AP Environmental Science

1. Energy flow and nutrient cycles in ecosystems

Learning goal: Trace energy through trophic levels and follow carbon, nitrogen and phosphorus through the environment.

Unit 1 starts with how ecosystems work. Producers use sunlight in photosynthesis to build sugars; consumers eat other organisms; decomposers break down dead matter and return nutrients. Energy flows one way through an ecosystem and is lost as heat at each step, while matter cycles. Net primary productivity is the energy captured by producers minus the energy they use in respiration, and it is what remains to support consumers.

Only a fraction of energy passes from one trophic level to the next. A common teaching approximation is that about 10% moves up each step, although real efficiencies vary. This is why food chains are short and why eating lower on the food chain feeds more people from the same land. In a pyramid of energy each level is smaller than the one below it. Biomass and numbers pyramids can sometimes be inverted, but an energy pyramid cannot.

The carbon cycle moves carbon between the atmosphere, living things, soil, oceans and rocks. Photosynthesis removes carbon dioxide from the air, respiration and decomposition return it, and burning fossil fuels moves carbon from long-term storage to the atmosphere quickly. Carbon stored in forests, soils and the ocean is a sink, and a source is a process that releases carbon. Deforestation can turn a sink into a source.

The nitrogen cycle matters because plants need usable nitrogen but most of the air is nitrogen gas that they cannot use. Nitrogen fixation by bacteria and lightning converts it to ammonia, nitrification turns ammonia into nitrate, plants take up nitrate, and denitrification returns nitrogen gas to the air. Fertiliser adds nitrogen and can run off into water. Phosphorus has no significant gas phase, moves slowly through rock and water, and often limits growth in fresh water. Examples in this lesson use invented numbers.

Worked example

Producers fix 20,000 units of energy. Using the 10% model, how much reaches secondary consumers?

1. Primary consumers get 10% of 20,000, which is 2,000.
2. Secondary consumers get 10% of 2,000.
3. That is 200 units.
4. The rest was lost as heat or used in life processes.

Mental model: Energy flows and is lost as heat. Matter cycles.

Common trap: Treating energy as recycled like nutrients.

2. Biodiversity, niches and ecosystem services

Learning goal: Explain biodiversity, keystone species, succession and threats to species.

Unit 2 covers the variety of life. Biodiversity has three levels: genetic diversity within a species, species diversity in an area and ecosystem diversity across a region. Genetic diversity helps populations adapt to disease and change. Ecosystem services are benefits people get from nature, such as pollination, water purification, flood control and recreation. Many of these services have no market price, so they are easy to undervalue.

A niche is the role and conditions a species uses in its environment, including what it eats, where it lives and when it is active. Two species cannot share exactly the same niche for long in the same place, because competition favours one. A keystone species has an effect on its ecosystem far larger than its numbers suggest, and removing it changes the community. An indicator species shows the health of the environment by its presence or absence. Invasive species can outcompete natives because they lack natural predators in the new place.

Ecological succession is the change in a community over time. Primary succession starts on bare rock with no soil, and pioneer species such as lichens build soil first. Secondary succession starts where soil remains after a disturbance, such as a fire or abandoned farm, and proceeds faster. Early successional species grow and spread quickly; later ones are slower growing and shade the earlier ones out.

The main threats to biodiversity can be remembered as habitat loss, invasive species, overexploitation, pollution and climate change. Habitat fragmentation splits habitat into small patches and cuts gene flow. Conservation tools include protected areas, wildlife corridors, captive breeding and laws that restrict trade in endangered species. On an exam, name the threat, say how it reduces population size or diversity, and give one matched solution. Examples here use invented scenarios.

Worked example

A burned forest has soil intact. What kind of succession follows and how does the rate compare?

1. Soil is still present after the fire.
2. That makes it secondary succession.
3. Seeds and roots in the soil speed regrowth.
4. It is faster than primary succession on bare rock.

Mental model: Niches, keystone effects, succession and threats.

Common trap: Assuming more species always means more ecosystem stability.

