AP Environmental Science Key Terms & Vocabulary

247 essential AP Environmental Science terms, defined — aligned to the College Board CED.

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10% rule: The approximation that only about 10% of energy is passed from one trophic level to the next, with the rest lost as heat. It explains why food chains are short and top predators are rare.
A
Abiotic factor: A nonliving component of an ecosystem like temperature, sunlight, water, soil, and pH. These set the limits on which organisms can survive in a place.
Acid deposition: The fallout of acidic compounds (wet as acid rain or dry as particles) caused by nitrogen oxides and sulfur oxides in the atmosphere. It acidifies soils and waters and corrodes structures.
Acid rain: Precipitation made acidic by sulfuric and nitric acids formed from SO2 and NOx emissions. It mainly harms communities downwind of coal-burning power plants.
Active solar: A solar system that uses pumps, fans, and other equipment to collect, move, and store heat from the sun. The mechanical parts let it store energy for later.
Adaptation: A heritable trait that improves an organism's survival or reproduction, accumulating through incremental genetic changes over time. Sudden environmental change can outpace it, threatening a species.
Age structure diagram: A graph showing the distribution of a population across age groups and sexes. Its shape predicts growth — a wide base means rapid growth, a narrow base means decline.
Air Quality Index (AQI): A scale that reports daily air quality based on pollutants like particulates, ozone, CO, SO2, and NO2. Tells people when air is unhealthy enough to limit outdoor activity.
Albedo: The reflectivity of a surface. Ice and snow have high albedo and bounce solar energy back to space; when they melt, darker surfaces absorb more heat, worsening warming.
Algal bloom: A rapid overgrowth of algae fueled by excess nutrients from fertilizer or wastewater runoff. Its decay strips oxygen from the water and can produce toxins.
Aquaculture: Farming fish and other aquatic organisms. It's efficient and needs little land or fuel, but escaped fish and crowded conditions can spread disease and contaminate water.
Aquatic biome: A water-based biome, split into freshwater (streams, rivers, ponds, lakes) and marine (oceans, coral reefs, estuaries, marshland) types.
Aquifer depletion: Pumping groundwater from an aquifer faster than it recharges, drawing it down. Heavy agricultural use has severely depleted aquifers like the Ogallala in the central United States.
Asbestos: A fibrous mineral once used in insulation that becomes an indoor particulate hazard when disturbed. Inhaling fibers causes mesothelioma, a cancer of the lung lining.
Axial tilt: The roughly 23.5-degree tilt of Earth's rotation axis. It causes the seasons and the changing hours of daylight as Earth orbits the sun.
B
Battery electric vehicle: A vehicle powered entirely by an electric battery rather than fuel combustion. Switching to BEVs is a key large-scale energy-conservation strategy in transportation.
Bioaccumulation: The buildup of a substance, usually fat-soluble, in the tissues of a single organism over its lifetime as it absorbs more than it can excrete.
Biodiversity: The variety of life in an ecosystem, measured at three levels: genetic, species, and habitat diversity. Higher biodiversity makes ecosystems more resilient to disruption.
Biogeochemical cycle: The movement of an element or molecule between living organisms and the physical environment. These cycles demonstrate the conservation of matter — atoms are recycled, not created.
Biomagnification: The increasing concentration of a persistent substance at each higher trophic level of a food chain. Top predators get the highest doses, causing effects like eggshell thinning.
Biomass energy: Energy from burning organic matter like wood and crop waste. It's cheap but emits carbon dioxide and other pollutants, and overharvesting trees for it causes deforestation.
Biome: A large region defined by a characteristic climate and the communities of plants and animals adapted to it. Climate, especially temperature and precipitation, determines which biome forms.
Biotic factor: Any living component of an ecosystem — plants, animals, fungi, bacteria. Paired with abiotic factors to define what an ecosystem actually is.
Biotic potential: The maximum reproductive rate of a population under ideal, unlimited conditions. Real growth is always lower because of environmental resistance.
Buffering capacity: The ability of soils and bedrock to neutralize acid deposition. Limestone bedrock buffers acid rain, so regions with limestone suffer less damage to their lakes and ponds.
C
CAFO: A concentrated animal feeding operation, or feedlot, where livestock are densely packed and fattened quickly on grain. It's cheap but crowds animals and generates large amounts of waste that can pollute water.
Capacity factor: The ratio of the energy a power source actually produces to the maximum it could produce if it ran full-time. Intermittent sources like solar and wind have lower capacity factors than steady ones.
Carbon cycle: The movement of carbon between sources and sinks like the atmosphere, oceans, living things, and fossil fuels. Photosynthesis and respiration move carbon quickly; fossil fuel deposits store it for millions of years.
