United Grade 12 AP Environmental Science
Chapters: 9
1. The Living World: Ecosystems
Introduction to ecosystems · Terrestrial biomes · Aquatic biomes · The carbon cycle · The nitrogen cycle · The phosphorus cycle · The hydrologic (water) cycle · Primary productivity · Trophic levels · Energy flow and the 10% rule · Food chains and food webs
- Ecosystem: Structure, Productivity, Energy Flow and Pyramids – An ecosystem is a working unit of nature where living things (biotic) and non-living things (abiotic) interact. Plants make food at a rate called productivity: gross (GPP) minus respiration gives net (NPP = GPP − R). Dead matter is broken down in five steps: fragmentation, leaching, catabolism, humification and mineralisation. Energy enters as sunlight, flows one way through trophic levels and only about 10% passes on each time. Ecological pyramids of number, biomass and energy show this; number and biomass pyramids can be inverted, but the energy pyramid is always upright.
- Ecosystems in Depth: Biomes, Nutrient Cycles and Disruption – Biomes are large regions with similar climate and life. On land, temperature and rainfall decide the biome; in water, salt, depth, light and flow decide the zones. Matter cycles through ecosystems: nitrogen moves between air, soil, organisms and back through fixation, nitrification, assimilation, ammonification and denitrification; phosphorus cycles slowly between rock, soil, water, organisms and sediment with no gas stage. Decomposers recycle matter with oxygen (aerobic) or without it (anaerobic). Human actions such as fertiliser runoff, habitat loss, invasive species and climate change disrupt these systems; ecosystems respond with resistance and resilience.
- The Carbon Cycle: Where Carbon Is Stored and How It Moves – Carbon moves between the air, living things, soil, oceans and rocks. These places are called stores (or sinks and sources), and the movements are called flows (fluxes). Photosynthesis takes carbon dioxide out of the air; respiration, decomposition and burning put it back. The ocean takes in and gives out huge amounts. Over millions of years carbon is locked into rocks and fossil fuels, and volcanoes and weathering slowly return it. People now burn fossil fuels and clear forests, adding carbon faster than natural sinks can take it up, so the CO₂ in the air rises and the Earth warms.
- Water in the Atmosphere – Air always holds some water as invisible vapour; the amount is called humidity. Absolute humidity is the actual mass of vapour in a volume of air; relative humidity is how full the air is compared with the most it could hold at that temperature. Water becomes vapour by evaporation (taking heat) and turns back to water by condensation (giving heat) when air cools to its dew point. Condensation near the ground gives dew, frost, mist and fog; higher up it gives clouds: cirrus, cumulus, stratus and nimbus. When drops or crystals grow heavy they fall as precipitation: rain, snow, sleet or hail. Rain forms in three main ways: convectional, orographic (relief) and cyclonic (frontal). Rainfall is heaviest near the equator and on windward coasts, and least in subtropical deserts, continental interiors and polar lands.
- Ecosystem, Food Chains and Energy Flow – An ecosystem is all the living things in a place plus the non-living things around them, working together. Energy enters as sunlight, passes one way up a food chain, and only about 10% moves to each next level.
2. The Living World: Biodiversity
Introduction to biodiversity · Ecosystem services · Island biogeography · Ecological tolerance · Natural disruptions to ecosystems · Adaptations · Ecological succession
- Biodiversity and its Conservation – Biodiversity is the variety of life at three levels: genetic, species and ecological. It is highest near the equator and grows with area (log S = log C + Z log A). Every species matters, like rivets on a plane. We are losing species fast because of the 'evil quartet': habitat loss and fragmentation, over-exploitation, alien species invasions and co-extinction. The IUCN Red List (Red Data Book) ranks species from Least Concern to Extinct. We protect life in situ (hotspots, national parks, sanctuaries, biosphere reserves, sacred groves, Ramsar wetlands) and ex situ (zoos, botanical gardens, seed and gene banks).
- Diversity and Classification of Living Organisms – Earth has millions of kinds of living things, and India is one of the richest places. To study them, we sort them into groups by asking simple questions about their bodies: Is there a nucleus? One cell or many? Can it make its own food? This gives five kingdoms: Monera, Protista, Fungi, Plantae and Animalia. Animals split into invertebrates and vertebrates. Every species gets a two-part scientific name. Viruses are not cells, so they fit in no kingdom.