3. Populations: growth, limits and demography

Learning goal: Distinguish exponential and logistic growth, and read age structure diagrams.

Unit 3 carries 10% to 15% of the multiple-choice section. Population size changes with births, deaths, immigration and emigration. Exponential growth happens when resources are unlimited and appears as a J-shaped curve. Logistic growth levels off at the carrying capacity, K, the maximum population an environment can support, and appears as an S-shaped curve. Limiting factors that depend on density, such as disease and competition, act more strongly when the population is crowded.

The rule of 70 estimates doubling time: divide 70 by the percentage growth rate. At 2% growth the doubling time is about 35 years. Growth rate is calculated as births minus deaths over the population, expressed as a percentage, and net migration is added for a region. These formulas are simple models, and real populations often overshoot carrying capacity and then decline.

Species fall along a range of life history strategies. R-selected species have many offspring, little parental care and short lives, and tend to thrive in unstable environments. K-selected species have few offspring, much parental care and long lives, and tend to live near carrying capacity. Many pest and weed species are r-selected, while large mammals tend to be K-selected, though the categories are two ends of a spectrum.

Survivorship curves show the pattern of deaths by age: type I shows most individuals living to old age, type II shows constant deaths, and type III shows most deaths when young.

Human demography uses age structure diagrams. A wide base means many young people and rapid future growth, a straight-sided shape means stable growth, and a narrow base means decline. Total fertility rate is the average number of children per woman, and replacement level is about 2.1. The demographic transition describes a shift from high births and deaths to low births and deaths as a country develops. The numbers here are invented for practice.

Worked example

A population grows at 2% per year (invented). Estimate its doubling time.

1. Use the rule of 70.
2. Doubling time ≈ 70 / 2.
3. That is 35.
4. About 35 years, assuming the rate stays constant.

Mental model: J curve without limits, S curve with limits.

Common trap: Treating carrying capacity as a fixed number that never changes.

4. Earth systems, soils and the atmosphere

Learning goal: Explain plate tectonics, soil properties, weather and the layers of the atmosphere.

Unit 4 covers 10% to 15% of the multiple-choice section. Earth's crust is divided into tectonic plates that move slowly. Earthquakes and volcanoes cluster at plate boundaries, where plates diverge, converge or slide past each other. Over long periods this recycles rock and creates mountains and ocean basins. Rocks form and change through the rock cycle: igneous, sedimentary and metamorphic rocks can become each other.

Soil forms slowly from weathered rock and organic matter. Its layers, or horizons, run from the surface organic layer down to the parent material. Soil texture comes from sand, silt and clay: sand drains fast but holds few nutrients, clay holds water and nutrients but drains slowly, and loam mixes them. Good soil has organic matter, which improves structure and water holding. Erosion removes topsoil faster than it forms, so cover crops and terracing protect it.

The atmosphere has layers. The troposphere is the lowest and holds most weather. The stratosphere above it contains the ozone layer, which absorbs much of the Sun's ultraviolet radiation. The atmosphere is mostly nitrogen and oxygen, with small amounts of argon, carbon dioxide and water vapour. Heating is uneven: more solar energy reaches the equator than the poles, which drives winds and ocean currents. The Coriolis effect, from Earth's rotation, bends moving air to the right in the northern hemisphere.

Weather is short-term atmospheric conditions and climate is the long-term pattern. Warm air rises, creating low pressure, and cool air sinks, creating high pressure. A rain shadow forms on the downwind side of mountains where air has lost its moisture. El Niño and La Niña are shifts in Pacific Ocean temperature patterns that alter weather in many regions. Seasons come from Earth's axial tilt, not from distance from the Sun. Examples here are invented for practice.

Worked example

Describe why the leeward side of a mountain range is often dry.

1. Moist air is forced upward on the windward side.
2. It cools and its water vapour condenses as rain.
3. The air that crosses the top is drier.
4. It sinks and warms on the leeward side, so little rain falls there.

Mental model: Plates, soils and uneven heating shape environments.

Common trap: Blaming Earth's distance from the Sun for seasons.