Carbon monoxide (CO): A colorless, odorless gas from incomplete combustion that acts as an asphyxiant by binding to blood and blocking oxygen. A major outdoor pollutant from vehicles and a deadly indoor pollutant.
Carrying capacity (K): The maximum population size an environment can sustain given its resources. Exceed it and the population overshoots, then crashes.
Catalytic converter: An emissions control device on vehicles that converts CO, NOx, and hydrocarbons in exhaust into less harmful gases like CO2, N2, and water vapor.
Cellular respiration: The process where organisms break down organic compounds to release energy, returning CO2 to the atmosphere. It's the counterpart to photosynthesis in the carbon cycle.
Chlorofluorocarbons (CFCs): Synthetic chemicals once used as refrigerants and propellants that destroy stratospheric ozone. They are also potent greenhouse gases with the highest global warming potential.
Clean Air Act: U.S. legislation that empowers the EPA to regulate air pollutant emissions to protect human health. It famously phased out leaded gasoline, sharply cutting atmospheric lead.
Clearcutting: Logging that removes all trees from an area at once. It's cheap and profitable but causes soil erosion, warmer soil and streams, flooding, and the release of stored carbon.
Coal: A solid fossil fuel that comes in grades — lignite, bituminous, and anthracite — set by the heat, pressure, and burial depth that formed it. Higher grades pack more energy but mining and burning it is highly polluting.
Cogeneration: Using a single fuel source to produce both useful heat and electricity at once. It captures waste heat and so squeezes more value from the fuel.
Commensalism: A symbiotic relationship where one species benefits and the other is unaffected, like barnacles hitching a ride on a whale.
Competition: An interaction within or between species over limited resources. It pushes species toward resource partitioning to reduce conflict.
Composting: Letting organic matter like food scraps and yard waste decompose into a usable fertilizer. A waste-reduction method whose drawbacks are odor and rodents.
Contour plowing: Plowing across a slope along its natural contour lines rather than up and down. It slows water runoff and prevents soil erosion.
Convergent boundary: A plate boundary where two tectonic plates collide. It builds mountains, volcanoes, and island arcs and triggers earthquakes — the source of much of Earth's most dramatic geology.
Coral bleaching: When stressed corals expel the algae living in them and turn white, often due to warming water. Some corals recover; many die, collapsing reef ecosystems.
Coral reef: A marine biome built by coral organisms in warm, shallow, clear water. Among the most biodiverse ecosystems on Earth and very sensitive to temperature change.
Coriolis effect: The apparent deflection of moving air (and water) caused by Earth's rotation. It bends global winds into curved paths instead of straight north–south flows.
Crop rotation: Alternating the crops grown in a field across seasons. It improves soil fertility and breaks pest and disease cycles.
Cultural services: Nonmaterial ecosystem benefits like recreation, tourism, spiritual value, and aesthetic enjoyment.
D
DDT: A persistent pesticide that bioaccumulates and biomagnifies, famously thinning the eggshells of top predator birds. A textbook example of a POP's ecosystem impact.
Dead zone: An area of dangerously low oxygen in oceans or lakes caused by excess nutrient pollution. Fish and other organisms cannot survive there.
Demographic transition: The shift from high birth and death rates to low ones as a country industrializes, shown in a four-stage model (DTM). It explains why population growth surges then levels off during development.
Density-dependent factor: A limiting factor whose effect intensifies as population density rises, such as disease transmission, food scarcity, or territory size.
Density-independent factor: A limiting factor that affects a population regardless of its density, such as storms, fires, droughts, or heat waves.
Desertification: The degradation of low-precipitation land toward becoming a desert, often driven by overgrazing and the loss of protective vegetation.
Developing country: A less-industrialized nation typically marked by higher infant mortality, higher fertility, and more children in the workforce than developed countries.
Dieback: A sharp, often catastrophic drop in population after overshoot, driven by famine, disease, or conflict from resource shortages.
Disease vector: An organism, like a mosquito, that transmits a pathogen to humans. Vectors for malaria, West Nile, and Zika spread as climate zones shift.
Divergent boundary: A plate boundary where two plates pull apart, producing seafloor spreading, rift valleys, volcanoes, and earthquakes. This is where new crust is created.
Dose-response curve: A graph showing how the effect on an organism, or the mortality rate in a population, changes with the dose of a toxin or drug.
Doubling time: The time it takes a population to double in size at its current growth rate, estimated with the rule of 70. Useful for projecting future resource demand.
Drip irrigation: Irrigation that delivers small amounts of water directly to plant roots through perforated hoses. It's the most water-efficient method but expensive, so it's used less often.
Dysentery: An intestinal infection caused by untreated sewage contaminating streams and rivers. A marker of poor water sanitation.
E
E-waste: Discarded electronics like phones, TVs, and computers. They contain heavy metals such as lead and mercury that can leach into groundwater if not recycled properly.
Earthquake: Ground shaking that happens when stress overcomes a locked fault and stored energy is suddenly released. Most occur along plate boundaries, which is why mapping boundaries predicts where they strike.
Ecological footprint: A measure of the land and resources a person or society needs to supply their demands and absorb their waste. It's a quick way to compare how heavily different lifestyles tax the planet.
Ecological succession: The gradual change in species composition of an ecosystem over time. Comes in two types: primary and secondary succession.
Ecological tolerance: The range of conditions — temperature, salinity, sunlight, flow rate — an organism or species can survive before injury or death. Conditions outside this range force adaptation, migration, or death.
Ecosystem: A community of living organisms interacting with each other and their nonliving environment. The whole AP course builds from the idea that biotic and abiotic factors are inseparable.
Ecosystem services: The benefits humans get from ecosystems, grouped into four categories: provisioning, regulating, cultural, and supporting. They link ecological health directly to human economies.
El Niño: A phase of the El Niño–Southern Oscillation marked by unusually warm surface water in the Pacific Ocean. It shifts global rainfall, wind, and ocean circulation, disrupting marine food chains.
Electrostatic precipitator: A device that uses an electric charge to pull particulate matter out of smokestack exhaust, commonly used at coal-burning power plants.
Endangered species: A species at risk of extinction due to factors like overhunting, limited diet, invasive competitors, or narrow habitat needs. Species that can adapt or relocate are less at risk.
Endangered Species Act: U.S. legislation that protects threatened and endangered species and their habitats. An example of strategies—alongside anti-poaching laws—used to combat extinction.
Endocrine disruptor: A chemical that interferes with an animal's hormone (endocrine) system. Linked to birth defects, developmental disorders, and gender imbalances in fish and other species.
Energy conservation: Reducing energy use through efficiency and behavior — from adjusting a thermostat to driving electric vehicles and using public transit. It cuts both costs and environmental impact.
Energy return on energy investment: The ratio of energy a source delivers to the energy spent to produce it. A low EROEI — like ethanol's — means much of the gain is eaten up by production.
ENSO: The El Niño–Southern Oscillation, the back-and-forth swing between El Niño and La Niña conditions in the Pacific. It's one of the biggest natural drivers of year-to-year climate variability.
Estuary: A coastal zone where freshwater rivers meet saltwater oceans, creating brackish, highly productive habitat. Acts as a nursery for many fish and shellfish.
Ethanol: A plant-based fuel that can substitute for gasoline. Burning it adds no net new carbon, but its energy return on energy investment is low.
Eutrophication: The over-enrichment of a body of water with nutrients, triggering an algal bloom. When the algae die, microbes consume the oxygen, causing fish die-offs from low dissolved oxygen.
F
Flood irrigation: Irrigation that floods an entire field with water. It's simple and cheap but loses a large share to evaporation and runoff and can waterlog the soil.
Food chain: A single linear path showing who eats whom and how energy flows from producers to top consumers.
Food web: A model of interlocking food chains showing energy and nutrient flow through an ecosystem. Removing or adding one species can ripple through the entire web.
Fossil fuels: Coal, oil, and natural gas formed from ancient organic matter. They're the world's most-used energy source, and burning them releases carbon dioxide that drives climate change.
Fracking: Hydraulic fracturing — injecting high-pressure fluid to crack rock and release trapped oil and gas. It can contaminate groundwater and release volatile organic compounds.
Free-range grazing: Letting animals graze on grass for their whole lifecycle. The meat avoids feedlot chemicals and waste fertilizes the land, but it needs far more land and costs consumers more.
Furrow irrigation: Irrigation that fills channels cut between crop rows with water. It's inexpensive but a significant portion is lost to evaporation and runoff.
G
Generalist species: A species with a broad niche that can use many resources and tolerate varied conditions. Advantaged in changing environments and common among invasive species.
Genetic diversity: The variety of genes within a population. More genetic diversity means a population can better respond to environmental stressors like disease or climate shifts.
Geothermal energy: Energy tapped from heat stored in Earth's interior, used to make steam that drives a generator. It's clean but expensive to access and only available in certain regions.
Global climate change: Long-term shifts in Earth's climate, now driven by excess greenhouse gases. Effects include rising temperatures, melting ice, rising seas, and shifting disease vectors.
Global warming potential (GWP): A measure of how much heat a greenhouse gas traps compared to CO2, which is set at 1. CFCs have the highest GWP, followed by nitrous oxide, then methane.
