AP Environmental Science Study Guide

Ecosystems are communities of organisms interacting with each other and with the nonliving (abiotic) environment. AP Environmental Science expects you to trace energy and matter as they move through these systems — and to explain what happens when human activity disrupts those flows.

Biomes: Terrestrial and Aquatic

A biome is a large region with a characteristic climate, vegetation, and animal community. Climate — principally temperature and precipitation — determines which biome exists in a given location.

Biome	Climate	Key Features
Tropical Rainforest	Hot, wet year-round	Highest biodiversity; nutrient-poor soils; rapid decomposition
Temperate Deciduous Forest	Moderate temp, seasonal rainfall	Leaf litter; fertile soils; four seasons
Grassland/Savanna	Warm, low-moderate rainfall	Fire-maintained; large herbivore populations
Desert	Extreme heat/cold; <25 cm rain/yr	CAM plants; nocturnal animals; high evaporation
Taiga (Boreal Forest)	Cold, low precipitation	Conifers; acidic, nutrient-poor soils; short growing season
Tundra	Bitterly cold; very dry	Permafrost; low biodiversity; carbon storage

Aquatic biomes include freshwater (lakes, rivers, wetlands) and marine (ocean zones, coral reefs, estuaries). Estuaries — where rivers meet the ocean — are among the most productive ecosystems on Earth.

Biogeochemical Cycles

Matter cycles; energy flows. The four cycles most heavily tested on APES are carbon, nitrogen, phosphorus, and water.

• Carbon Cycle: Photosynthesis removes CO₂ from air; respiration and combustion return it. Oceans absorb ~30% of anthropogenic CO₂. Fossil fuel combustion is the primary human disruption.
• Nitrogen Cycle: N₂ gas must be "fixed" into ammonium (NH₄⁺) by nitrogen-fixing bacteria before plants can use it. Nitrification converts NH₄⁺ → NO₂⁻ → NO₃⁻. Denitrification converts NO₃⁻ back to N₂. Fertilizer over-application disrupts the cycle and causes eutrophication.
• Phosphorus Cycle: No atmospheric reservoir — phosphorus moves through rocks, soil, and water only. Weathering releases phosphate (PO₄³⁻); runoff carries it into aquatic systems where it is a common limiting nutrient for algal growth.
• Water (Hydrologic) Cycle: Evaporation, transpiration (combined = evapotranspiration), condensation, precipitation, infiltration, runoff, and groundwater recharge. Deforestation reduces transpiration and increases runoff.

Energy Flow: Trophic Levels and the 10% Rule

Energy enters most ecosystems through photosynthesis by producers. It is then passed to primary consumers (herbivores), secondary consumers, tertiary consumers, and finally decomposers. At each transfer, approximately 90% of energy is lost as metabolic heat — only ~10% is stored in biomass and available to the next level. This is the 10% rule.

Biodiversity refers to the variety of life on Earth at three levels: genetic diversity (variation within a species), species diversity (number and relative abundance of species in an area), and ecosystem diversity (variety of habitats, communities, and ecological processes). Higher biodiversity generally makes ecosystems more resilient to disturbance.

Ecosystem Services

Healthy ecosystems provide services humans depend on:

• Provisioning: Food, freshwater, timber, medicine (tangible goods)
• Regulating: Climate regulation, water purification, pollination, flood control, carbon sequestration
• Cultural: Recreation, tourism, aesthetic and spiritual value
• Supporting: Nutrient cycling, soil formation, primary production (the foundation for the other three)

Ecological Niches and Tolerance

A species' fundamental niche is the full range of conditions it can theoretically tolerate. Its realized niche is the narrower range it actually occupies due to competition and predation. Tolerance curves show that each species has an optimal range for environmental factors (temperature, pH, salinity) with zones of physiological stress at the extremes and lethal limits beyond that.

Ecological Succession

Communities change over time in a predictable pattern called succession:

• Primary succession occurs on bare rock or new substrate with no soil (e.g., after a volcanic eruption or glacier retreat). Pioneer species — typically lichens and mosses — break down rock and accumulate organic matter, slowly building soil. Takes hundreds to thousands of years to reach a climax community.
• Secondary succession occurs after a disturbance removes the community but leaves soil intact (e.g., after fire, logging, or agricultural abandonment). Faster than primary succession because soil and seed banks remain. Grasses → shrubs → pioneer trees → climax forest.