5. Land and water use

Learning goal: Evaluate agriculture, forestry, mining, urbanisation and fishing, and their trade-offs.

Unit 5 is about how people use land and water. The tragedy of the commons describes how shared resources are overused when each user gains personally and the cost is shared by all. Solutions include regulation, quotas, privatisation and community management. Agriculture uses most freshwater and a large share of land. The Green Revolution raised yields through high-yield crops, fertiliser, pesticides and irrigation, but with costs such as water use and pollution.

Irrigation methods differ in efficiency. Flood irrigation loses much water to evaporation and runoff, while drip irrigation delivers water at the roots and wastes less. Over-irrigation can cause salinisation, as salts left by evaporated water build up in soil and reduce yields. Monoculture is efficient but vulnerable to pests and disease. Practices like crop rotation, no-till farming, contour plowing and integrated pest management reduce erosion and chemical use.

Forests supply timber and habitat and store carbon. Clear-cutting is cheap but causes erosion and habitat loss, while selective cutting leaves more cover. Sustainable forestry harvests no faster than regrowth. Mining removes minerals and can leave acid drainage and large waste piles. Surface mining disturbs more land than underground mining, though underground mining is more hazardous for workers. Urban sprawl converts farmland and habitat, and impervious surfaces increase runoff.

Fisheries can be overharvested when catch exceeds reproduction. Bycatch is the unintended capture of other species. Aquaculture, or fish farming, reduces pressure on wild stocks but can pollute water and spread disease. Sustainable yield means taking only what the population can replace. When answering exam questions on trade-offs, name a benefit, a cost and a way to reduce the cost, and tie the cost to a specific mechanism. Examples here use invented scenarios.

Worked example

A farm switches from flood to drip irrigation. Give one benefit and one possible cost.

1. Benefit: less water lost to evaporation and runoff.
2. Benefit: less salt buildup in some conditions.
3. Cost: higher installation cost.
4. Cost: maintenance of the lines.

Mental model: Every use has a benefit, a cost and a mitigation.

Common trap: Naming a benefit without naming the cost.

6. Energy resources and consumption

Learning goal: Compare fossil fuels, nuclear, renewable and biomass energy and the trade-offs in each.

Unit 6 covers energy sources. Fossil fuels, coal, oil and natural gas, formed from ancient organisms and supply most of the world's energy today. They are nonrenewable on human time scales and release carbon dioxide when burned. Coal produces more carbon dioxide per unit of energy than natural gas and releases sulfur and mercury. Natural gas burns cleaner but leaks methane, a powerful greenhouse gas.

Nuclear power uses fission to heat water for steam, producing electricity without direct carbon dioxide from the reaction. It creates radioactive waste that needs long-term storage and has accident risks, though accidents are rare. Renewable sources include solar, wind, hydroelectric, geothermal and biomass. Solar and wind are intermittent, so they need storage or backup. Hydroelectric dams provide steady power but change river ecosystems and displace people.

Biomass includes wood, crop residue and biofuels such as ethanol. It can be near carbon neutral if the plants regrow, but growing fuel crops competes with food and uses land and water. Geothermal uses heat from the Earth and is steady where available. Energy efficiency, getting the same service with less energy, is often the cheapest way to cut demand. Examples are insulation, efficient appliances and cogeneration, which captures waste heat.

When comparing sources, use the same criteria for each: cost, reliability, emissions, land and water use and waste. Net energy ratio compares energy gained to energy used to obtain it. Conservation changes behaviour, while efficiency changes technology. An exam answer should give an advantage, a disadvantage and a reason for each source you discuss. The numbers in this lesson are invented for practice, and real costs vary by place and year.

Worked example

List one advantage and one disadvantage of wind power.

1. Advantage: no air pollution while operating.
2. Advantage: renewable and low emissions over its lifetime.
3. Disadvantage: output varies with wind.
4. Disadvantage: can harm birds and bats and affect landscapes.

Mental model: Compare sources on the same criteria.

Common trap: Assuming renewable means free of environmental cost.

