynthetic area, so they outcompete and replace the older species.
• The plants colonising early in succession (the pioneer species) tend to be small and fast growing, with shallow roots and wind-dispersed seeds.
• The plants colonising late in succession tend to be tall and slow growing, with deep roots and animal-dispersed seeds.
• Successive stages are called seral stages, or seral communities.
• Whole succession is called a sere.
• Takes a few hundred years to reacha stable climax community.
• The climax community is usually a forest 

• As the succession proceeds the habitat becomes less harsh and the abiotic factors less hostile.
• For example daily temperature fluctuations decrease (due to shade); water is more easily available (since it is retained in soil) and nitrates increase (due to nitrogen fixation and decay).
• These changes are what allow more plant species to colonise.
• As the plant community becomes more diverse, the animal community also becomes
  more diverse, since there is a greater variety of food for primary consumers and therefore a greater variety  of food for secondary consumers.
• There is also a greater diversity of niches in the more complex ecosystem.
• The climax community supports a complex food web, which also aids stability.

Are two kinds of succession.

Primary succession

• Starts with bare rock or sand.

Secondary succession

• Starts with soil, but no (or only a few) species, such as in a forest clearing, following a forest fire.

Primary succession from bare rock

• Bare rock stores very little water and has few available nutrients.
• The only species that can survive there are lichens – a mutualistic relation between an -alga and a fungus – who start the process of succession.
• The climax can beoak, beech or pine forest.

I Lichens and mosses

• The first pioneers are lichens, who can absorb the scarce water from the bare rock.
• Mosses can then grow on top of the lichens.
•  These species are very small, slow-growing, wind-dispersed and tolerant of extreme conditions.
• They start to weather the rock by secreting acids, and so begin to form a very thin soil.

II Grasses and Herbs

• The next colonisers are grasses and ferns, dandelion and nettles.
• These species have a larger leaf area, so they grow fast and out- compete the pioneers.
• Their larger roots weather the rock and add more detritus, adding inorganic and organic matter to the soil, which now holds more water.

III Shrubs and Bushes

• Larger plants (shrubs) such as bramble, gorse, hawthorn, broom and rhododendron
  can now grow in the thicker soil.
• These species have larger, animal-dispersed seeds and they grow faster and taller,
  out-competing the smaller herbs.

IV Woodland

• Trees grow slowly, but eventually shade and out-compete the shrubs, which
  are replaced by shade-tolerant forest-floor species.
• A complex layered community is now established with many trophic levels and intercations. This is the climax community.

Primary succession from water - a hydrosere

• Light cannot penetrate far through water, so only floating phytoplankton can survive in deep water.
• Their detritus starts the process of succession, which ends with woodland or a peat bog.

I Plankton

• Floating phyto plankton colonise deep water using wind-dispersed spores.
• When they die they sink to the bottom, forming humus, which combines with silt  deposited by rivers to form mud that builds up on the bottom.

II Rooted Aquatic Plants

• As the mud builds up, the water becomes shallower, allowing rooted plants to g r o w .
• These include submerged species, like pondweed, and species with floating leaves, like lilies.
• Their root systems trap more silt and their faster growth results in more detritus settling to the bottom.

III Swamp and Marsh

• Eventually the sediment rises out of the water to form a waterlogged soil.
•  Reed grasses and sedges colonise to form a reed marsh.
• Their roots bind the mud together to form semisolid soil, and the increased rate of
  transpiration starts to dry the soil.

IV Woodland

• As the soil dries it can be colonised by more terrestrial species.
• First herbs replace the marsh vegetation then shrubs replace the herbs and eventually trees replace the shrubs.

Human effects on succession

Humans interfere with succession, and have done so  since the development of farming, by cutting down forests to make farmland.

All landscapes today like woodland, grassland, moorland, farmland and gardens are all maintained at pre-climax stages by constant human interventions, including ploughing, weeding, herbicides, burning, mowing, crop planting, grazing animals and dredging waterways.

These interventions cause a deflected succession, resulting in a plagioclimax.

Why does grazing stop succession at the grassland stage?

• Herbs, shrubs, trees and the later species of plants are mostly dicotyledons (broad-leafed plants), which have strong, vertical stems and grow from apical meristemsat the tips of their shoots and leaves. Grazing animals eat these apical meristems or uproot the whole plant, so the plants die.
• Grasses on the other hand are monocotyledons (narrow-leaved plants), which have horizontal stems and grow from intercalary meristemsat the base of their shoots and leaves. Grazers cannot eat these intercalary meristems or uproot the plants, so grasses can continue to grow.