- Evolution: Origin of Life, Mechanisms and Human Evolution – Life began on the early Earth from simple chemicals: Oparin and Haldane proposed it and Miller made amino acids in a flask. Evidence of evolution comes from fossils, homologous and analogous organs, embryos, molecules and changes we can watch (industrial melanism, drug resistance). Darwin explained it by natural selection acting on variation in populations; the modern synthetic theory adds genes: mutation, recombination, gene flow, genetic drift and natural selection change allele frequencies. If none of these act, frequencies stay constant: Hardy–Weinberg, p² + 2pq + q² = 1. Selection can be stabilising, directional or disruptive. One ancestor spreading into many habitats gives adaptive radiation (Darwin’s finches, Australian marsupials). Humans evolved from Dryopithecus-like apes through Australopithecus, Homo habilis, Homo erectus and Neanderthals to Homo sapiens.
- Ecosystem: Structure, Productivity, Energy Flow and Pyramids – An ecosystem is a working unit of nature where living things (biotic) and non-living things (abiotic) interact. Plants make food at a rate called productivity: gross (GPP) minus respiration gives net (NPP = GPP − R). Dead matter is broken down in five steps: fragmentation, leaching, catabolism, humification and mineralisation. Energy enters as sunlight, flows one way through trophic levels and only about 10% passes on each time. Ecological pyramids of number, biomass and energy show this; number and biomass pyramids can be inverted, but the energy pyramid is always upright.
3. Populations
Generalist and specialist species · K-selected and r-selected species · Survivorship curves · Carrying capacity · Population growth and resource availability · Age structure diagrams · Total fertility rate · Human population dynamics · Demographic transition
- Population Ecology: r and K Species, Survivorship and Human Populations – Species follow different life plans. r-selected species have many small young and give no care; K-selected species have few big young and care for them. Survivorship curves show who dies when (Type I late, Type II steady, Type III early). Generalists use many resources; specialists need one. For humans, age pyramids, the total fertility rate (replacement about 2.1) and the four stages of the demographic transition explain how populations grow, level off or shrink.
- Organisms and Populations: Growth and Interactions – A population is a group of the same species living in one area at one time. It has features that a single organism does not have: density, birth rate, death rate, sex ratio and age structure. N grows by births and immigration and shrinks by deaths and emigration. With unlimited food it grows fast (J-curve, dN/dt = rN); with limited food it slows and stops at the carrying capacity K (S-curve, dN/dt = rN(1 − N/K)). Different species affect each other: mutualism (+/+), competition (−/−), predation and parasitism (+/−), commensalism (+/0) and amensalism (−/0).
4. Earth Systems and Resources
Plate tectonics · Soil formation and erosion · Soil composition and properties · Earth's atmosphere · Global wind patterns · Watersheds · Solar radiation and seasons · Earth's geography and climate · El Nino and La Nina
- Distribution of Oceans and Continents: Drift, Spreading and Plates – In 1912 Alfred Wegener said all continents were once one landmass, Pangaea, surrounded by one ocean, Panthalassa, and that they drifted apart. Matching coastlines, rocks, fossils, glacier deposits and placer gold supported him, but he could not explain the force. Mapping the ocean floor showed ridges, plains and trenches; Harry Hess then proposed sea-floor spreading: new crust forms at mid-ocean ridges and old crust sinks at trenches. This led to plate tectonics: the lithosphere is broken into rigid plates that move on the soft asthenosphere and meet at divergent, convergent and transform boundaries. The Indian plate broke away from the south, moved north, and collided with Asia to raise the Himalayas.
- Soil, Watersheds, Solar Radiation and Seasons – Soil forms slowly as rock weathers and living things add organic matter; climate, organisms, relief (slope), parent material and time control it. It builds layers called horizons (O, A, E, B, C, R). Its texture (sand, silt, clay) sets porosity, permeability and water-holding. Wind and water erode soil, faster on bare, steep land. A watershed is all the land that drains to one outlet, bounded by a ridge (divide). Earth's 23.5° tilt changes the Sun's angle and day length through the year; direct rays deliver more energy per square metre, which gives seasons.
- Composition and Structure of the Atmosphere – The atmosphere is a thick blanket of air held to the Earth by gravity. It is mostly nitrogen (78%) and oxygen (21%), with small amounts of argon, carbon dioxide, ozone, water vapour and dust. It has five layers: troposphere (where weather happens), stratosphere (ozone layer), mesosphere (coldest), thermosphere (ionosphere, very hot) and exosphere (the outer edge). Weather is the state of air for a short time; climate is the average over about 30 years. Both are described by the same elements: temperature, pressure, wind, humidity, clouds and precipitation.
