AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/5/5-12)
Unit 5 · Topic 5.12
5.12 Introduction to Sustainability
Sustainability means using resources in ways that meet people's needs today without leaving future generations with less. Sustainable yield is the most of a renewable resource you can harvest year after year while the resource stays the same size, and environmental indicators show whether we're on a sustainable path.
Key terms
- sustainability
- sustainable yield
- environmental indicator
- renewable resource
What sustainability means
A sustainable practice can continue indefinitely without degrading the resources or environment it depends on. Sustainable development is development that improves people's lives now while protecting the ability of future generations to meet their needs.
Sustainability is often described as balancing three goals: environmental (healthy ecosystems and resources), economic (people can earn a living) and social (people's needs are met fairly). A fishery that collapses fails all three.
Sustainable yield
Renewable resources like fish, forests and groundwater regenerate, but only at a certain rate. Sustainable yield is the amount you can harvest each year without shrinking the resource, which means taking no more than it regrows or is replenished. If a forest adds 300 m³ of wood a year, harvesting 300 m³ a year or less is sustainable; harvesting more shrinks the forest.
For a population that grows logistically, the number added each year is largest at about half of carrying capacity. Harvesting exactly that maximum amount is called the maximum sustainable yield (MSY). In practice, MSY is risky: populations fluctuate, estimates are uncertain, and harvesting at the maximum leaves no margin for error. Many fishery collapses happened when managers aimed for MSY using overestimated numbers. Setting catches below MSY is safer.
Environmental indicators
Environmental indicators are measurements that show the condition of the environment and whether it's improving or getting worse. Key indicators include:
- Biological diversity: the number of species and the rate of extinctions.
- Food production: how much food is grown per person, and whether soils and water used to grow it are being depleted.
- Average global surface temperature and atmospheric CO₂ concentration: signs of climate change.
- Human population: the number of people and how fast it's growing.
- Resource depletion: how fast resources like groundwater, fish and fossil fuels are being used compared with how fast they're replaced.
Putting it together
Most environmental indicators today point in an unsustainable direction: extinction rates are well above natural background rates, CO₂ and temperatures are rising, and many aquifers and fisheries are being drawn down. Some trends are positive, like the recovery of the ozone layer and falling birth rates in many countries. Sustainability questions on the exam usually ask whether a practice can continue long term and why.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
A simple sustainable harvest
A managed forest holds 10,000 m³ of timber and grows by about 3% per year. What is the sustainable yield? What happens if loggers harvest 500 m³ a year?
Show the solutionHide the solution
- Step 1: Growth per year = 10,000 × 0.03 = 300 m³.
- Step 2: Sustainable yield is the amount that matches growth: 300 m³ per year or less.
- Step 3: Harvesting 500 m³ removes 200 m³ more than the forest adds each year, so the forest shrinks. A smaller forest also grows less wood, so the decline speeds up.
Answer: Sustainable yield is about 300 m³/yr; harvesting 500 m³/yr would steadily deplete the forest.
- Example 2
Finding maximum sustainable yield
Scientists estimate how many fish a population adds per year at different population sizes (carrying capacity = 10,000): 2,000 fish → 640 added; 4,000 → 960; 5,000 → 1,000; 6,000 → 960; 8,000 → 640. What is the maximum sustainable yield, and at what population size? Why would managers set the catch lower?
Show the solutionHide the solution
- Step 1: The most fish added per year is 1,000, at a population of 5,000.
- Step 2: That's half of the carrying capacity, as expected for logistic growth.
- Step 3: Maximum sustainable yield = 1,000 fish per year, harvested from a population kept near 5,000.
- Step 4: Managers set catches lower because the estimates are uncertain and bad years happen. If the population dips below 5,000 while 1,000 fish are still caught, it adds fewer than 1,000, so it shrinks further and can collapse.
Answer: MSY is 1,000 fish per year at a population of 5,000 (half of K). A lower catch gives a safety margin against uncertainty and bad years.
Common mistakes
- Thinking any harvest of a renewable resource is sustainable. It's sustainable only if the harvest doesn't exceed regrowth.
- Putting maximum sustainable yield at carrying capacity. Growth is near zero at K; MSY occurs around half of K.
- Listing vague indicators. Use specific ones like atmospheric CO₂ concentration or species extinction rates.
On the exam
- Be ready to define sustainable yield and decide whether a harvest is sustainable using growth data.
- Questions may ask you to name and explain environmental indicators of sustainability.
Connected topics
Videos
Check yourself
4 questions on 5.12 Introduction to Sustainability. Pick an answer to see if you got it, and why.
A managed forest grows by about 5,000 cubic meters of wood each year. Which yearly harvest is closest to the forest's sustainable yield?
Which trend would most clearly indicate that a region's use of its forests is sustainable?
| Year | Fish stock at start of year (thousand metric tons) | Growth of the stock during the year (thousand metric tons) | Catch (thousand metric tons) |
|---|---|---|---|
| 1 | 800 | 60 | 50 |
| 2 | 810 | 60 | 70 |
| 3 | 800 | 58 | 90 |
| 4 | 768 | 55 | 110 |
| 5 | 713 | 50 | 120 |
Hypothetical data. Each year's starting stock equals the previous year's stock plus its growth, minus the catch.
Fishery managers want the stock to grow from its Year 5 size of 713 back toward 800 thousand metric tons. Based on the table, which plan would do this?
If the pattern in the table continues, what will the fish stock be at the start of Year 6?
0 of 4 answered