Keystone species have an outsized influence on community structure relative to their biomass (e.g., sea otters controlling sea urchin populations, which in turn protects kelp forests). Indicator species signal changes in environmental quality — their presence or absence reflects ecosystem health (e.g., mayfly larvae indicate clean water).

Island Biogeography

The theory of island biogeography (MacArthur and Wilson) predicts that species richness on an island is determined by immigration rate (higher for islands closer to a mainland) and extinction rate (lower on larger islands). The same principles apply to habitat fragments — isolated forest patches function like "islands" and lose species over time.

Population ecology examines how and why population sizes change. APES tests both the underlying biology (growth models, life history strategies) and the human demographic patterns that drive environmental impact.

Life History Strategies: r vs. K Selection

Trait	r-selected (opportunistic)	K-selected (equilibrium)
Body size	Small	Large
Offspring per year	Many (hundreds to thousands)	Few (1–5)
Parental care	Little to none	Extensive
Time to reproductive maturity	Short	Long
Lifespan	Short	Long
Survivorship curve	Type III (high early mortality)	Type I (low early mortality)
Examples	Insects, mice, dandelions	Elephants, whales, humans, oaks

Survivorship curves: Type I (humans, elephants) — most individuals survive to old age, then die in a short window. Type II (birds, lizards) — constant mortality rate at all ages. Type III (fish, oysters, most insects) — extremely high mortality in early life; survivors tend to live long.

Population Growth Models

Exponential growth occurs when resources are unlimited: the population grows at a constant per-capita rate. The J-shaped curve has no upper limit. Logistic growth occurs when resources are limited: growth slows as the population approaches carrying capacity (K), producing an S-shaped (sigmoidal) curve. Limiting factors — food, water, shelter, disease, predation — impose K.

Human Population and the Demographic Transition Model

The Demographic Transition Model (DTM) describes how societies move from high birth and death rates (pre-industrial) to low birth and death rates (post-industrial):

• Stage 1: High CBR, high CDR → slow population growth. Pre-industrial, agrarian societies.
• Stage 2: CBR remains high; CDR drops (due to sanitation, medicine, food security) → rapid population growth. Many developing nations entered this stage in the 20th century.
• Stage 3: CBR begins to fall (urbanization, education, women's rights, family planning) → growth slows. Middle-income countries.
• Stage 4: Both CBR and CDR low → near-zero population growth. Most developed nations.
• Stage 5 (some models): CDR exceeds CBR → population decline. Germany, Japan, Italy.

Age structure diagrams (population pyramids) reveal a country's growth trajectory. A wide base = young population, rapid growth (Stage 2–3). Equal widths = stable population (Stage 4). Wider top = aging population, potential decline (Stage 5).

Earth is a set of interacting physical systems — the lithosphere, hydrosphere, atmosphere, and biosphere. Understanding their structure and dynamics explains where soil forms, why climates differ by latitude, and how ocean temperature anomalies affect weather globally.

Plate Tectonics and Soil Formation

Earth's crust is divided into tectonic plates that move via convection currents in the mantle. Divergent boundaries create new crust (mid-ocean ridges, rift valleys). Convergent boundaries cause subduction and mountain building (Himalayas, Andes, volcanic arcs). Transform boundaries produce earthquakes (San Andreas Fault). Volcanic activity concentrates in subduction zones and rift zones.

Soil formation (pedogenesis) begins with weathering of parent rock material. Physical weathering breaks rock into smaller pieces; chemical weathering alters minerals. Organic matter from decomposing organisms builds the upper soil layers. Fully developed soil has distinct horizons:

Horizon	Name	Characteristics
O	Organic	Decomposing leaf litter and humus; surface layer
A	Topsoil	Rich in organic matter and microorganisms; most fertile; lost first to erosion
B	Subsoil	Mineral-rich; receives leached materials from above; less organic matter
C	Parent material	Partially weathered rock; little organic matter
R	Bedrock	Unweathered rock; parent of the soil above

Soil texture is determined by the relative proportions of sand, silt, and clay. Loam (balanced mix) is ideal for agriculture — it has good water retention, aeration, and nutrient-holding capacity. Clay soils hold water but drain poorly; sandy soils drain rapidly but hold little water or nutrients.

