AP Environmental Science

## Learning Outcomes

- Distinguish between **wetland loss** and **wetland degradation**
- Identify the main human threats to wetlands and mangroves
- Explain how draining, filling, dredging, and water diversion damage these habitats
- Understand how nutrient runoff, pollution, and oil contamination reduce wetland function
- Compare inland wetland threats with coastal wetland and mangrove threats
- Explain why sea-level rise and **coastal squeeze** are major risks to coastal wetlands
- Link habitat damage to lost natural services such as flood control, nursery habitat, and water filtration

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand threats to wetlands and mangroves, with a focus on the following syllabus points:

- Know that wetlands and mangroves are highly productive habitatss that provide major environmental services
- Identify direct threats such as draining, filling, dredging, shoreline development, and mangrove clearing
- Explain how altered hydrology changes water depth, salinity, sediment delivery, and species composition
- Understand how nutrient runoff and other pollution can degrade wetlands through water-quality decline and eutrophication
- Compare wetland loss, meaning physical conversion, with wetland degradation, meaning reduced ecological function
- Know that mangroves are especially threatened by oil spills, coastal development, and storm damage
- Understand how sea-level rise can cause wetland migration, but development can block that movement and create coastal squeeze

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the difference between wetland loss and wetland degradation?
2. How does draining a wetland for farming or development affect the water table and soil?
3. Why does altered hydrology often damage wetlands even if the wetland is not completely removed?
4. How can nutrient runoff cause ecological damage in wetlands and mangroves?
5. What is coastal squeeze, and why does it matter for sea-level rise?

## Introduction

Wetlands and mangroves are among the most productive environments on Earth. They support biodiversity, store water, trap sediment, filter pollutants, protect shorelines, and provide nursery habitat for fish and shellfish. That productivity is exactly why they matter on the AP exam: when humans damage these systems, the ecological effects spread far beyond the wetland itself.

In AP Environmental Science, you need to go beyond naming a threat. You need to explain the chain of cause and effect. A question may ask how urban development affects a wetland, why mangrove clearing harms fisheries, or how sea-level rise interacts with human infrastructure. Strong answers connect the human action to hydrologic change, habitat damage, biodiversity loss, and reduced ecological services.

> **Key Term: wetland**
> An area where water covers the soil or saturates it long enough to support water-tolerant vegetation and hydric soils.

### Why wetlands and mangroves are so vulnerable

Wetlands are controlled by water. That sounds obvious, but it is the key idea behind nearly every threat. If humans change how much water enters, how long it stays, how salty it is, or where it flows, the wetland system can shift quickly. Plants adapted to saturated soils may die. Sediment supply may change. Oxygen levels in soil and water may drop. Species that used the area for breeding or feeding may disappear.

Mangroves are a special kind of coastal wetland found in tropical and subtropical regions. They grow in brackish water and have root systems adapted to low-oxygen mud, tides, and salt. Those adaptations let them survive in stressful environments, but they also make them sensitive to certain human disturbances. If oil coats their roots, if dredging buries or floods those roots, or if development removes the forest entirely, recovery may be slow.

> **Key Term: mangrove**
> A salt-tolerant shrub or tree that grows in coastal brackish water, especially in tropical and subtropical estuaries.

A common AP trap is to assume wetlands are "wasteland" or unused land waiting for development. In reality, they perform major regulating services. Wetland vegetation slows runoff, reduces flooding, stores carbon, filters water, traps sediment, and protects shorelines from erosion and storm surge. Mangroves do all of this while also supporting coastal food webs and fisheries.

> **Test Tip:** If a question asks why wetland loss matters, do not stop at "loss of habitat." Add at least one ecological service, such as reduced flood control, poorer water filtration, weaker shoreline protection, or loss of fish nursery habitat.

### Direct habitat conversion: draining, filling, and clearing

The most obvious threat is physical removal. Wetlands are often drained or filled so the land can be used for agriculture, housing, roads, ports, or other development. This is **wetland loss** in the clearest sense: the area is converted into something else.

> **Key Term: wetland loss**
> The physical destruction or conversion of a wetland into another land use, such as cropland, housing, or roads.

Draining a wetland usually involves cutting ditches or installing drainage systems that carry water away. This lowers the water table. Once the soil is no longer saturated, the conditions that supported wetland plants disappear. The soil may compact as it dries. In some areas, salinization can follow. Water-loving plants lose their advantage, and upland plants may move in.

