AP® Environmental Science Cheat Sheets: Units, Formulas, Flashcards & Quiz

Populations

The Populations unit connects ecology to human demography. At the ecological level, students must know how population size changes through births, deaths, immigration, and emigration. At the human level, students must interpret age structure diagrams, demographic transition stages, total fertility rate, population momentum, and development differences.

r-selected species and K-selected species represent two ends of a life-history strategy continuum. r-selected organisms reproduce quickly, produce many offspring, mature early, and invest little parental care. They often thrive in unstable environments. K-selected organisms reproduce more slowly, produce fewer offspring, mature later, and invest more parental care. They are more common near carrying capacity in stable environments. Most organisms are not perfectly one or the other, but the comparison helps students predict survival and reproduction patterns.

Population growth can be exponential or logistic. Exponential growth occurs when resources are unlimited and the population grows by a constant proportion. The curve is J-shaped. Logistic growth occurs when limiting factors slow growth as the population approaches carrying capacity, producing an S-shaped curve. Carrying capacity is the maximum population size that the environment can support sustainably. Overshoot occurs when a population exceeds carrying capacity, often followed by dieback.

Density-dependent limiting factors increase with population density. Examples include disease, competition, predation, and waste buildup. Density-independent factors affect populations regardless of density, such as drought, hurricanes, fires, floods, and extreme temperatures. APES questions often ask students to explain why a factor belongs in one category and predict how it affects population growth.

Human population growth is often analyzed through age structure diagrams. A wide base indicates many young people and future growth. A more rectangular structure indicates stable population. A narrow base and larger older population indicate decline. Population momentum occurs when a large cohort of young people enters reproductive age, causing continued growth even if fertility rates fall. This is why population policies do not always produce immediate stabilization.

The demographic transition model describes changes in birth and death rates as a society industrializes. Stage 1 has high birth and death rates. Stage 2 has falling death rates due to improved sanitation, food, and medicine, while birth rates remain high, causing rapid growth. Stage 3 has falling birth rates as education, urbanization, and access to contraception increase. Stage 4 has low birth and death rates and stable or slow-growing populations. Some countries may enter a fifth stage with birth rates below death rates.

Total fertility rate is the average number of children born per woman. Replacement level is about 2.1 in many developed countries, though it can be higher where mortality is higher. Factors that lower TFR include education for women, access to contraception, later marriage, urbanization, child labor restrictions, and economic security. Factors that raise TFR include cultural expectations, limited education, limited access to healthcare, and high infant mortality.

Earth Systems and Resources

Earth Systems and Resources explains the physical foundation of environmental science. Plate tectonics shapes landforms, earthquakes, volcanoes, mountain building, ocean basins, and mineral resources. Weathering creates soil. Atmospheric circulation drives climate, wind patterns, ocean currents, and precipitation. Hydrology determines water availability and water pollution movement.

Plate boundaries are high-yield. Convergent boundaries occur where plates move toward each other. Oceanic-continental convergence can create trenches and volcanic arcs. Continental-continental convergence can create mountains. Divergent boundaries occur where plates separate and new crust forms, such as mid-ocean ridges. Transform boundaries occur where plates slide past each other, causing earthquakes. These processes influence hazards and resource distribution.

Soil is not just dirt; it is a complex mixture of minerals, organic matter, water, air, and organisms. Soil texture depends on sand, silt, and clay. Sand drains quickly and has large particles. Clay holds water and nutrients but drains slowly. Silt is intermediate. Porosity measures pore space. Permeability measures how fast water moves. Soil fertility depends on nutrients, organic matter, pH, texture, and biological activity.

Soil horizons matter on APES exams. The O horizon is organic material. The A horizon is topsoil. The E horizon is leached by eluviation. The B horizon accumulates materials by illuviation. The C horizon is partially weathered parent material. The R horizon is bedrock. Erosion removes topsoil and reduces fertility. Practices such as contour plowing, terracing, cover crops, no-till agriculture, windbreaks, and crop rotation reduce soil loss.

