Mangroves

1. Introduction to Mangroves

  • The lecturer frames mangroves as “one of my absolute favourite ecosystems… the most amazing, probably the most interesting and beautiful system on the planet.”

  • Mangroves are used to illustrate ecosystem functioning, resilience, and biodiversity–function relationships across several lectures.

Global distribution

  • Found mainly in tropics and subtropics, but extend into warm‑temperate regions (e.g., New Zealand, southern Americas).

  • Distribution strongly linked to temperature, competition, and evolutionary history.

  • Quote: “Mangroves don’t exist outside those realms… a lot to do with temperature… and competition.”

2. Biodiversity Patterns

Global species richness

  • Highest diversity in Southeast Asia, shown in the red areas of the map.

  • Linked to evolutionary history:

    • “The centre of evolution was over here… after those landmasses separated, mangroves evolved.”

  • Americas have low diversity because mangroves arrived late and via difficult dispersal routes (e.g., around the Cape).

Implications for research

  • Much early mangrove research came from the Americas, where forests often contain 1–3 species.

  • Quote: “Be a little bit sceptical… old literature from the Americas… it is quite biased.”

3. Environmental Conditions

Climate

  • Mangroves thrive in wet climates with high rainfall and river runoff.

  • Runoff delivers organic matter and nutrients.

Exceptions

  • Mangroves also occur in arid regions (e.g., Qatar) despite “rainfall practically zero”.

  • In these systems, nutrients come from the sea inward, reversing the usual land‑to‑sea flow.

4. Mangrove Foundation Species (Tree Families)

Three major families highlighted:

1. Avicenniaceae (Avicennia spp.)

  • Opportunistic, tolerant of cooler climates (e.g., New Zealand).

  • Produce small, plum‑like propagules.

  • Have pneumatophores (vertical “breathing roots”).

2. Rhizophoraceae (Rhizophora, Ceriops, Bruguiera)

  • Classic mangroves with prop roots.

  • Quote: “These are the classic mangrove trees… cigar‑shaped propagules.”

  • Create dense, complex root networks that strongly influence habitat structure.

3. Sonneratiaceae (Sonneratia spp.)

  • Occur lower on the shore.

  • Produce cable roots and pneumatophores.

5. Root Morphology and Function

Mangrove roots are essential for stability, gas exchange, and ecosystem engineering.

Key root types

  • Pneumatophores (“fingerling type roots… breathing roots”)

    • Allow gas exchange in anoxic sediments.

    • Contain lenticels that open at low tide and close at high tide (“the tree holding its breath”).

  • Prop roots (Rhizophora)

    • Provide structural support in soft sediments.

    • Create complex habitats for marine life.

  • Cable roots (Sonneratia)

  • Radiate horizontally, stabilising the tree and increasing oxygen uptake.

Sediment conditions

  • Mangrove sediments are highly anoxic due to waterlogging and microbial oxygen consumption.

  • Trees must transport oxygen from aboveground tissues to roots.

6. Biodiversity Supported by Mangroves

Mangroves host marine, semi‑terrestrial, and terrestrial species simultaneously.

Marine fauna

  • Fish, prawns, crabs, snails.

  • Quote: “Loads and loads and loads of crabs… foundation to the system.”

Semi‑terrestrial fauna

  • Mudskippers (“highly modified fish… spends most of its time up in the air”).

Terrestrial fauna

  • Snakes, birds, lizards, ants, spiders living in the canopy.

Visitors

  • From the sea: fish and prawns entering with the tide.

  • From land: birds, monkeys, monitor lizards.

  • Humans: “You will struggle to find many mangroves not influenced by people.”

7. Ecosystem Services Provided by Mangroves

Mangroves deliver provisioning, regulating, supporting, and cultural services.

7.1 Provisioning services

  • Fisheries: mangroves act as nurseries and feeding grounds.

    • Larger mangrove area → higher fish catch.

    • Quote: “Commercial species catches more than doubled when there was a mangrove nearby.”

  • Timber and firewood:

    • Mangrove wood is dense, durable, and widely used in developing countries.

  • Building materials: used for houses, charcoal, poles.

7.2 Regulating services

  • Coastal protection (major theme):

    • Mangroves reduce wave energy and prevent erosion.

    • Quote: “As it comes into the mangrove… hardly a ripple on it.”

  • Climate regulation (Blue Carbon):

  • Mangroves store carbon at far higher rates than terrestrial forests.

  • Blue carbon concept formalised in 2009 (IUCN report).

7.3 Supporting services

  • Nutrient cycling:

  • Mangroves absorb nutrients and prevent eutrophication.

  • Example: sewage dumping site in Tanzania where mangroves acted as primary treatment.

7.4 Cultural services

  • Important for local livelihoods, traditional practices, and coastal communities.

8. Coastal Protection in Detail

Mechanisms

  1. Wave attenuation

    • Roots and trunks absorb wave energy.

    • Demonstrated in tank experiments: “Look at the wave… hardly a ripple.”

  2. Erosion prevention

  • Roots bind sediment and prevent scouring.

Forest properties that enhance protection

  • Tree density

    • Dense forests reduce wave energy within 25–250 m.

    • Sparse forests may require >1000 m.

  • Forest width (depth)

    • Wider forests provide stronger protection.

  • Structural diversity

  • Mixed species with varied root types increase turbulence and energy dissipation.

9. Biodiversity and Coastal Protection: Evidence from Japan

Study of the 2011 tsunami:

Forest types compared

  • Monoculture pine plantations (straight rows)

  • Monoculture pine with irregular spacing

  • Mixed‑species forests

Findings

  • Greatest damage in monoculture pine forests.

  • Least damage in mixed, structurally diverse forests.

  • Quote: “Biodiversity really matters… coastal protection is one of those things we want to boost.”

10. Key Takeaways

  • Mangroves are ecosystem engineers with complex root systems enabling survival in anoxic, saline, tidal environments.

  • They support exceptionally high biodiversity across marine–terrestrial gradients.

  • They provide critical ecosystem services, especially fisheries, blue carbon, nutrient cycling, and coastal protection.

  • Forest structure, density, and species diversity strongly influence resilience and protective capacity.

  • Mangroves are deeply intertwined with human livelihoods, especially in developing countries.