7. Air, water and land pollution

Learning goal: Identify major pollutants, their sources and effects, and the controls that reduce them.

Units 7 and 8 cover pollution. Primary air pollutants come directly from sources: carbon monoxide, sulfur dioxide, nitrogen oxides, particulates and volatile organic compounds. Secondary pollutants form in the air, as ground-level ozone does when nitrogen oxides and volatile organic compounds react in sunlight. Sulfur dioxide and nitrogen oxides form acid deposition, which harms lakes, forests and buildings. Controls include scrubbers, catalytic converters and cleaner fuels.

Ozone in the stratosphere is helpful, but ground-level ozone harms lungs. Chlorofluorocarbons broke down stratospheric ozone, and an international agreement phased them out. Indoor air can be polluted by radon, mould and combustion gases. A temperature inversion traps cool air and its pollutants near the ground, worsening smog.

Water pollutants include nutrients, pathogens, sediment, heavy metals, oil and heat. Nutrients from fertiliser and sewage cause eutrophication: algae bloom, die and decompose, and the decomposers use up oxygen, leading to dead zones. Dissolved oxygen is a key indicator of water health, and biochemical oxygen demand measures how much oxygen decomposers need. Point sources come from a single location, such as a pipe, while non-point sources are spread out, such as farm runoff, and are harder to control.

Solid waste goes to landfills, incinerators, recycling and composting. Modern landfills have liners and gas collection, and leachate is the liquid that drains through waste. Hazardous waste needs special handling. Bioaccumulation is the buildup of a substance in an organism, and biomagnification is the rising concentration at higher trophic levels, as with some pesticides and mercury. LD50 is the dose that kills half of a test population. Pollution prevention is generally cheaper than cleanup. Examples use invented scenarios.

Worked example

Explain how fertiliser runoff can kill fish.

1. Runoff adds nitrogen and phosphorus to the water.
2. Algae grow rapidly in a bloom.
3. The algae die and decomposers consume oxygen.
4. Low dissolved oxygen kills fish.

Mental model: Know the source, the pathway and the control.

Common trap: Confusing stratospheric ozone with ground-level ozone.

8. Global change: climate, ozone and sea level

Learning goal: Explain the greenhouse effect, evidence of climate change, impacts and responses.

Unit 9 carries 15% to 20% of the multiple-choice section, the largest unit. The greenhouse effect is natural: gases such as water vapour, carbon dioxide and methane absorb outgoing heat and warm the planet. Human activity adds carbon dioxide mainly from burning fossil fuels and deforestation, and adds methane from livestock, rice paddies, landfills and gas leaks. Nitrous oxide comes mainly from fertilised soils. Gases differ in how strongly they warm and how long they last.

Evidence for climate change comes from many sources: thermometer records, ocean heat content, shrinking ice sheets and glaciers, sea level rise and the timing of seasonal events. Ice cores and tree rings give information about the past. The IPCC assesses the scientific literature, and its reports are public. Check the current IPCC and national agency pages for up-to-date figures; this lesson gives no numerical values because they change as new data arrive.

Impacts include sea level rise from thermal expansion of water and melting land ice, shifts in species ranges, more intense heat waves and changes in rainfall. Ocean acidification occurs when oceans absorb carbon dioxide, lowering pH and making it harder for shell-forming organisms to build shells. Melting ice reduces albedo, the reflectivity of the surface, which adds warming in a positive feedback. Not every feedback amplifies change: negative feedbacks reduce it.

Responses combine mitigation, which reduces emissions or removes carbon, and adaptation, which reduces harm from changes already happening. Mitigation includes renewable energy, efficiency, protecting forests, carbon pricing and capturing carbon. Adaptation includes sea walls, drought-resistant crops and early warning systems. Policies differ in cost and fairness, and countries differ in responsibility and capacity. Ozone depletion is a separate issue from climate change, handled by the Montreal Protocol, though some ozone-depleting gases also warm the planet. Examples here use invented scenarios.

Worked example

Explain how melting sea ice can speed up warming.

1. Ice is bright and reflects sunlight.
2. When it melts, dark water is exposed.
3. Dark water absorbs more energy.
4. More warming melts more ice, a positive feedback.

Mental model: Causes, evidence, impacts and responses.

Common trap: Treating ozone depletion and climate change as the same problem.

9. Sustainability, footprints and decision trade-offs

Learning goal: Use ecological footprints, cost-benefit thinking and environmental laws to compare options.

Sustainability means meeting present needs without reducing the ability of future generations to meet theirs. An ecological footprint estimates the land and water area needed to supply a person's or group's resources and absorb their waste. Footprints are larger where consumption is higher, and a footprint can be compared with the area of biologically productive land available. The measure is a model with assumptions, so treat any single figure as an estimate and check the source.

The IPAT idea says environmental impact is the product of population, affluence and technology. Impact rises with more people, more consumption per person and more damaging technology, and falls with cleaner technology. This is why policies can target any of the three. A resource can be renewable, but still be depleted if it is used faster than it renews, as with forests or fish. Nonrenewable resources are fixed on human time scales, and recycling extends their useful life.

Decisions about the environment involve trade-offs. A cost-benefit analysis compares the monetary costs and benefits of an option, which is hard when benefits such as clean air have no market price. Externalities, costs imposed on others, such as pollution, are often left out of prices. Policy tools include regulation, taxes, tradable permits, subsidies and voluntary standards. Each has strengths: regulation is certain, while taxes and permits can reach a goal at lower cost.

Environmental laws set rules for air, water, waste and species. Many countries have laws that require assessment of a project's environmental effects before it goes ahead, and laws that protect endangered species and limit toxic releases. The specific laws differ by country and change over time, so check the current text for the country you study. On an exam, answer trade-off questions with a clear choice, a reason tied to evidence and one acknowledged cost. Examples here are invented for practice.

Worked example

Population is unchanged but consumption per person doubles while technology is unchanged. What happens to impact in the IPAT model?

1. Impact is population times affluence times technology.
2. Affluence doubles and the others stay the same.
3. The product doubles.
4. Impact doubles unless technology becomes cleaner.

Mental model: Impact comes from people, consumption and technology.

Common trap: Treating one footprint number as exact.

10. Exam day: data, graphs and free response

Learning goal: Use the exam format and practice weightings to answer data and design questions.

The College Board's AP Environmental Science exam page lists two sections. Section I has 80 multiple-choice questions in 1 hour 30 minutes, worth 60% of the exam score. Section II has three free-response questions in 1 hour 10 minutes, worth 40%. The questions are: design an investigation, analyse and interpret quantitative data, and analyse an environmental problem and propose a solution with calculations. Check the exam page for your year, since format and policies can change.

The course page names the science practices and their weighting. Concept explanation carries 30% to 38% of the multiple-choice section and 13% to 20% of the free response; visual representations 12% to 19% and 6% to 10%; text analysis 6% to 8% in the multiple-choice section only; and scientific experiments 2% to 4% and 10% to 14%. That means explaining concepts and reading graphs carry most of the points.

For data questions, read the title, axes and units before the numbers, then describe the trend with a comparison, such as 'increases from X to Y'. For calculations, show the formula, substitute values, include units and round only at the end. Common calculations include percent change, rates, dimensional analysis and converting between units. The numbers in this course are invented for practice.

For design questions, state a testable question, name the independent and dependent variables, say what you hold constant, include a control group and describe repeated trials. For solution questions, name a specific action, say how it reduces the problem and mention one trade-off. Avoid vague answers such as 'reduce pollution'. Use the reading time to underline command words such as 'identify', 'describe', 'explain' and 'justify', and answer each exactly.

Worked example

A lake's phosphorus rose from 20 to 30 units (invented). Find the percent change.

1. Change = 30 − 20 = 10.
2. Divide by the original: 10 / 20.
3. That equals 0.5.
4. The increase is 50%.

Mental model: Describe trends with numbers. Design with variables and controls.

Common trap: Giving a vague solution with no mechanism or trade-off.