AP Environmental Science Unit 5 ReviewLand & Water Use

AP Environmental Science Unit 5, Land and Water Use, is about how humans take resources from the land and water (farming, logging, mining, fishing, building cities) and what those choices do to ecosystems. The single biggest idea is the tragedy of the commons, the pattern where individuals overuse shared resources for personal gain until the resource collapses, and almost every topic in the unit is either an example of that tragedy or a sustainable fix for it. Unit 5 makes up 10-15% of the AP exam, which makes it one of the heavier units in the course.

What this unit covers

The core problem: shared resources get overused

• The tragedy of the commons explains why open-access resources like fisheries, groundwater, and public grazing land get depleted. Each user gains the full benefit of taking more while the cost is split among everyone, so the rational individual choice wrecks the collective resource.
• Overfishing is the classic real-world case. Some fish species have become extremely scarce, which reduces aquatic biodiversity and hurts communities that depend on fishing for food and income.
• Clearcutting trades short-term economic gain for soil erosion, flooding, and warmer soil and stream temperatures. Cutting and burning trees also releases stored carbon dioxide, so deforestation feeds climate change.
• Mining follows the same logic. As high-grade ores run out, operations chase lower-grade ores, which takes more energy and produces more waste. Surface mining (like strip mining) removes the overburden, the soil and rock above the ore, leaving land bare and erosion-prone. Leftover slag and tailings can contaminate water, and coal mining destroys habitat and releases dust and methane.

Feeding the world: the Green Revolution and its trade-offs

• The Green Revolution boosted food production through mechanization, GMOs, synthetic fertilizers, irrigation, and pesticides. More food, yes, but also more fossil fuel dependence, water depletion, and chemical pollution.
• Damaging practices to know by name are tilling (exposes soil to erosion), slash-and-burn farming (clears land fast but destroys soil fertility and releases CO2), and heavy fertilizer use (runoff drives eutrophication downstream).
• Irrigation is the single largest human use of freshwater, about 70%. Furrow irrigation is cheap but loses roughly one third of its water to evaporation and runoff. Flood irrigation loses water too and can waterlog soil. Spray irrigation is more efficient but costs more. Drip irrigation is the most efficient and the most expensive.
• Two irrigation side effects matter on the exam. Waterlogging happens when too much water sits in the soil and raises the water table, suffocating plant roots. Salinization happens when irrigation water evaporates and leaves salts behind in the soil.
• Pesticides work until they don't. Spraying kills susceptible pests, the resistant few survive and reproduce, and the population evolves resistance through artificial selection. This drives the pesticide treadmill, where farmers need new or stronger chemicals over time. GMO crops can resist pests but reduce genetic diversity.
• Meat production is energetically inefficient. It takes roughly 20 times more land to produce the same calories from meat as from plants (think back to the 10% rule between trophic levels). CAFOs (feedlots) fatten livestock quickly and cheaply but crowd animals, feed them grain instead of grass, and generate huge amounts of waste that can contaminate ground and surface water. Free-range grazing avoids those problems but uses far more land and can cause overgrazing.

Cities: urbanization and its footprint

• Urbanization depletes local resources and can cause saltwater intrusion, where overpumped coastal aquifers fill with seawater.
• Cities run on fossil fuels and produce landfill waste, both of which pump CO2 into the atmosphere and alter the carbon cycle.
• Impervious surfaces (roads, parking lots, buildings, sidewalks) block water from soaking into the soil, so rain becomes runoff and flooding instead of groundwater recharge.
• Urban sprawl spreads low-density development outward from city centers, eating farmland and habitat and locking in car dependence.
• An ecological footprint measures the resource demand and waste production of a person or society, which lets you compare how sustainably different populations live.

The fixes: sustainable land and water practices

• Sustainability means using resources without depleting them for future generations. Sustainable yield is the amount of a renewable resource you can harvest without shrinking the supply.
• Urban runoff solutions all aim to get water back into the ground. Permeable pavement, planting trees, building up instead of out, and public transportation (less pavement needed) all increase infiltration.
• Integrated pest management (IPM) combines biological controls (natural predators, biocontrol), physical methods, crop rotation, and intercropping with only limited chemical use. It protects wildlife, water, and human health, but it is complex and expensive to run.
• Soil conservation methods to know by name are contour plowing, terracing, windbreaks, perennial crops, no-till agriculture, and strip cropping. Crop rotation, green manure, and limestone improve soil fertility, and rotational grazing prevents overgrazing.
• Aquaculture (fish farming) is efficient, needs little space and fuel, and takes pressure off wild stocks. But escaped fish can compete or interbreed with wild populations, wastewater carries contamination, and crowded pens breed disease that can spread to wild fish.
• Sustainable forestry includes reforestation, buying sustainably harvested wood, reusing wood, IPM for forest pests, and prescribed burns, which are controlled fires that clear out fuel so catastrophic natural fires are less likely.

Unit 5, Land & Water Use at a glance

Why Unit 5, Land & Water Use matters in APES

Unit 5 is where APES stops describing how natural systems work and starts analyzing how humans break them and how we could stop. It carries two of the course's big ideas at once, that Earth is one interconnected system and that humans alter natural systems, then introduces the third, that sustainable practices can reduce those impacts.

• Almost every FRQ scenario in this course involves a land or water use decision, so the trade-off thinking you build here (cheap furrow irrigation vs. efficient drip, CAFO vs. free-range) is the core skill of the whole class.
• The tragedy of the commons is a lens you can apply to nearly any environmental problem, from groundwater pumping to atmospheric CO2.
• This is the first unit organized around solutions, not just problems. IPM, soil conservation, and sustainable forestry are the templates for the mitigation strategies you will propose on exam day.

