AP Syllabus focus:
‘Threats to wetlands and mangroves include commercial development, dam construction, overfishing, and pollution from agriculture and industrial waste.’
Wetlands and mangroves are productive transition zones between land and water.
Diagram illustrating how a wetland’s position within a watershed influences the functions it performs (e.g., hydrologic storage/flow moderation, water-quality improvement, and habitat support). This helps connect human-driven changes in drainage, channelization, and land use to predictable shifts in wetland services and vulnerability. Source
Their shallow waters and saturated soils make them highly sensitive to human disturbance, especially activities that alter water flow, habitat structure, and water quality.
Major Threat Categories (AP focus)
Commercial development
Commercial development threatens wetlands and mangroves primarily by direct conversion and hydrologic disruption.
Filling and draining for buildings, roads, ports, and tourism infrastructure removes habitat outright.
Dredging for navigation channels increases turbidity and can bury root systems and benthic habitat.
Coastal armoring (seawalls, bulkheads) changes natural shoreline processes, reducing sediment deposition needed to maintain wetland elevation.
Fragmentation breaks continuous habitat into smaller patches, increasing edge effects (more light/heat exposure, invasive species access, and human disturbance).
Key ecological outcomes:
Loss of nursery habitat for fish and shellfish.
Reduced storm-buffering capacity where mangrove belts are narrowed or removed.
Lower biodiversity as specialist species lose breeding and feeding areas.
Dam construction
Dam construction threatens wetlands and mangroves by changing the timing, amount, and sediment load of freshwater reaching downstream ecosystems.
Reduced freshwater flow can increase salinity in downstream wetlands and estuaries, stressing salt-intolerant vegetation and shifting species composition.
Sediment trapping behind dams deprives downstream deltas and coastal wetlands of sediments that replenish soils; wetlands may subside or erode when sediment inputs drop.
Altered flooding regimes reduce the seasonal inundation patterns many wetlands depend on for nutrient delivery and habitat renewal.
Changed nutrient delivery can either starve downstream systems of nutrients or concentrate impacts in new locations, depending on reservoir dynamics and release patterns.
Common downstream effects:
Shrinking or “drowning” coastal wetlands when soil building can’t keep pace with erosion.
Simplified food webs as vegetation structure and water chemistry change.
Overfishing
Overfishing threatens wetlands and mangroves through food-web disruption and physical damage from some harvesting practices.
Photograph of a red mangrove stand highlighting arching prop roots, the aboveground root structures that stabilize sediments and create complex nursery habitat. Seeing the root architecture makes it easier to understand why trampling, prop-root breakage, and sediment burial can rapidly reduce habitat quality and shoreline protection. Source
Removing key predators or grazers can cause trophic cascades, shifting community structure (for example, unchecked growth of certain prey species that alters vegetation or benthic communities).
Harvesting methods can damage habitat:
Trampling and prop-root breakage in mangroves during collection.
Disturbance of shallow sediments, increasing turbidity and reducing light for aquatic plants.
Loss of fish and invertebrate populations reduces ecological interactions that help maintain wetland productivity and resilience.
Management relevance:
Mangrove and wetland fisheries are often juvenile-dependent; degrading nursery habitat and overharvesting adults can compound population declines.
Pollution from agriculture and industrial waste
Pollution is a major driver of wetland and mangrove degradation because these ecosystems intercept runoff and can become sinks for contaminants.
Agricultural pollution pathways:
Fertiliser and manure runoff elevates nutrient levels, increasing algal growth and plant overgrowth that can alter habitat structure.
Conceptual diagram of eutrophication: nutrient inputs from farms/lawns enter surface waters, stimulate algal blooms, and—after algal die-off—microbial decomposition consumes dissolved oxygen, creating hypoxic conditions and fish kills. This directly supports why nutrient pollution can shift wetlands/estuaries toward low-oxygen, lower-diversity states. Source
Pesticides and herbicides can be toxic to non-target aquatic organisms and reduce biodiversity.
Sediment runoff from tilled fields clouds water, buries eggs and benthic habitat, and can smother mangrove pneumatophores (aerial roots).
Industrial waste pathways:
Heavy metals and persistent chemicals can accumulate in sediments, creating long-term contamination hotspots.
Organic and chemical discharges can lower dissolved oxygen through microbial decomposition or direct toxicity.
Typical consequences:
Reduced reproduction and survival of fish, amphibians, and invertebrates.
Contaminated seafood and wildlife, increasing risk for higher trophic levels (including people where subsistence harvest occurs).
Shifts toward pollution-tolerant species, lowering ecosystem complexity.
How threats interact (compounding impacts)
Development often increases runoff volume and pollutant loads, intensifying agricultural/industrial contamination effects.
Dams can reduce sediment supply, making coastal wetlands more vulnerable when development removes protective vegetation.
Overfishing can reduce ecosystem stability, making wetlands less able to recover from pollutant pulses or habitat alteration.
FAQ
Decisions typically use an “avoid–minimise–compensate” hierarchy.
Key factors can include:
Rarity and connectivity of the habitat
Flood risk and shoreline stability implications
Expected permanence of impacts
Feasibility of restoring equivalent habitat nearby
Reduced river discharge allows seawater to push further upstream (saltwater intrusion).
Lower peak flows also mean:
Less flushing of salts from soils
Longer residence time of saline water in tidal channels
Damage usually comes from access and gear use rather than fishing itself, such as:
Cutting branches/roots to clear pathways
Trampling prop roots during collection
Dragging gear through shallow root zones
Persistent contamination often involves:
Heavy metals (e.g., lead, cadmium)
Some synthetic organic chemicals that bind to fine particles Low-oxygen sediments can slow breakdown, extending persistence.
Common approaches include:
Regrading shorelines to restore tidal flow
Replanting native mangrove species
Removing barriers (culverts/berms) that block inundation Success depends strongly on restoring hydrology before planting.
Practice Questions
Identify two threats to wetlands and mangroves and state one way each threat can damage the ecosystem. (2 marks)
1 mark for each valid threat + matching damage mechanism (max 2):
Commercial development: habitat loss via draining/filling, fragmentation, dredging.
Dam construction: reduced freshwater/sediment delivery, altered flooding.
Overfishing: trophic cascade, loss of nursery-dependent populations, physical damage during harvesting.
Agricultural/industrial pollution: токсicity, nutrient enrichment, sedimentation, contaminated sediments.
A mangrove-lined estuary is downstream of a new dam and an expanding coastal resort. Explain how these activities could jointly threaten mangroves and associated fisheries. (5 marks)
(Any five, 1 mark each):
Dam reduces freshwater flow, increasing salinity stress on mangroves.
Dam traps sediment, reducing soil building and increasing erosion/submergence risk.
Dam alters flooding regime, disrupting nutrient delivery and habitat conditions.
Resort development removes/fragment mangroves through clearing, filling, dredging, or shoreline hardening.
Increased runoff from development raises pollutants/sediments, smothering roots and reducing water quality.
Loss of mangrove nursery habitat reduces juvenile survival, lowering fishery recruitment.
Combined stressors reduce resilience, slowing recovery after disturbance.