- Atmospheric Circulation and Weather Systems – Air has weight, so it presses down: this is air pressure (about 1013 mb at sea level). Wind blows from high pressure to low pressure, pushed by the pressure gradient force, turned by the Coriolis force (right in the north, left in the south) and slowed by friction near the ground. Uneven heating makes pressure belts (equatorial low, subtropical highs, subpolar lows, polar highs) and three circulation cells, giving planetary winds: trade winds, westerlies and polar easterlies. Belts shift with the seasons, giving seasonal winds like the monsoon; local winds include land and sea breezes and mountain and valley winds. Big bodies of air with the same temperature and moisture are air masses; where two meet is a front. Cyclones are low-pressure storms: tropical cyclones form over warm seas; extratropical ones form along fronts. Thunderstorms and tornadoes are small but violent storms.
- Earth's Geography and Climate: Rain Shadows, El Niño and La Niña – Climate depends on geography. Latitude sets how much sunlight arrives; air rising at the equator and sinking near 30° makes rainforests and deserts (the Hadley cell). Altitude cools air about 6.5 °C per km. Oceans warm and cool slowly, so coasts have milder climates, and ocean currents carry heat. Mountains force moist wind upward, giving rain on the windward side and a dry rain shadow behind. El Niño is when Pacific trade winds weaken and warm water spreads east, shifting rain and changing weather worldwide; La Niña is the opposite, with stronger trade winds and a colder east Pacific.
5. Land and Water Use
The tragedy of the commons · Clearcutting · The Green Revolution · Impacts of agricultural practices · Irrigation methods · Pest control methods · Meat production methods · Impacts of overfishing · Impacts of mining · Impacts of urbanization · Ecological footprints · Introduction to sustainability · Methods to reduce urban runoff · Integrated pest management · Sustainable agriculture · Aquaculture · Sustainable forestry
- Resource Security and Sustainable Use of Land and Water – Natural resources are things from nature that people use: water, soil, forests, fish, minerals and energy. Non-renewable resources (coal, oil, metal ores) are stocks that run down; renewable ones (water, forests, fish) refill, but only at a limited speed. Resource security means having enough of a resource, reliably and at a fair price. Shared resources often suffer the tragedy of the commons: each user takes a little more and the resource collapses. Water security and energy security are the two biggest worries; farming uses about 70% of fresh water, and about 80% of world energy still comes from fossil fuels. Sustainable methods, such as drip irrigation, integrated pest management, selective forestry, careful aquaculture, fishing quotas, mine restoration and reducing urban runoff, let us use resources today while leaving enough for the future. The ecological footprint measures how much land and water our lifestyle needs.
- Agriculture and the Environment: Agroecosystems, Green Revolution and Sustainability – A farm is an agroecosystem: a simplified ecosystem where people grow few species and add energy and materials (fertiliser, water, fuel, pesticides) to raise yields. Better crop and livestock genetics and the Green Revolution package (HYV seeds, irrigation, fertilisers, pesticides, machines) multiplied food output. But intensive farming can cause eutrophication, salinisation, soil erosion and degradation, pesticide resistance, biodiversity loss and greenhouse gases. Social and economic forces shape farming choices. Sustainable methods (rotation, IPM, conservation tillage, efficient irrigation, agroforestry) aim to keep yields high while protecting land and water.
- Urbanisation: Why Cities Grow and How to Make Them Work – Urbanisation is the rise in the share of a country's or the world's people who live in towns and cities. In 1950 about 30% of the world was urban; since 2007 more than half is, and about 68% is expected by 2050. Cities grow by rural–urban migration (push factors from the countryside, pull factors to the city) and by natural increase. Today the fastest growth is in lower-income and newly emerging countries of Asia and Africa, where megacities of 10 million or more are forming; in many rich countries growth is slow and people also move out to suburbs (suburbanisation and counter-urbanisation). Fast growth brings opportunities (jobs, services) and problems (informal housing, traffic, pollution, waste, flooding, the urban heat island). Sustainable urban development tries to fix these with public transport, green spaces, clean water, recycling, affordable housing and renewable energy.