How this unit connects across the course

• The 10% rule of energy transfer between trophic levels (Unit 1) is exactly why meat production takes about 20 times more land than growing plants for the same calories.
• Overfishing, clearcutting, and GMO monocultures all reduce biodiversity and genetic diversity, paying off the habitat loss and population resilience ideas from Unit 2.
• Soil properties, the hydrologic cycle, and groundwater from Unit 4 explain the mechanisms here, like why tilling causes erosion and why overpumping coastal aquifers causes saltwater intrusion.
• Mechanized farming, mining, and urban sprawl all run on fossil fuels, setting up energy resources (Unit 6) and the air pollution that comes from burning them (Unit 7).
• Fertilizer runoff, CAFO waste, and mining tailings become the eutrophication and water pollution stories of Unit 8, and deforestation's CO2 release feeds directly into global climate change (Unit 9).

Key equations and processes

• Percent change, $\frac{\text{new} - \text{old}}{\text{old}} \times 100$, shows up constantly in math FRQs about crop yields, fish catch declines, or water savings from switching irrigation methods.
• Energy efficiency of food production. Use the 10% trophic transfer rule to explain why plant calories require far less land than meat calories (about a 20x land difference).
• Irrigation water-loss reasoning. Know that furrow loses about 1/3 of water to evaporation and runoff, and be ready to calculate water saved by switching to drip in a quantitative problem.
• Sustainable yield. The harvest rate that keeps a renewable resource's supply constant; harvesting above it depletes the stock.
• Pesticide resistance via artificial selection. Resistant individuals survive spraying, reproduce, and shift the population, which is why repeated pesticide use loses effectiveness over time.
• Salinization and waterlogging. Evaporating irrigation water concentrates salts in topsoil; standing water raises the water table and blocks root oxygen uptake. Be able to explain both mechanisms step by step.

Unit 5, Land & Water Use on the AP exam

Unit 5 is 10-15% of the exam, near the top of the weighting range. On the multiple-choice section, expect scenario-based questions where you identify the best practice for a goal (which irrigation method conserves the most water, which soil conservation technique fits a hilly farm) and data questions reading graphs of fish catch, crop yield, or runoff. This unit is also a favorite for the free-response section, especially the "propose a solution" style FRQ. A typical prompt describes an environmental problem like urban flooding or pest outbreaks, then asks you to describe a solution, explain how it works, and identify a drawback. That describe-the-mechanism-and-the-trade-off move is exactly what IPM, drip irrigation, aquaculture, and prescribed burns are built for. Practice writing answers that name the practice, explain the cause-and-effect chain, and acknowledge the cost, because "drip irrigation saves water" earns less than "drip irrigation delivers water directly to roots, reducing evaporation loss, though it is expensive to install."

Essential questions

• Why do rational individuals destroy shared resources, and what rules or incentives can prevent it?
• Was the Green Revolution worth its environmental costs, and how do we judge that trade-off?
• How can we produce enough food, water, and materials for a growing population without depleting them for future generations?
• How do cities change the way water, carbon, and resources move through the environment?

Key terms to know

• Tragedy of the commons: the depletion of a shared resource because individuals use it in their own self-interest rather than for the common good.
• Clearcutting: removing all trees from an area at once, which causes erosion, flooding, warmer streams, and CO2 release.
• Green Revolution: the shift to mechanization, GMOs, fertilizers, irrigation, and pesticides that raised food production with mixed environmental results.
• Waterlogging: water saturating soil and raising the water table so plant roots cannot absorb oxygen.
• Salinization: salt buildup in soil left behind when irrigation water evaporates.
• Pesticide resistance: pests evolving tolerance to chemicals through artificial selection, making pesticides less effective over time.
• CAFO (concentrated animal feeding operation): a feedlot that fattens crowded livestock quickly while generating waste that can contaminate water.
• Overburden: the soil and rock removed in surface mining to reach the ore underneath.
• Tailings and slag: mining wastes left over after minerals are extracted from ore.
• Impervious surface: pavement, roofs, and other structures that block water infiltration and increase runoff and flooding.
• Urban sprawl: low-density development spreading outward from city centers, consuming farmland and habitat.
• Ecological footprint: a measure comparing the resource demands and waste production of an individual or society.
• Sustainable yield: the amount of a renewable resource you can harvest without reducing the supply.
• Integrated pest management (IPM): combining biological, physical, and limited chemical methods to control pests with minimal environmental disruption.
• Prescribed burn: a controlled fire set intentionally to reduce fuel and prevent severe natural wildfires.

Common mix-ups

• Waterlogging vs. salinization. Both come from irrigation, but waterlogging is too much water suffocating roots, while salinization is salt accumulating as water evaporates. FRQs reward you for naming the right one.
• Drip vs. furrow irrigation efficiency. Drip is the most efficient and most expensive; furrow is the cheapest but loses about a third of its water. Don't reverse the trade-off.
• Aquaculture is a solution with drawbacks, not a pure fix. It relieves wild fisheries but spreads disease, escapes can interbreed with wild fish, and wastewater gets contaminated.
• Crop rotation vs. rotational grazing. Crop rotation alternates plant species on the same field to protect soil fertility; rotational grazing moves livestock between pastures to prevent overgrazing. Same word, different practice.