Notes
The tragedy of the commons
When a resource is shared and open to all (fisheries, grazing land, the atmosphere), individuals acting in their own interest can overuse it and ruin it for everyone. Solutions include regulations, permits, quotas and private ownership.
Forestry
- Clear-cutting removes all trees: cheap and fast but causes erosion, habitat loss and more runoff and stream warming. Selective cutting removes some trees; reforestation replants.
- Forests store carbon, so deforestation adds $\mathrm{CO_2}$ to the air.
Agriculture
- Green Revolution: high-yield crops, synthetic fertilizers, pesticides, irrigation and mechanization raised yields but increased fossil fuel use, pollution and loss of genetic diversity.
- Problems: monocultures, soil erosion, nutrient depletion, salinization from irrigation, pesticide resistance.
- Sustainable practices: crop rotation, contour plowing, terracing, no-till farming, windbreaks, integrated pest management (IPM), and cover crops.
- Irrigation types: flood and furrow (cheap but waste water), spray, and drip irrigation (most efficient, over 95%).
- Meat production: CAFOs (feedlots) produce meat cheaply but concentrate waste and use antibiotics; free-range grazing needs more land and can cause overgrazing.
Water use and fishing
- Agriculture uses the most fresh water worldwide. Aquifers can be depleted when water is pumped faster than it recharges; near coasts this causes saltwater intrusion.
- Overfishing and bycatch (catching unwanted species) harm marine ecosystems. Trawling damages the seafloor.
Mining and urban land use
- Surface mining removes the land above ore; it destroys habitat and can cause acid mine drainage. Laws require reclamation.
- Urbanization adds impervious surfaces, increasing runoff and flooding. Urban sprawl spreads development outward.
- Ecological footprint: the land and water needed to supply a person's resources and absorb their waste.
- Sustainable cities use mass transit, green roofs, permeable pavement, and mixed-use zoning.
Practice
Shared resources are overused when no one is responsible for them.
Water goes directly to the roots with little evaporation.
Water evaporates, leaving dissolved salts behind.
IPM uses many strategies to reduce pesticide use.
Salt water moves into the depleted aquifer.
Without roots and cover, soil washes into streams.
Impervious surfaces stop water from soaking in.
Bycatch is unintended capture of other species.
Free response
Write your answer first, then open the scoring guide.
Free-response question
A farmer on a sloped field notices soil washing away and declining crop yields.
(a) Describe two farming practices that would reduce erosion on this field.
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Contour plowing (plowing across the slope so furrows catch water) and terracing (cutting flat steps into the slope). No-till farming or cover crops also work.
Check off the points you earned:
(b) Explain how continued erosion would affect crop yields.
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Erosion removes the nutrient-rich topsoil (A horizon), leaving less fertile soil, so yields drop unless more fertilizer is used.
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(c) Identify one environmental effect of the eroded soil entering a nearby stream.
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Sediment makes the water cloudy (turbid), reducing photosynthesis and harming fish by clogging gills and covering spawning beds.
Check off the points you earned: