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Unit 5
10–15% of examThis unit is about how people use land and water to get food, wood, fish and minerals, and the damage that can cause. You'll also learn practical fixes, like integrated pest management, drip irrigation, sustainable forestry and permeable pavement, that let people meet their needs without using resources up.
Longer videos that cover the whole unit. Good for a first pass or a final review.
The tragedy of the commons happens when a shared resource, like a pasture, a fishery or clean air, gets overused because each person acts in their own interest and no one owns or manages it. It helps explain problems like overfishing, overgrazing and pollution, and why rules or ownership can help.
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Clearcutting means cutting down all the trees in an area at once. It's cheap and efficient for logging, but it causes soil erosion, warmer soil and streams, flooding and habitat loss, and it releases the carbon that trees had been storing.
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The Green Revolution, starting in the mid-1900s, boosted food production with high-yield crop varieties, synthetic fertilizers, pesticides, irrigation and farm machinery. It fed far more people, but it also increased fossil fuel use and pollution and reduced the genetic variety of crops.
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Some common farming practices damage the environment: tilling loosens soil so it erodes more easily, slash-and-burn clears forests and releases carbon, and fertilizer that isn't used by crops runs off into rivers and lakes. Synthetic fertilizers give plants nutrients quickly, while organic fertilizers like manure release them more slowly.
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About 70% of the fresh water people use goes to farming, so how crops are watered matters. Furrow and flood irrigation are cheap but lose a lot of water, spray irrigation wastes less but costs more and needs energy, and drip irrigation is the most efficient but the most expensive. Too much water can waterlog soil, and when irrigation water evaporates it can leave salts behind (salinization), which can leave soil too salty for crops.
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Pesticides, herbicides and similar chemicals kill pests but can also harm other living things, and pests can evolve resistance, so stronger doses or new chemicals are needed over time. Crops can also be genetically engineered to resist pests, but planting the same crop everywhere lowers genetic diversity.
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Meat comes from feedlots and CAFOs (concentrated animal feeding operations), where many animals are packed together and fed grain and soy, and from pasture-based systems like free-range and rotational grazing, where herds are moved between pastures so plants can regrow. Per gram of protein, meat needs far more land, water and energy than plant foods and adds nutrient pollution and methane. CAFOs keep prices low but pile up manure that can pollute streams, and routine antibiotics there help bacteria become resistant; grazing needs more land, and too many animals on a pasture (overgrazing) strips plants, erodes and packs down soil and can turn dry land into desert (desertification).
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Overfishing means catching fish faster than they can reproduce. It has pushed some species to very low numbers, cuts ocean biodiversity and hurts people who rely on fish for food and income, and some methods also catch lots of unwanted animals (bycatch) or damage the seafloor.
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As the easy-to-reach ores run out, mining moves to lower-grade ores, which takes more energy and creates more waste. Surface mining strips away the overburden (soil and rock above the ore), subsurface mining is more dangerous and expensive, and both leave waste like tailings and slag that can pollute water.
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Urbanization, the growth of cities, changes the land and water around them. Impervious surfaces like roads and parking lots stop rain from soaking in, so more water runs off and floods, urban sprawl spreads low-density suburbs into farmland and habitat, and pumping too much groundwater near coasts can let salt water seep in.
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An ecological footprint estimates how much land and water it takes to provide everything a person or country uses and to absorb its waste. Footprints are usually much larger in wealthy countries, and a carbon footprint, which measures greenhouse gas emissions, is one piece of the bigger picture.
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Sustainability means using resources in a way that doesn't use them up for future generations. Sustainable yield is the most of a renewable resource, like fish or timber, you can harvest year after year while the supply stays the same size, and environmental indicators like biodiversity, food production, global temperature and CO₂ levels help show whether we're on track.
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Cities can cut runoff by helping rain soak into the ground instead of rushing into storm drains. Options include permeable pavement that lets water through, more trees and green space, building taller instead of spreading out, and better public transit so less land has to be paved for roads and parking.
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Integrated pest management (IPM) combines several methods, like natural predators (biocontrol), crop rotation and intercropping, and uses chemicals only when needed. It lowers the risks pesticides pose to wildlife, water and people, but it can take more time, knowledge and money to run.
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Sustainable farming keeps soil in place and fertile. Erosion is cut by plowing along the contours of a slope, strip cropping (alternating strips of different crops), planting rows of trees as windbreaks, terracing hillsides and skipping tilling altogether, while crop rotation, green manure (plants grown to be plowed back into the soil) and limestone for acidic soil help keep it fertile.
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Aquaculture is raising fish and other seafood in controlled areas of water. It's efficient and needs little space and fuel, but crowded fish farms can pollute water with waste, spread disease to wild fish, and let farmed fish escape and compete with wild ones.
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Sustainable forestry manages forests so they keep providing wood, habitat and carbon storage. Approaches include replanting (reforestation), buying wood from sustainably managed forests, reusing wood, fighting pests with IPM or by removing infected trees, and using prescribed burns to clear built-up fuel and lower the risk of huge wildfires.
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