Global wind patterns: Large-scale, predictable wind belts driven by intense solar heating at the equator, the resulting density differences, and the Coriolis effect. They redistribute heat and moisture worldwide.
Green building design: Designing buildings to use less energy and resources, through features like insulation, efficient systems, and smart orientation. It's a large-scale strategy for cutting energy demand.
Green Revolution: The mid-20th-century shift to high-yield agriculture using mechanization, GMOs, fertilizers, irrigation, and pesticides. It boosted food production but raised reliance on fossil fuels and chemical inputs.
Greenhouse effect: The warming that occurs when atmospheric gases absorb and re-radiate heat from Earth's surface. It keeps the planet warm enough for life; excess gases amplify it.
Greenhouse gases: Gases that trap heat in the atmosphere—chiefly carbon dioxide, methane, water vapor, nitrous oxide, and CFCs. Rising levels drive global climate change.
Gross primary productivity (GPP): The total rate of photosynthesis in a given area, before any energy is used by the producers themselves. The raw amount of energy captured.
H
Habitat corridor: A protected strip of habitat connecting fragmented areas so wildlife can move between them. A key mitigation strategy against fragmentation.
Habitat diversity: The variety of habitats present in a given area. More habitat types support more species and more complex interactions.
Habitat fragmentation: The breaking of a large habitat into smaller, isolated patches by roads, agriculture, or logging. It harms species that need large, connected ranges.
Half-life: The time it takes for half of a radioactive sample to decay. It's used to calculate decay rates and how long nuclear waste stays hazardous.
HIPPCO: The acronym for the main drivers of biodiversity loss: Habitat destruction, Invasive species, Population growth, Pollution, Climate change, and Overexploitation.
Hot spot: A fixed plume of rising magma that punches through a plate, creating volcanoes that are far from any plate boundary. As the plate drifts overhead, it leaves a trail of volcanic islands.
Hydroelectric power: Electricity generated by moving water spinning a turbine, usually via a dam and reservoir. It produces no air pollution but dams are costly and can flood or alter habitats.
Hydrofluorocarbons (HFCs): Replacement chemicals for CFCs that do not deplete the ozone layer. The catch: some are strong greenhouse gases, trading one problem for another.
Hydrogen fuel cell: A device that combines hydrogen and oxygen to make electricity, emitting only water. It's clean but expensive, and producing the hydrogen still takes energy.
Hydrologic cycle: The sun-powered movement of water between oceans, atmosphere, ice, and groundwater through evaporation, precipitation, and runoff. Oceans are the primary reservoir of surface water.
Hypoxic: Describing water that is low in dissolved oxygen, often a result of eutrophication. Hypoxic conditions suffocate fish and aquatic life.
I
Ice cores: Cylinders of layered glacial ice that trap ancient air bubbles, letting scientists reconstruct past CO2 levels and temperatures. Key evidence that climate has changed over geologic time.
Impervious surface: A human-made surface like a road, parking lot, or roof that blocks water from soaking into the ground. It increases runoff and urban flooding.
Incineration: Burning solid waste at high temperatures to greatly reduce its volume. The tradeoff is the release of air pollutants, though the heat can generate electricity.
Indicator species: A species whose presence, abundance, or condition reveals the health or quality of an ecosystem. Like a canary in a coal mine for environmental change.
Indoor air pollutants: Pollutants concentrated inside buildings from sources like combustion, building materials, and natural gases. Includes CO, radon, asbestos, mold, VOCs, formaldehyde, and tobacco smoke.
Infant mortality rate: The number of infant deaths per number of live births, tied to maternal healthcare and nutrition. A key signal of a country's development and a driver of fertility choices.
Insolation: Incoming solar radiation — Earth's main energy source. How much a location receives depends on latitude and season, which drives temperature and climate.
Integrated pest management: A mix of biological, physical, and limited chemical methods — like biocontrol, intercropping, crop rotation, and natural predators — to control pests with minimal environmental disruption. Effective but more complex and costly than spraying alone.
Invasive species: A species living and often thriving outside its native habitat where it threatens native species. Frequently generalist, r-selected organisms that outcompete natives.
Island biogeography: The study of how species richness on islands depends on island size and distance from the mainland. Larger, closer islands hold more species; smaller, remote ones hold fewer.
K
K-selected species: Species that are large, long-lived, produce few offspring, and invest heavily in each one. They thrive near carrying capacity but are hit hard by invasive species.
Keystone species: A species whose activities have an outsized effect on community structure relative to its abundance. Remove it and the whole ecosystem can collapse or reorganize.
L
La Niña: The cool-water counterpart to El Niño, with below-average Pacific surface temperatures. Like El Niño, it reshuffles global weather, just in the opposite direction.
Leachate: Liquid that drains through landfill waste, picking up contaminants. Sanitary landfills collect it to keep it from polluting groundwater.
Lethal dose 50% (LD50): The dose of a chemical that kills 50% of a test population of a species. A standard measure of toxicity—lower LD50 means more toxic.
Limiting nutrient: The nutrient in shortest supply that caps how much an ecosystem can grow. In many undisturbed systems, phosphorus is the limiting factor.
M
Malthusian theory: The idea that human population grows faster than resource supply, eventually hitting limits set by famine, disease, and conflict. It frames Earth's carrying capacity for humans.
Mesothelioma: A cancer of the lining of the lungs caused mainly by inhaling asbestos fibers. A direct link between an indoor pollutant and a human health outcome.
Methylmercury: A highly toxic form of mercury created when bacteria convert elemental mercury that enters aquatic environments. It bioaccumulates and biomagnifies up the food chain.
Mutualism: A symbiotic relationship where both species benefit, like bees pollinating flowers while getting nectar. A win-win interaction.
N
Natural disruption: A non-human disturbance like a wildfire, flood, volcanic eruption, or drought. Their environmental impact can equal or exceed many human-caused disruptions.
Natural gas: The cleanest-burning fossil fuel, made mostly of methane. It produces less carbon dioxide per unit of energy than coal or oil.
Net primary productivity (NPP): GPP minus the energy producers use in respiration — the energy actually available to the next trophic level. NPP = GPP - respiration.
Nitrogen cycle: The movement of nitrogen between the atmosphere, soil, and living things. The atmosphere is the major reservoir, but most other nitrogen reservoirs hold it only briefly.
Nitrogen fixation: The conversion of atmospheric nitrogen gas into ammonia, a form plants can absorb. Done by bacteria (and lightning); without it, plants couldn't access the huge atmospheric nitrogen supply.
Nitrogen oxides (NOx): Reactive gases released when fossil fuels burn at high temperatures, mainly from vehicles and power plants. They drive both photochemical smog and acid deposition.
No-till agriculture: Planting crops without turning over the soil. It preserves topsoil structure, cuts erosion, and retains soil moisture and carbon.
Noise pollution: Sound at levels high enough to cause physiological stress and hearing loss. In wildlife it masks communication and hunting calls and can alter migration routes.
Nonpoint source pollution: Diffuse pollution with no single origin, such as urban runoff or pesticide spraying across farmland. Hard to trace and therefore hard to control.
Nonrenewable energy: Energy from sources that exist in a fixed amount and can't be replaced on a human timescale, like fossil fuels and uranium. They're widely used because they're energy-dense and currently cheap.
Nuclear fission: Splitting Uranium-235 atoms by striking them with neutrons, releasing large amounts of heat. That heat makes steam to spin a turbine and generate electricity.
O
Ocean acidification: The drop in ocean pH as seawater absorbs excess atmospheric CO2 and forms carbonic acid. It makes it hard for corals and shellfish to build calcium carbonate shells.
Ocean conveyor belt: The system of deep ocean currents that carries heat around the globe. Climate change can disrupt it, with big effects on coastal regional climates.
Ocean warming: The rise in ocean temperatures driven by increased greenhouse gases. It causes habitat loss, metabolic and reproductive changes in marine species, and coral bleaching.
Oligotrophic: Describing nutrient-poor water with stable algae populations and high dissolved oxygen. The healthy opposite of a eutrophic waterway.
Overburden: The soil and rock removed to expose an ore deposit in surface mining. It becomes waste that must be managed once mining ends.
Overexploitation: Harvesting a species faster than it can reproduce, through overhunting or overfishing. The 'O' in HIPPCO and a leading cause of endangerment.
Overfishing: Harvesting fish faster than populations can replenish. It can drive species to extreme scarcity, cut aquatic biodiversity, and hurt people who depend on fishing.
Overgrazing: When too many animals feed on an area, stripping its vegetation. The exposed soil erodes and, in dry regions, the land can degrade into desert.
Overshoot: When a population temporarily exceeds its carrying capacity, depleting resources. It sets up a dieback as the environment can no longer support the numbers.
Oxygen sag curve: A graph of dissolved oxygen levels plotted against distance downstream from a pollution source. Oxygen dips near the source as microbes consume waste, then recovers downstream.
Ozone depletion: The thinning of the stratospheric ozone layer, caused by chemicals like CFCs and natural factors. Less ozone means more UV reaches the surface, raising skin cancer and cataract risk.
P
Parasitism: A symbiotic relationship where one species (the parasite) benefits at the expense of the host. The host is harmed but usually not immediately killed.