Earth's Atmosphere

The atmosphere is composed of ~78% N₂, ~21% O₂, ~0.93% Ar, and ~0.04% CO₂ plus trace gases. Four main layers:

• Troposphere (0–12 km): Where weather occurs; temperature decreases with altitude; contains 75% of atmospheric mass.
• Stratosphere (12–50 km): Temperature increases with altitude (ozone absorbs UV); contains the ozone layer (15–35 km).
• Mesosphere (50–85 km): Coldest layer; meteors burn up here.
• Thermosphere (85–600 km): Extremely hot; very thin; aurora borealis occurs here.

Global Wind Patterns and Climate

Differential solar heating drives circulation cells: Hadley cells (0°–30°) create trade winds and the ITCZ (Intertropical Convergence Zone) — a band of heavy tropical rainfall. Ferrel cells (30°–60°) drive prevailing westerlies. Polar cells (60°–90°) drive polar easterlies. The Coriolis effect deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, creating the major wind belts.

Climate is determined by latitude, altitude, ocean currents, prevailing winds, and distance from the ocean (continentality). Rain shadows form on the leeward side of mountain ranges, where descending dry air creates deserts (e.g., the Great Basin east of the Sierra Nevada).

El Niño occurs when trade winds weaken, allowing warm Pacific water to push east. This suppresses upwelling off South America, reducing fish populations, increases eastern Pacific rainfall, and causes drought in Australia and Indonesia. La Niña is the opposite: stronger trade winds push warm water west, intensifying typical patterns.

Unit 5 covers the environmental consequences of how humans extract and manage natural resources. APES consistently tests your ability to identify the specific environmental impact of each land-use practice and propose concrete, realistic solutions.

The Tragedy of the Commons

Ecologist Garrett Hardin's 1968 concept: when a resource is shared and open access, rational self-interest drives each user to maximize their own extraction, leading to collective overexploitation and eventual collapse. Classic examples include ocean fisheries, shared grazing land, and the global atmosphere. Solutions involve privatization, regulation, or community-based management (Elinor Ostrom's "governing the commons").

Agriculture: Practices, Impacts, and Alternatives

Practice	Environmental Impact	Sustainable Alternative
Monoculture	Soil depletion, high pesticide use, vulnerability to pests/disease	Crop rotation, polyculture, agroforestry
Flood/furrow irrigation	Salinization, waterlogging, groundwater depletion	Drip irrigation (most efficient); sprinkler systems
Synthetic fertilizers	Nitrogen/phosphorus runoff → eutrophication	Organic fertilizers, precision agriculture, buffer strips
Broad-spectrum pesticides	Non-target species harm, resistance, bioaccumulation	Integrated Pest Management (IPM), biological control
CAFOs (feedlots)	Waste runoff, greenhouse gases, antibiotic resistance	Pasture-based grazing, reduced meat consumption
Tillage	Soil erosion, carbon release from soil	No-till farming, contour plowing, terracing

The Green Revolution (1950s–1970s) dramatically increased global food production through high-yield crop varieties, synthetic fertilizers, pesticides, and large-scale irrigation. It averted mass starvation but created long-term environmental costs: soil degradation, groundwater depletion, and increased chemical runoff.

Forestry

Clearcutting removes all trees in an area, maximizing short-term timber yield but causing erosion, habitat loss, stream sedimentation, and loss of biodiversity. Selective cutting removes only mature or targeted trees, maintaining forest structure. Shelterwood cutting removes trees in phases, allowing regeneration. Reforestation (planting trees) and afforestation (planting on previously unforested land) can restore ecosystem services.

Mining, Urbanization, and Overfishing

Surface mining (strip mining, open-pit) destroys habitat and generates acid mine drainage when sulfide minerals oxidize in air and water, producing sulfuric acid that leaches heavy metals into streams. Subsurface mining has lower surface impact but risks collapse and methane release.

Urbanization replaces permeable land with impervious surfaces (roads, rooftops), increasing stormwater runoff, flooding, and pollution. Green infrastructure — permeable pavement, rain gardens, bioswales, green roofs — reduces these impacts.

Overfishing reduces populations below sustainable yield, collapses food webs, and harms non-target species through bycatch. The Maximum Sustainable Yield (MSY) concept tries to set harvest limits at the level where fish populations can replenish themselves annually.

Energy resources are either nonrenewable (finite stock that replenishes on geological timescales: coal, oil, natural gas, uranium) or renewable (replenish on human timescales: solar, wind, hydro, geothermal, biomass). APES expects you to compare energy sources across multiple dimensions: environmental impact, reliability, cost, and feasibility.