Atmospheric layers have different temperature trends and functions. The troposphere contains weather and most water vapor. The stratosphere contains ozone that absorbs ultraviolet radiation. The mesosphere is very cold. The thermosphere contains auroras and very high temperatures because particles absorb high-energy radiation. Students should know that ozone is beneficial in the stratosphere but harmful at ground level.

Climate patterns are driven by unequal solar heating, Earth's rotation, atmospheric circulation, and ocean currents. The equator receives intense solar radiation, causing warm moist air to rise near the ITCZ. Hadley, Ferrel, and Polar cells create major wind belts. The Coriolis effect deflects moving air and water. Ocean currents redistribute heat. El Niño and La Niña alter normal Pacific conditions and affect global weather patterns. Rain shadows form when air rises over mountains, cools, drops rain, and descends dry on the leeward side.

Land, Water, and Energy Use

Land and water use questions often ask students to evaluate trade-offs. A practice may increase food production, energy supply, or economic output while also causing pollution, habitat loss, greenhouse gas emissions, or resource depletion. Strong APES answers name both benefits and costs, then propose realistic solutions with clear mechanisms.

The tragedy of the commons occurs when individuals acting in their own short-term interest overuse a shared resource. Examples include overfishing, groundwater depletion, air pollution, and overgrazing. Solutions include regulations, quotas, permits, property rights, community management, and economic incentives. The key is to connect the solution to the mechanism: limits reduce extraction, permits cap emissions, and pricing makes overuse costly.

Agriculture has major environmental impacts. The Green Revolution increased yields through fertilizers, pesticides, irrigation, high-yield varieties, and mechanization. It reduced famine risk but increased runoff, soil degradation, water use, fossil fuel use, and genetic uniformity. Irrigation can increase crop yield but may cause salinization, waterlogging, aquifer depletion, and river flow reduction. Drip irrigation reduces water loss by delivering water near roots. Flood irrigation is cheaper but less efficient.

Pest control is another trade-off. Synthetic pesticides are cheap and effective but can cause resistance, toxicity, bioaccumulation, and harm to non-target species. Integrated pest management uses multiple methods, including biological control, crop rotation, habitat modification, resistant crops, and limited pesticide use. Organic agriculture can improve soil health and reduce synthetic chemical use, but may have lower yields or higher labor costs. GMOs can increase yield or drought tolerance but raise concerns about gene flow and monoculture.

Urbanization changes land cover. Impervious surfaces increase runoff, flooding, and pollutant transport. Urban heat islands form because dark surfaces absorb heat and vegetation is reduced. Sprawl increases habitat fragmentation, vehicle use, and infrastructure costs. Green roofs, rain gardens, permeable pavement, urban trees, public transit, and smart growth can reduce these impacts.

Energy resources must be compared by cost, availability, emissions, reliability, land use, waste, and energy return. Coal is abundant and cheap but has high CO₂, SO₂, mercury, and particulate emissions. Oil is portable and energy dense but causes spills, emissions, and geopolitical concerns. Natural gas emits less CO₂ than coal but still emits CO₂ and can leak methane. Nuclear energy has low operational carbon emissions and high output but creates long-lived radioactive waste and high construction costs. Renewables reduce emissions but may be intermittent or geographically limited.

Energy conservation is often the cheapest way to reduce environmental impact. Insulation, efficient appliances, LEDs, public transit, building design, and behavioral changes reduce demand. On FRQs, do not simply say “use renewable energy.” Name a specific energy source, explain its advantage, identify a limitation, and justify a solution such as storage, transmission, or demand management.

Atmospheric, Aquatic, and Terrestrial Pollution

Pollution units are high-yield because they connect chemistry, health, ecology, policy, and solutions. Students should be able to identify the pollutant, source, environmental effect, health effect, and control strategy. A strong FRQ answer will not say “reduce pollution.” It will name the pollutant and explain how a specific technology or policy reduces it.

Air pollution includes primary and secondary pollutants. Primary pollutants are emitted directly, such as carbon monoxide, sulfur dioxide, nitrogen oxides, particulate matter, volatile organic compounds, and lead. Secondary pollutants form through reactions in the atmosphere, such as ozone, PAN, sulfuric acid, and nitric acid. Photochemical smog forms when NOₓ and VOCs react in sunlight to produce ozone. Gray smog is associated with coal combustion, soot, and sulfur dioxide.