6. Energy Resources and Consumption
Renewable and nonrenewable resources · Global energy consumption · Fuel types and uses · Distribution of natural energy resources · Fossil fuels · Nuclear power · Energy from biomass · Solar energy · Hydroelectric power · Geothermal energy · Hydrogen fuel cells · Wind energy · Energy conservation
- Energy Resources: Use, Global Mix and a Sustainable Future – Societies need energy to grow food, make goods, move people and run homes. Richer countries use far more energy per person. About 80% of the world's primary energy still comes from fossil fuels (oil, coal, gas), which are non-renewable and release CO₂. Renewable sources (solar, wind, hydro, geothermal, biomass) are flows that keep returning; nuclear is non-renewable but low-carbon. Each source differs in cost, reliability, location and pollution. Energy resources are unevenly spread, so countries work for energy security by diversifying sources, trading, storing energy, saving energy and investing in new technology such as hydrogen and fuel cells.
- Radioactivity: Alpha, Beta, Gamma and Half-Life – Some atomic nuclei are unstable. They give out radiation at random to become more stable. This is radioactive decay. Alpha (2 protons + 2 neutrons), beta (a fast electron) and gamma (a wave of energy) are the three main kinds. Half-life is the time for half of the unstable nuclei to decay.
7. Atmospheric Pollution
Introduction to air pollution · Photochemical smog · Thermal inversion · Atmospheric CO2 and particulates · Indoor air pollutants · Reduction of air pollutants · Acid rain · Noise pollution
- Pollution: Air, Water, Land and Noise – Pollution is the release of harmful substances or energy into air, water or land faster than nature can remove them. Main air pollutants are particulates, carbon monoxide, sulfur dioxide, nitrogen oxides and ozone; they cause acid rain and smog. Water pollution by nutrients, sewage and toxins causes eutrophication and biomagnification. We reduce pollution by cutting emissions at the source, cleaning waste before release, and the 3Rs.
8. Aquatic and Terrestrial Pollution
Sources of pollution · Human impacts on ecosystems · Endocrine disruptors · Human impacts on wetlands and mangroves · Eutrophication · Thermal pollution · Persistent organic pollutants · Bioaccumulation and biomagnification · Solid waste disposal · Waste reduction methods · Sewage treatment · Lethal dose 50% (LD50) · Dose-response curve · Pollution and human health · Pathogens and infectious diseases
- Pollution: Air, Water, Land and Noise – Pollution is the release of harmful substances or energy into air, water or land faster than nature can remove them. Main air pollutants are particulates, carbon monoxide, sulfur dioxide, nitrogen oxides and ozone; they cause acid rain and smog. Water pollution by nutrients, sewage and toxins causes eutrophication and biomagnification. We reduce pollution by cutting emissions at the source, cleaning waste before release, and the 3Rs.
9. Global Change
Stratospheric ozone depletion · Reducing ozone depletion · The greenhouse effect · Increases in greenhouse gases · Global climate change · Ocean warming · Ocean acidification · Invasive species · Endangered species · Human impacts on biodiversity
- Climate Change and the Greenhouse Effect – Climate is the average weather of a place over about 30 years. Greenhouse gases like carbon dioxide and methane trap some of the heat the Earth gives off. Humans have added a lot more of these gases by burning fossil fuels and cutting forests, so the Earth is warming. This causes melting ice, rising seas and more extreme weather. We can cut emissions (mitigation) and prepare for changes (adaptation).
- Biodiversity and its Conservation – Biodiversity is the variety of life at three levels: genetic, species and ecological. It is highest near the equator and grows with area (log S = log C + Z log A). Every species matters, like rivets on a plane. We are losing species fast because of the 'evil quartet': habitat loss and fragmentation, over-exploitation, alien species invasions and co-extinction. The IUCN Red List (Red Data Book) ranks species from Least Concern to Extinct. We protect life in situ (hotspots, national parks, sanctuaries, biosphere reserves, sacred groves, Ramsar wetlands) and ex situ (zoos, botanical gardens, seed and gene banks).
- Diversity and Classification of Living Organisms – Earth has millions of kinds of living things, and India is one of the richest places. To study them, we sort them into groups by asking simple questions about their bodies: Is there a nucleus? One cell or many? Can it make its own food? This gives five kingdoms: Monera, Protista, Fungi, Plantae and Animalia. Animals split into invertebrates and vertebrates. Every species gets a two-part scientific name. Viruses are not cells, so they fit in no kingdom.