Fossil Fuels

Coal, oil, and natural gas are formed from ancient organic matter compressed over millions of years. They provide ~80% of global primary energy but release CO₂ and other pollutants when burned.

• Coal: Most carbon-intensive; surface mining causes habitat destruction and acid mine drainage; combustion releases SO₂, NOₓ, particulates, mercury, and CO₂.
• Oil: Extracted by conventional drilling or hydraulic fracturing (fracking). Spills damage marine and terrestrial ecosystems. Refining produces various fuels and chemical feedstocks.
• Natural Gas: Cleanest-burning fossil fuel; primarily methane. Fracking extracts gas from shale but risks groundwater contamination and methane leaks (methane is a potent greenhouse gas).

Nuclear Power

Nuclear fission splits heavy atoms (uranium-235) releasing enormous heat used to generate steam and electricity. Produces no direct CO₂ emissions during operation. Concerns: radioactive waste (remains hazardous for 10,000+ years), high construction costs, low probability but high consequence accident risk (Chernobyl, Fukushima), and uranium mining impacts. Produces ~10% of global electricity.

Renewable Energy: Comparison

Source	How it works	Advantages	Disadvantages
Solar (PV)	Photovoltaic cells convert sunlight to electricity	No emissions; widely deployable; declining costs	Intermittent; land use; manufacturing impacts
Wind	Turbines convert wind kinetic energy to electricity	No emissions; low land-use impact (dual-use farmland)	Intermittent; bird/bat mortality; noise; visual impact
Hydroelectric	Dams or run-of-river systems use water flow	Reliable; long lifespan; flood control; storage	Habitat disruption; fish migration blocked; community displacement; silting
Geothermal	Earth's internal heat boils water for steam turbines	Reliable baseload; low emissions; small footprint	Geographically limited; induced seismicity; high upfront cost
Biomass	Burning organic material (wood, crops, waste)	Carbon-neutral in theory; uses waste material	Air pollution from combustion; land/water competition; net carbon neutrality disputed
Hydrogen Fuel Cell	H₂ + O₂ → H₂O + electricity	Only emission is water; high energy density	H₂ production currently energy-intensive; storage/distribution challenges

Primary pollutants are emitted directly from a source: CO, SO₂, NOₓ, particulate matter (PM), VOCs. Secondary pollutants form in the atmosphere through chemical reactions between primary pollutants and sunlight: ground-level ozone (O₃), sulfuric acid (H₂SO₄), nitric acid (HNO₃), and photochemical smog.

Photochemical Smog

Photochemical smog forms in sunny, urban areas with heavy traffic. The sequence:

1. Cars and industrial sources emit NOₓ and volatile organic compounds (VOCs).
2. Sunlight drives photochemical reactions that convert NO₂ into NO + O (atomic oxygen).
3. Atomic oxygen reacts with O₂ to form ground-level ozone (O₃).
4. O₃ reacts with VOCs to produce a mix of secondary pollutants — this brown haze is smog.

Health impacts: respiratory irritation, asthma exacerbation, reduced lung function, eye irritation. Ecological impacts: crop damage, forest decline, reduced photosynthesis. Controls: catalytic converters on vehicles, reduced VOC emissions, vapor recovery at gas stations.

Thermal Inversions

Normally, temperature decreases with altitude in the troposphere, allowing warm surface air to rise and carry pollutants upward (dispersion). A thermal inversion occurs when a layer of warm air sits above cooler surface air, trapping pollutants at ground level. Common in valleys surrounded by mountains (Los Angeles, Salt Lake City) and during calm, clear nights. Air quality crises can result — the 1952 London "Great Smog" killed ~4,000 people in four days.

Acid Deposition

Sulfur dioxide (SO₂, from coal combustion and smelters) and nitrogen oxides (NOₓ, from vehicles and power plants) react in the atmosphere with water vapor to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃), which fall as acid rain, snow, or dry deposition. Normal rain pH ≈ 5.6 (slightly acidic from natural CO₂); acid rain pH < 5.6, sometimes below 4.

Environmental impacts: acidification of lakes and streams (kills fish and amphibians), leaching of calcium and magnesium from soils (weakening trees), damage to limestone statues and buildings. Controls: scrubbers (remove SO₂ from flue gas), catalytic converters, switching to low-sulfur fuels, cap-and-trade programs.

