Plant succession, mangroves and coral reefs

Sand dune succession (psammosere)

Succession is the gradual change in plant communities over time, as each stage alters conditions so that new species can survive. On a sandy coast:

1. Embryo dunes: sand trapped behind driftwood or debris is colonised by salt-tolerant pioneer species such as sea couch grass.

2. Fore (yellow) dunes: marram grass thrives on being buried, so dunes grow taller; much bare sand, alkaline, dry.

3. Grey (fixed) dunes: dead plant matter forms humus, the soil darkens, mosses and lichens and then more species grow; less bare sand.

4. Dune slacks: damp hollows where the water table reaches the surface.

5. Climax heath or woodland far inland, with acidic soils and full plant cover.

As distance from the sea increases, bare sand falls, species diversity and soil organic matter rise, and the dunes become fixed. This is why a transect through dunes is a classic fieldwork investigation (the June 2025 paper used % bare sand against distance from the beach).

Salt marsh succession (halosere)

In sheltered, low-energy estuaries and behind spits, mud settles out (clay flocculates in salt water). Pioneer plants such as glasswort and cordgrass tolerate submergence by every tide and trap more mud. The marsh surface rises, flooding less often, so less salt-tolerant species take over (low marsh to high marsh), and finally land plants. Salt marshes and dunes stabilise sediment and protect the coast: dunes act as a sand store and barrier against storm waves; marshes absorb wave energy and reduce flood heights.

Mangroves and coral reefs

Mangroves are salt-tolerant trees of tropical intertidal coasts. The Global Mangrove Alliance (State of the World's Mangroves 2024) reports:

• 147,256 km2 of mangroves worldwide in 2020.
• On average about 394 tonnes of carbon per hectare stored in biomass and the top metre of soil.
• Mangroves reduce median flood depths from 1-in-100-year storms by 15 to 20%, and by over 70% in some areas.

The Sundarbans (about 10,000 km2 across Bangladesh and India) is the world's largest contiguous mangrove forest. Millions of people benefit from its coastal protection; a UNDP estimate put the cost of replacing it with 2,200 km of embankments at US$294 million capital plus US$6 million a year maintenance (IUCN World Heritage Outlook).

Coral reefs break waves offshore, protecting beaches and mangroves behind them, and provide fish nurseries, food, tourism income and biodiversity. Together these are ecosystem services: benefits people get from ecosystems, often of great local economic value.

Worked example

Using the UNDP estimate, what would the Sundarbans embankment replacement cost over its first 25 years, including capital and maintenance?

1. Capital: US$294 million.
2. Maintenance: 25 x US$6 million = US$150 million.
3. Total = 294 + 150 = US$444 million.

Answer: US$444 million, a measure of the protection the mangroves provide free.

Check yourself

1. Why does marram grass dominate yellow dunes?
   • It needs acidic, waterlogged soil
   • It only grows in shade
   • It grows well when buried by sand and tolerates dry conditions
   • It cannot survive salt spray
   Show answer

   It grows well when buried by sand and tolerates dry conditions. Burial stimulates growth, so it keeps pace with accumulating sand.

2. Moving inland across a dune system, which normally increases?
   • Soil organic matter and species diversity
   • The percentage of bare sand
   • Soil alkalinity from shell fragments
   • Wind speed at ground level
   Show answer

   Soil organic matter and species diversity. Humus builds up and more species can survive.

3. In a salt marsh, pioneer plants such as glasswort are important because they
   • shade out all other species permanently
   • only grow on cliffs
   • trap mud, raising the marsh surface
   • increase wave energy
   Show answer

   trap mud, raising the marsh surface. Trapped sediment raises the surface so it floods less often.

4. Mangroves store about 394 tonnes of carbon per hectare. How many tonnes are stored in 100 hectares?
   Show answer

   39400 tonnes. 394 x 100 = 39,400 tonnes.

5. Global mangrove area was 147,256 km2 in 2020. The Sundarbans covers about 10,000 km2. What percentage of the world's mangroves is that? Give your answer to 1 decimal place.
   Show answer

   6.8 %. 10,000 / 147,256 x 100 = 6.8%.

6. Which is a regulating ecosystem service provided by coral reefs?
   • Selling souvenirs
   • Reducing wave energy before it reaches the shore
   • Building hotels
   • Dredging channels
   Show answer

   Reducing wave energy before it reaches the shore. Breaking waves offshore protects the coast.

7. Explain how mangroves protect coastal communities from storm surges.
   Show answer

   Their dense tangled roots and trunks create friction that slows water and absorbs wave energy, reducing flood depths and trapping sediment, so less water and erosion reach homes behind them.. Structure, then how it reduces energy, then the effect.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Geography specification, Issue 3 (September 2025)
• Pearson IAL Geography Getting Started Guide
• Global Mangrove Alliance: The State of the World's Mangroves 2024, executive summary
• IUCN World Heritage Outlook: Sundarbans benefits case study
• Pearson WGE02/01 mark scheme, June 2025

Last updated 8 November 2026. Spotted a mistake? Tell us.