Filling is different in method but similar in result. Soil, rock, or other material is dumped into the wetland to raise the land surface. This buries hydric soils and eliminates the shallow water conditions needed by wetland organisms. A filled wetland cannot function as a flood buffer or nursery habitat in the same way because the wetland itself no longer exists.

Mangroves are also directly removed for coastal development and resource use. Shoreline construction, housing expansion, roads, and port activity can clear mangrove forests. In some parts of the world, shrimp farming has been a major driver of mangrove deforestation. When mangroves are removed, the coast loses root systems that held soil in place and reduced wave energy. The result is often more erosion, less flood protection, and a decline in offshore fisheries that depended on mangrove nursery habitat.

A strong AP answer should show the sequence clearly:

development or agriculture -> draining/filling/clearing -> habitat loss -> fewer species and weaker ecological function services

This is more analytical than simply saying "humans damage wetlands."

### Functional degradation: altered hydrology without full removal

Not all damage means complete destruction. A wetland may still be present on a map but work much less effectively than before. That is **wetland degradation**.

> **Key Term: wetland degradation**
> A decline in wetland function caused by changes to hydrology, water chemistry, soils, or biological communities, even when the wetland is not fully removed.

Hydrology is the central issue. Wetlands depend on the timing, depth, duration, and source of water. If humans alter any of these, the system changes.

Dams and levees are classic examples. They can stop nutrient-rich sediments from reaching downstream floodplains and wetlands. This matters because many wetlands depend on regular sediment renewal. Without fresh sediment, soils are not replenished, nutrients decline, and coastal areas may erode more quickly. Fish nurseries and bird breeding areas can decline as habitat structure changes.

Water diversion also harms wetlands. If freshwater that once flowed into a marsh or estuary is redirected for cities, farms, or industry, the wetland may shrink or become saltier. In coastal systems, reduced freshwater inflow can allow saltwater intrusion farther inland. Species adapted to one salinity range may be replaced by others or lost entirely.

Dredging and diking can damage mangroves in a more specific way. Mangrove roots need access to oxygen in difficult, muddy conditions. If dredging changes water depth or diking causes standing water to cover aerial roots for too long, oxygen transport is disrupted. The roots can no longer function normally, and trees may die even though the site was not "cleared" in the usual sense.

Roads and railways can also degrade wetlands by interrupting surface water and groundwater flow. A road embankment may act like a barrier that changes drainage patterns on both sides. One side may flood too much, while the other side dries out. Either way, the original wetland conditions are disrupted.

This is why APES often emphasizes altered hydrology. Water flow is not just background information. It controls wetland structure and function.

> **Test Tip:** If a prompt mentions dams, levees, ditching, drainage, or diversion, your first idea should be altered hydrology. Then explain what changes: water table, salinity, flooding pattern, sediment delivery, or oxygen availability.

### Pollution, runoff, and eutrophication

Wetlands often receive runoff from surrounding land, so they are vulnerable to pollutants carried by water. Much of this is **nonpoint-source pollution**, meaning it comes from many diffuse locations rather than one discharge pipe.

> **Key Term: nonpoint-source pollution**
> Pollution that comes from many diffuse sources, often carried by runoff from farms, roads, lawns, and developed land.

Agricultural runoff may carry nitrates, phosphates, pesticides, and sediment. Urban runoff may carry oil, motor fluids, litter, metals, and other contaminants. Because wetlands sit low in the terrain, they tend to collect these substances.

Nutrient pollution is especially important because it can trigger eutrophication.

> **Key Term: eutrophication**
> The enrichment of water by excess nutrients, especially nitrates and phosphates, causing excessive plant or algal growth and often low oxygen levels.

The process matters more than the definition. Excess nitrates and phosphates enter wetland or estuarine water. Algae and aquatic plants grow rapidly. At first, students often think "more plants means healthier water," but that is the mistake. When the algae die, decomposers break them down and use dissolved oxygen. Oxygen levels drop. That can stress or kill fish and invertebrates, especially in warm or slow-moving water.

In wetlands, high nutrient inputs may also change species composition. Fast-growing or tolerant species can outcompete native plants. Turbidity may rise, light penetration may fall, and submerged vegetation may be lost. So nutrient runoff affects far more than just algae.