Indoor Air Pollution

• Radon (Rn): Radioactive gas from uranium decay in soils and rock; seeps into basements. #1 cause of lung cancer in non-smokers.
• Carbon monoxide (CO): Colorless, odorless; from incomplete combustion (gas appliances, cars). Displaces O₂ from hemoglobin.
• Asbestos: Fibrous mineral insulation; inhaled fibers cause mesothelioma (lung cancer) decades later.
• VOCs and formaldehyde: Off-gassed from building materials, paints, adhesives, furniture. Irritants and potential carcinogens.

Key legislation: The U.S. Clean Air Act (1970, amended 1990) established the National Ambient Air Quality Standards (NAAQS) for six criteria pollutants: particulate matter, ozone, CO, SO₂, NO₂, and lead. The EPA regulates these under two tiers: primary standards (protect human health) and secondary standards (protect environmental and social welfare).

Pollution enters ecosystems from point sources (single, identifiable locations: factory pipes, sewage outfalls) or nonpoint sources (diffuse, widespread: agricultural runoff, urban stormwater, atmospheric deposition). Nonpoint source pollution is harder to regulate and is the leading cause of water quality impairment in the United States.

Eutrophication

Eutrophication is the nutrient enrichment of water bodies, most commonly caused by nitrogen and phosphorus runoff from agriculture and sewage. The process:

1. Excess nutrients → explosive algal growth (algal bloom)
2. When algae die, decomposers consume them and use up dissolved oxygen (O₂)
3. Hypoxic (low O₂) or anoxic (no O₂) dead zones form, killing fish and other aerobic organisms
4. Only anaerobic bacteria survive; they produce sulfides and methane, which make water smell

The Gulf of Mexico dead zone, fed by the Mississippi River's agricultural drainage, is one of the largest in the world (~22,000 km² at its peak). Solutions include riparian buffer strips, reducing fertilizer application, constructed wetlands to filter runoff, and upgraded sewage treatment.

Bioaccumulation and Biomagnification

Bioaccumulation refers to the buildup of a persistent toxic substance within an individual organism faster than it can be eliminated. Biomagnification (biological magnification) is the increase in concentration of a toxin at each successive trophic level in a food chain. Fat-soluble, persistent compounds (DDT, PCBs, mercury, dioxins) biomagnify because they are not metabolized and accumulate in fatty tissues. Top predators — including humans eating large marine fish — receive the highest concentrations.

Persistent Organic Pollutants (POPs)

POPs are synthetic organic compounds that resist environmental degradation, accumulate in fatty tissues, and biomagnify through food webs. Regulated globally by the Stockholm Convention (2001). Key examples:

• DDT: Insecticide; thinned bird eggshells (bald eagle decline); Rachel Carson's Silent Spring (1962) documented its effects and launched the modern environmental movement. Banned in the U.S. in 1972.
• PCBs: Industrial fluids; carcinogens; persist in sediments.
• Dioxins: Industrial byproducts; endocrine disruptors.

Sewage Treatment

• Primary treatment: Physical — screens and sedimentation tanks remove solid debris and settle suspended solids (~60% of solids removed).
• Secondary treatment: Biological — aerobic bacteria decompose dissolved organic matter in aeration tanks; ~90% of organic matter removed. Produces activated sludge.
• Tertiary treatment: Advanced chemical and physical processes remove nitrogen, phosphorus, and pathogens. Required before discharge into sensitive water bodies. Includes chlorination, UV treatment, or ozonation for disinfection.

Solid Waste

The waste hierarchy ranks options from most to least preferred: Reduce → Reuse → Recycle → Recover (energy) → Dispose (landfill/incinerate). Modern sanitary landfills have clay and plastic liners, leachate collection systems, and methane capture for energy. Incineration reduces waste volume by ~90% and can generate electricity but produces air pollutants if not properly controlled.

Unit 9 carries the greatest exam weight and requires the deepest understanding. Global environmental changes are interconnected — climate change, ozone depletion, ocean acidification, and biodiversity loss are all driven or worsened by human activity, and their solutions often overlap.