Criteria pollutants regulated under NAAQS include CO, Pb, NO₂, O₃, PM, and SO₂. Carbon monoxide reduces oxygen transport by binding to hemoglobin. Particulate matter damages respiratory and cardiovascular systems. Sulfur dioxide and nitrogen oxides contribute to acid deposition. Ground-level ozone irritates lungs and damages plants. Lead harms neurological development. Control technologies include catalytic converters, scrubbers, electrostatic precipitators, low-sulfur fuels, fuel switching, and emissions standards.

Indoor air pollution can be as important as outdoor air pollution. Radon is a radioactive gas linked to lung cancer. Asbestos is linked to mesothelioma. Carbon monoxide is dangerous because it is colorless and odorless. VOCs such as formaldehyde can come from building materials. Mold can worsen respiratory symptoms. In developing regions, indoor cooking fires produce particulate matter and cause major health burdens.

Water pollution includes point and nonpoint sources. Point sources are easier to regulate because they come from an identifiable location. Nonpoint sources are harder because they spread across landscapes, such as fertilizer runoff. Eutrophication begins when nitrogen and phosphorus stimulate algal blooms. Decomposition of dead algae increases BOD and reduces dissolved oxygen, causing hypoxic or anoxic dead zones. Thermal pollution decreases dissolved oxygen because warmer water holds less oxygen.

Terrestrial pollution includes solid waste, hazardous waste, pesticides, heavy metals, and persistent organic pollutants. Landfills produce methane and leachate. Incineration reduces waste volume but requires emissions controls. The waste hierarchy prioritizes reducing consumption, reusing materials, recycling, composting, incineration, and landfilling last. Toxicology terms include LD₅₀, dose-response, threshold, bioavailability, persistence, bioaccumulation, biomagnification, and synergism.

Legislation matters because APES often asks students to connect laws to environmental problems. The Clean Air Act regulates air pollution and NAAQS. The Clean Water Act regulates discharges and water quality standards through NPDES permits. The Safe Drinking Water Act sets drinking water standards. RCRA regulates hazardous waste from cradle to grave. CERCLA, or Superfund, addresses abandoned hazardous waste sites.

Global Change

Global Change includes ozone depletion, climate change, ocean acidification, biodiversity loss, invasive species, and conservation policy. This unit is weighted heavily because it synthesizes many earlier topics: energy use, greenhouse gases, atmospheric chemistry, land-use change, agriculture, oceans, population growth, and sustainability.

Stratospheric ozone protects life by absorbing UV-B radiation. CFCs and related compounds were widely used as refrigerants and propellants. In the stratosphere, ultraviolet radiation breaks them down and releases chlorine radicals. Chlorine radicals catalyze ozone destruction. The Antarctic ozone hole forms because polar stratospheric clouds and extreme cold create conditions that accelerate ozone loss. The Montreal Protocol is a major environmental policy success because it phased out many ozone-depleting substances.

The greenhouse effect is natural, but human activities have enhanced it by increasing greenhouse gas concentrations. Carbon dioxide comes from fossil fuel combustion, deforestation, and cement production. Methane comes from livestock, rice paddies, landfills, and natural gas leaks. Nitrous oxide comes from agriculture and combustion. CFCs and HCFCs have very high global warming potentials. GWP compares how much heat a gas traps relative to CO₂ over a time horizon, commonly 100 years.

Climate change effects include warming, sea-level rise, stronger heat waves, shifting precipitation, glacier melt, ocean warming, ocean deoxygenation, ocean acidification, ecosystem range shifts, coral bleaching, and increased risk of extreme events in many regions. Sea level rises through thermal expansion and melting land ice. Ocean acidification occurs when CO₂ dissolves in seawater and forms carbonic acid, lowering pH and reducing carbonate availability for shell-building organisms.

Feedback loops can amplify or reduce change. Ice-albedo feedback is positive because melting ice exposes darker surfaces that absorb more heat, causing more melting. Permafrost methane release is positive because warming releases methane, which causes more warming. Negative feedbacks oppose change, but many climate-related feedbacks discussed in APES amplify warming.