Parent material: The underlying rock or sediment that weathers to form a soil. Its makeup shapes the soil's texture, nutrients, and fertility.
Particulate matter (PM): Tiny solid or liquid particles suspended in air, such as dust, smoke, and soot. Small particles lodge deep in the lungs, making PM one of the most dangerous air pollutants for human health.
Passive solar: A solar design that absorbs the sun's heat directly through building orientation and materials, with no mechanical equipment. It can't store the energy it captures.
Pathogen: A disease-causing organism that cycles through the environment and adapts to infect human populations, often spreading where sanitation is poor.
Peat: Partially decomposed organic material that can be burned for fuel. It's an early stage in the formation of coal.
Permafrost: Permanently frozen ground in polar regions. As it thaws with warming, it releases stored greenhouse gases like methane, feeding a positive feedback loop.
Permeability: How easily water moves through a soil. Sandy soils are highly permeable and drain fast; clay soils are low-permeability and hold water.
Permeable pavement: Paving that lets water soak through into the soil instead of running off. It's a key tool for reducing urban runoff and recharging groundwater.
Persistent organic pollutants (POPs): Synthetic, carbon-based chemicals like DDT and PCBs that resist breakdown, dissolve in fat, and travel long distances by wind and water. Their persistence makes them biomagnify.
Pesticide resistance: When repeated pesticide use selects for surviving pests through artificial selection, so the chemical stops working. It pushes farmers toward stronger or more frequent applications.
Phosphorus cycle: The movement of phosphorus between rock, soil, water, and organisms. It has no atmospheric component, so phosphorus moves slowly and is often the limiting nutrient in ecosystems.
Photochemical smog: A brownish haze formed when nitrogen oxides and volatile organic compounds react with heat and sunlight, producing ozone and other pollutants. Worst on hot, sunny afternoons in car-heavy cities.
Photosynthesis: The process where producers convert sunlight, CO2, and water into organic compounds and oxygen. It's the entry point of carbon and energy into nearly every food web.
Photovoltaic cell: A solar device that converts sunlight directly into electricity. Its output is limited by how much sunlight is available.
Pioneer species: The first organisms to colonize a disturbed or barren area, like lichens and mosses. They modify conditions and pave the way for later successional species.
Point source pollution: Pollution from a single, identifiable source like a smokestack or discharge pipe. Easy to locate and regulate, unlike nonpoint sources.
Population bottleneck: A sharp reduction in population size that wipes out much of the genetic diversity. Even if numbers recover, the gene pool stays narrow and vulnerable.
Porosity: The fraction of a soil that is open pore space between particles. It controls how much water and air the soil can hold.
Positive feedback loop: A cycle that amplifies a change. In the Arctic, melting ice exposes dark surfaces that absorb more heat, melting more ice and accelerating warming.
Predator-prey relationship: An interaction where one organism (the predator) eats another (the prey). It's a key driver of population cycles, since predator and prey numbers rise and fall in response to each other.
Prescribed burn: Deliberately setting a forest fire under controlled conditions to clear fuel. It reduces the risk of larger, uncontrolled wildfires.
Primary pollutant: A pollutant emitted directly into the air from a source, like carbon monoxide or sulfur dioxide from burning fossil fuels. Contrast with secondary pollutants, which form afterward in the atmosphere.
Primary productivity: The rate at which producers convert solar energy into organic compounds via photosynthesis, measured per unit area per time (e.g., kcal/m2/yr). It sets the energy budget for the whole ecosystem.
Primary sewage treatment: The first stage of wastewater treatment: physically removing large solids with screens and grates, then settling solids in a tank.
Primary succession: Succession that begins on bare rock or lifeless ground with no soil, such as after a volcanic eruption or glacial retreat. Pioneer species must build soil first, so it's slow.
Provisioning services: Ecosystem services that supply tangible products like food, fresh water, timber, and medicine.
R
r-selected species: Species that are small, short-lived, mature early, and produce many offspring with little parental investment. Most invasive species are r-selected, and they bounce back fast.
Radioactivity: The process by which an unstable isotope's nucleus loses energy by emitting radiation. Understanding it corrects the common misconception that all radiation is harmful.
Radon-222: A naturally occurring radioactive gas from the decay of uranium in rocks and soil that seeps into homes through foundations. It is the second leading cause of lung cancer in America.
Rain shadow: A drier region on the leeward side of a mountain, created because the high elevation forces air up and wrings out precipitation before it reaches the far side.
Range of tolerance: The span of an environmental condition within which an organism can survive, with an optimum range where it thrives. Outside it, organisms face stress, reduced reproduction, or death.
Reforestation: Replanting trees in cleared or degraded areas. It restores carbon storage, habitat, and soil protection lost to deforestation.