Sediment pollution is another threat. Sediment can bury habitats, smother roots, and increase turbidity. In mangroves, root systems and mud surfaces can be especially sensitive to burial or clogging.

Oil pollution is particularly damaging in mangrove systems. Oil coats above-ground roots, reducing oxygen exchange. Since mangroves already live in low-oxygen mud, this extra barrier can be severe. Oil can also contaminate sediments, slow recovery, and disrupt food webs in both land and water portions of the system.

Chemical pollution can include herbicides and other toxic compounds. Some may directly damage vegetation; others may accumulate in sediments or organisms. Even when a wetland continues to exist physically, pollution can reduce its ecological quality and its value as habitat.

### Additional human pressures: extraction, recreation, and invasive species

Wetlands and mangroves also face pressure from overuse and disturbance. Groundwater pumping near wetlands can lower the water table and dry out nearby vegetation. Water withdrawal for human use can shrink wetland area and reduce hydroperiod, the seasonal pattern of flooding or saturation.

Logging in wetland areas decreases biodiversity and can increase flooding by removing vegetation that slowed runoff and stabilized soils. Grazing can compact soil, remove vegetation, and destabilize streambanks. Mining may dump spoils into floodplains or release pollutants into wetland waters.

Recreation sounds harmless, but it can still degrade sensitive habitats. Boating and all-terrain vehicles stir up sediments and disturb breeding areas for fish and wildlife. Noise can alter animal behavior. Repeated disturbance may reduce nesting or nursery success.

Invasive species are another major threat. Because many invasive species are fast-growing generalists with high reproductive rates, they can establish quickly in disturbed wetlands. Once present, they may outcompete native vegetation, alter food webs, and reduce habitat quality for specialist species. Disturbance often makes invasions easier, so habitat degradation and invasive spread often reinforce each other.

### Climate-linked threats: storms, sea-level rise, and coastal squeeze

Coastal wetlands and mangroves face all the local threats already discussed, plus climate-related pressures. Sea-level rise is especially important. Coastal wetlands can sometimes survive rising seas by migrating inland, but only if there is open space upslope.

> **Key Term: coastal squeeze**
> The loss of coastal wetlands when sea level rises but roads, seawalls, or development block inland migration.

If a wetland is trapped between rising water on one side and hard infrastructure or development on the other, it gets squeezed into a smaller area. Over time, it may drown or convert to open water. This is a major AP distinction: sea-level rise alone does not always destroy a coastal wetland immediately. The worst losses happen when migration is blocked.

Mangroves are also affected by stronger storms and storm surges. Their roots and vegetation normally help reduce wave energy, but repeated severe storms can damage forests, erode coastlines, and alter salinity patterns. If human development has already weakened the system, storm damage may become worse.

A useful AP comparison is this:

- **Inland wetlands** are often more threatened by drainage, groundwater withdrawal, grazing, dams, and runoff
- **Coastal wetlands and mangroves** face those same threats plus sea-level rise, storm surge, saltwater intrusion, and coastal squeeze

That comparison shows you understand both overlap and distinction.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Wetlands and mangroves are highly productive habitatss that support biodiversity and major ecological services
- Wetland loss means physical conversion or destruction, while wetland degradation means reduced function without total removal
- Draining wetlands lowers the water table and dries soils that once supported wetland vegetation
- Filling wetlands buries hydric soils and removes wetland habitat entirely
- Dams, levees, diversion, ditching, and dredging alter hydrology and can damage wetlands even when they remain in place
- Mangroves are sensitive to oil spills because oil can coat their roots and reduce oxygen exchange
- Nutrient runoff from farms and cities can cause eutrophication, algal growth, and low dissolved oxygen
- Sediment, chemical, and urban runoff reduce water quality and wetland ecological function
- Groundwater pumping, grazing, logging, mining, recreation, and invasive species can all degrade wetland systems
- Coastal wetlands and mangroves face added threats from sea-level rise, storm surge, and saltwater intrusion
- Coastal squeeze occurs when wetlands cannot migrate inland because development or hard infrastructure blocks them
- Damage to wetlands and mangroves reduces flood control, shoreline protection, nursery habitat, water filtration, and carbon storage

## Key Terms and Concepts

- wetland
- mangrove
- wetland loss
- wetland degradation
- nonpoint-source pollution
- eutrophication
- coastal squeeze


Human Impacts on Wetlands and Mangroves - Threats to wetlands and mangroves