Mitigation reduces the causes of climate change, such as reducing greenhouse gas emissions, increasing renewable energy, improving efficiency, protecting forests, restoring wetlands, and changing agricultural practices. Adaptation reduces vulnerability to impacts, such as sea walls, managed retreat, drought-resistant crops, heat planning, and resilient infrastructure. Strong APES responses distinguish mitigation from adaptation and justify why the strategy works.

Biodiversity loss is driven by habitat loss, invasive species, pollution, population growth, overharvesting, and climate change. The Endangered Species Act lists threatened and endangered species and protects critical habitat. CITES restricts international trade in endangered species. The Lacey Act targets illegal trade and poaching. Invasive species often spread because they lack predators, reproduce quickly, and outcompete native species, reducing biodiversity and altering ecosystem processes.

APES FRQ Strategy

The AP Environmental Science free-response section has three questions. Question 1 asks students to design an investigation. Question 2 asks students to analyze an environmental problem and propose a solution. Question 3 asks students to analyze an environmental problem and propose a solution while doing calculations. Students should expect authentic environmental scenarios with data, visuals, models, or quantitative information.

For investigation design, identify the independent variable, dependent variable, control group, constants, sample size, replication, and hypothesis. A strong hypothesis predicts a relationship and is testable. A strong procedure changes only one independent variable while measuring the dependent variable. Good experimental design includes repeated trials or adequate sample size to reduce random error.

For solution questions, use a three-part sentence: name the solution, explain the mechanism, and connect it to the environmental outcome. For example, “Installing permeable pavement reduces stormwater runoff because water infiltrates into the soil instead of flowing across impervious surfaces, which reduces pollutant transport to streams.” This is better than “use permeable pavement” because it explains why the solution works.

For calculation questions, show every step. Write the formula, substitute values, calculate, include units, and interpret the result. If asked for percent change, use the old value in the denominator. If asked for doubling time, confirm whether the growth rate is written as a percent or decimal. If asked for energy transfer, apply the 10% rule level by level. If asked for capacity factor or efficiency, multiply by 100 to express a percent.

Do not overgeneralize. If a prompt asks for one environmental benefit and one environmental cost, give one clear benefit and one clear cost. If it asks for a realistic limitation, name a specific limitation such as cost, intermittency, land availability, wildlife impacts, public resistance, or technology requirements. If it asks for a policy, name a policy mechanism such as cap-and-trade, carbon tax, permit system, quota, subsidy, or regulation.

FRQ pattern: identify the environmental problem, trace the cause-and-effect pathway, propose a specific solution, and justify the solution with a mechanism.

How to Study AP® Environmental Science

APES is easiest to study when you group topics by systems, impacts, and solutions. For each topic, ask four questions: What is the natural process? How do humans disrupt it? What are the environmental or health consequences? What solution would reduce the problem, and why?

1. Start with the cheat-sheet cards. Read one card and mark every term you cannot explain in one sentence.
2. Turn terms into cause-and-effect chains. For eutrophication, write: fertilizer runoff → algal bloom → decomposition → high BOD → low dissolved oxygen → dead zone.
3. Practice formulas daily. Use the Rule of 70, percent change, PGR, NPP, capacity factor, and energy efficiency until the setup is automatic.
4. Use flashcards for recall. Do not simply recognize the term; explain an example and one AP-style trap.
5. Take mixed quizzes. The exam mixes units, so practice connecting energy, pollution, biodiversity, and climate.
6. Write FRQ explanations. Always include a mechanism. A solution without a mechanism is usually too weak for full credit.
7. Estimate readiness. After practice, use the AP Environmental Science score calculator to plan your next study step.

A seven-day review plan can work well. Day 1: Living World and cycles. Day 2: populations and math. Day 3: Earth systems and soil. Day 4: land, agriculture, and energy. Day 5: air, water, and waste pollution. Day 6: climate, ozone, biodiversity, and laws. Day 7: mixed FRQs, calculations, and score review. If you have less time, prioritize formulas, pollution pathways, energy trade-offs, climate gases, and FRQ solution justification.