Regulating services: Ecosystem services that control natural processes, such as water filtration, flood control, pollination, and climate regulation.
Renewable energy: Energy from sources that nature replenishes at or near the rate we use them, like solar, wind, and hydro. They can be reused indefinitely with far less pollution.
Replacement-level fertility: The TFR at which a population exactly replaces itself, holding numbers stable. Below it, a population shrinks over time.
Reservoir (sink): A place where matter in a biogeochemical cycle is stored, sometimes for long periods. On the exam, the reservoir is the step that takes the longest to complete.
Residence time: How long a gas stays in the atmosphere. Water vapor is a greenhouse gas but its short residence time means it doesn't drive long-term climate change the way CO2 does.
Resource partitioning: When species reduce competition by using resources in different ways, places, or times. It lets similar species coexist instead of one wiping out the other.
Rotational grazing: Regularly moving livestock between pastures so no single area is overgrazed. It lets grass recover and protects the soil.
Rule of 70: A shortcut where dividing 70 by the percent growth rate gives a population's doubling time. A 2% growth rate means doubling in about 35 years.
S
Salinization: The buildup of salts in soil after irrigation water evaporates and leaves its dissolved salts behind. Over time it can make soil toxic to plants.
Saltwater intrusion: The movement of seawater into freshwater aquifers, often when coastal groundwater is overpumped. Urbanization can trigger it and contaminate drinking water.
Sanitary landfill: An engineered waste site with a bottom liner, leachate collection, stormwater controls, a cap, and methane collection to limit groundwater and air pollution.
Scrubber: A pollution control device that removes particulates and gases (like SO2) from industrial and power-plant exhaust streams before they reach the air.
Secondary pollutant: A pollutant that forms in the atmosphere when primary pollutants react with sunlight, heat, or other chemicals. Ground-level ozone in smog is the classic example.
Secondary sewage treatment: A biological stage where aerated bacteria break down organic matter in sewage into CO2 and settling sludge.
Secondary succession: Succession on land where soil already exists after a disturbance like a fire or abandoned farm. Faster than primary succession because the soil and seeds remain.
Selective pressure: Any factor that changes the behavior and fitness of organisms in an environment. Strong pressures, like habitat loss, can push a species toward endangerment.
Slash-and-burn agriculture: Clearing land by cutting and burning vegetation to farm it. It frees nutrients short-term but degrades soil and releases carbon dioxide.
Soil erosion: The wearing away of soil by wind or water. It strips fertile topsoil and is a major threat to agriculture, which is why protecting soil also protects the water it filters.
Soil formation: The process by which parent rock is weathered, transported, and deposited to build soil over long timescales. Climate factors like heat and moisture can speed it up or slow it down.
Soil horizon: A distinct layer in a soil profile, defined by its composition and organic content. Reading horizons tells you how a soil formed and how fertile it is.
Soil texture triangle: A diagram that classifies soil type from its percentages of sand, silt, and clay. It's the standard tool for comparing soils on the exam.
Solar radiation intensity: How concentrated the sun's energy is on a surface, set by the angle of the sun's rays. The equator receives the most per unit area; intensity decreases toward the poles.
Solid waste: Any discarded material that is not a liquid or gas, generated by homes, industry, business, and agriculture. Most ends up in landfills.
Specialist species: A species with a narrow niche and specific requirements for food or habitat. Advantaged in stable environments but vulnerable when conditions change.
Species diversity: The variety of different species in an ecosystem. Ecosystems with more species are more likely to recover from disruptions.
Species richness: The number of different species found in an ecosystem. A simple but core measure of biodiversity.
Spray irrigation: Irrigation that pumps groundwater through spray nozzles over a field. It's more efficient than flood or furrow methods but costs more and needs energy to run.
Stratosphere: The atmospheric layer above the troposphere that holds the ozone layer. Temperature rises with altitude here because ozone absorbs incoming UV radiation.
Stratospheric ozone layer: A layer of ozone in the stratosphere that absorbs harmful UV radiation, protecting life on Earth. Distinct from harmful ground-level ozone in smog.
Sulfur dioxide (SO2): A pollutant released mainly from burning coal and diesel fuel. It converts to sulfuric acid in the atmosphere, a key driver of acid deposition.
Supporting services: The underlying ecosystem services that make all others possible, like nutrient cycling, soil formation, and photosynthesis.
Surface mining: Removing large amounts of soil and rock (overburden) to reach ore near the surface. Strip mining is one form; it exposes land and makes it prone to erosion.
Survivorship curve: A graph of the proportion of a cohort surviving at each age. Type I (late losses), Type II (steady losses), and Type III (early losses) describe different life strategies.