Stratospheric Ozone Depletion

The ozone layer (15–35 km altitude in the stratosphere) absorbs 97–99% of incoming UV-B and UV-C radiation, protecting life from DNA damage, skin cancer, and cataracts. Chlorofluorocarbons (CFCs) — formerly used in refrigerants, aerosol propellants, and foam insulation — are the primary ozone-depleting substances. UV radiation breaks CFC molecules in the stratosphere, releasing chlorine radicals that catalytically destroy O₃ molecules in a chain reaction (one Cl atom can destroy 100,000 O₃ molecules). The Antarctic ozone hole forms each spring as polar stratospheric clouds catalyze ozone destruction.

The Montreal Protocol (1987) is the landmark international treaty phasing out ozone-depleting substances. It is considered the most successful environmental treaty ever enacted. CFCs have been largely replaced by HCFCs and HFOs. The ozone layer is recovering and is projected to return to 1980 levels by approximately 2070 over Antarctica.

The Greenhouse Effect and Climate Change

The natural greenhouse effect is essential to life: solar shortwave radiation passes through the atmosphere and warms Earth's surface; Earth re-emits longwave (infrared) radiation, which greenhouse gases (CO₂, H₂O vapor, CH₄, N₂O, O₃) absorb and re-radiate in all directions, warming the lower atmosphere. Without it, Earth's average temperature would be −18°C instead of +15°C.

The enhanced greenhouse effect results from human emissions amplifying this natural process:

• CO₂: ~420 ppm (2024), up from 280 ppm pre-industrial. Sources: fossil fuel combustion (primary), deforestation, cement production. Atmospheric half-life: hundreds of years.
• CH₄ (methane): 25–28× more potent than CO₂ over 100 years. Sources: livestock digestion, rice paddies, landfills, wetlands, fossil fuel extraction and leaks.
• N₂O (nitrous oxide): 265–310× more potent than CO₂. Sources: synthetic nitrogen fertilizers, animal waste, combustion.
• Fluorinated gases (HFCs, SF₆): Extremely potent (thousands of times CO₂); used in refrigeration and industrial processes.

Positive feedbacks amplify warming: ice-albedo feedback (melting ice exposes darker ocean/land → absorbs more heat → more melting), water vapor feedback (warming → more evaporation → more water vapor → more warming), and permafrost thaw feedback (frozen soil thaws → releases stored CH₄ and CO₂).

Observed and Projected Climate Impacts

• Global average temperature has risen ~1.2°C since pre-industrial; Arctic is warming 3–4× faster.
• Sea level rise from thermal expansion and ice melt (glaciers, Greenland, West Antarctic Ice Sheet); projected 0.3–1.0 m by 2100 under various scenarios.
• More frequent and intense extreme weather: heat waves, droughts, wildfires, hurricanes, flooding.
• Range shifts of species poleward and to higher elevations; phenological mismatches (flowers blooming before pollinators emerge).
• Coral reef bleaching: water temperatures 1–2°C above average for several weeks cause zooxanthellae expulsion; prolonged bleaching kills the coral.

Ocean Warming and Acidification

Oceans have absorbed >90% of the excess heat from the enhanced greenhouse effect and ~30% of anthropogenic CO₂. Ocean warming causes thermal stratification (reduces nutrient mixing), range shifts of marine species, and intensification of hurricanes. When CO₂ dissolves in seawater, it forms carbonic acid, which lowers ocean pH — a process called ocean acidification. Ocean pH has dropped from ~8.2 to ~8.1 since the industrial revolution (a 0.1 unit drop = 26% increase in acidity). This reduces carbonate ion availability, impairing shell formation in oysters, mussels, corals, and pteropods (sea butterflies), with cascading effects through marine food webs.

Invasive Species and Biodiversity Loss

Invasive species are non-native organisms that spread rapidly in their new environment — often because they lack natural predators, parasites, or competitors. Examples: zebra mussels (clog pipes, displace native mussels), kudzu vine (smothers forests in the southeastern U.S.), Burmese pythons (decimate mammals in the Everglades), Asian carp (outcompete native fish, threaten Great Lakes). Prevention — strict border controls on imported organisms — is far more cost-effective than eradication.

The current rate of species extinction is estimated at 100–1,000× the natural background rate, leading scientists to call this the Sixth Mass Extinction. Primary drivers: habitat loss and fragmentation, invasive species, pollution, overexploitation, and climate change. The Endangered Species Act (ESA, 1973) lists species as threatened or endangered and mandates recovery plans; it prohibits federal actions that jeopardize listed species or harm their critical habitat.