Sustainability: Using resources to meet present needs without depleting them for future generations. Indicators like biodiversity, food production, and CO2 levels help track whether we're on a sustainable path.
Sustainable yield: The amount of a renewable resource that can be harvested without shrinking the available supply. Stay at or below it and the resource keeps replenishing.
Symbiosis: A close, long-term interaction between two species. The three AP types are mutualism, commensalism, and parasitism.
T
Tailings: The leftover waste material after the valuable minerals have been extracted from ore. Along with slag, tailings are a major source of mining pollution.
Tar sands: A mix of clay, sand, water, and bitumen from which crude oil can be extracted. Recovering oil this way takes far more energy and processing than conventional drilling.
Terracing: Carving a steep slope into stepped, level platforms for farming. It reduces runoff and erosion on hilly land.
Terrestrial biome: A land-based biome such as taiga, tropical rainforest, temperate grassland, savanna, desert, or tundra. Each results from a distinct combination of temperature and rainfall.
Tertiary sewage treatment: The final stage using ecological or chemical processes to remove remaining pollutants, often followed by disinfection with chlorine, ozone, or UV before discharge.
Thermal inversion: A condition where a layer of warm air sits above cooler surface air, reversing the normal temperature gradient and trapping pollutants close to the ground. Why smog lingers in valleys and basins.
Thermal pollution: The release of heated water into an ecosystem, often from power-plant cooling, that harms aquatic organisms. Warm water holds less dissolved oxygen, stressing the system.
Tidal energy: Electricity generated by the flow of ocean tides turning a turbine. It's a renewable form of hydropower tied to predictable tidal cycles.
Tilling: Mechanically turning over soil to prepare for planting. It can damage the environment by loosening topsoil and accelerating erosion.
Total fertility rate (TFR): The average number of children a woman is expected to have in her lifetime. Influenced by age at first child, female education, family planning access, and government policy.
Tragedy of the commons: The idea that individuals using a shared resource for their own self-interest will deplete it, because no single user bears the full cost. It explains overfishing, overgrazing, and pollution of shared air and water.
Transform boundary: A plate boundary where two plates slide past each other horizontally. No crust is made or destroyed, but the grinding builds stress that releases as earthquakes.
Trophic level: A feeding position in a food chain, from producers up through primary, secondary, and tertiary consumers. Energy flows from the sun to producers and then upward.
Tropical rainforest: A hot, wet biome near the equator with the highest biodiversity of any terrestrial biome. High productivity but surprisingly nutrient-poor soils.
Troposphere: The lowest atmospheric layer, where weather happens and where we live. Temperature drops with altitude here, and most air pollution accumulates in it.
Tropospheric ozone: Ozone at ground level, a harmful secondary pollutant in smog that irritates lungs and worsens respiratory problems. The opposite of stratospheric ozone, which protects us.
Tundra: A cold, treeless biome with permafrost and low precipitation, found at high latitudes and altitudes. Low productivity because of the short growing season and frozen ground.
Type I survivorship curve: A curve showing high survival through youth and adulthood with most deaths late in life, typical of K-selected species like humans and large mammals.
Type III survivorship curve: A curve showing very high early mortality with the few survivors living long, typical of r-selected species like fish and insects that produce many offspring.
U
Uranium-235: The fissionable isotope used as fuel in nuclear reactors. It stays radioactive for a very long time, which is the core challenge of nuclear waste disposal.
Urban sprawl: The spread of low-density development from cities into surrounding rural land. It consumes habitat, increases driving, and strains resources.
V
Vapor recovery nozzle: An air pollution control device on a gas pump that captures fuel fumes during refueling instead of letting VOCs escape into the air.
Volatile organic compounds (VOCs): Carbon-based chemicals like formaldehyde and gasoline fumes that evaporate easily at room temperature. They react with nitrogen oxides to form photochemical smog; trees are a natural source.
W
Water holding capacity: The total amount of water a soil can retain. Higher capacity supports more plant growth, so it directly drives land productivity and fertility.
Waterlogging: When too much irrigation water sits in the soil, raising the water table and starving plant roots of oxygen. It's a common drawback of inefficient irrigation.
Watershed: All the land that drains into a common body of water. Its area, slope, soil, and vegetation determine how much water and pollution end up downstream.
Wetlands: Areas where water covers the soil all or part of the time. They provide water purification, flood protection, and habitat, making their loss to development costly.
Wind turbine: A machine that converts the kinetic energy of moving air into electricity. Wind is renewable and clean, though spinning blades can kill birds and bats.

Where these terms show up

Every AP Environmental Science term below belongs to one of the units on the exam. Each unit page has sample questions that put the vocabulary to work:

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