AP Environmental Science · 2026 · Final 48 Hours

Final Sprint — Everything That Matters

Read this file once the night before. Skim key sections on exam morning. Do not study anything new at this stage — your score is already built. This file only reminds you of what you already know.

No New Content 60 min to read fully 90 min MCQ · 70 min FRQ Calculator ✓ · FRQ 3 Ref Sheet ✓

Section 01

Key Formulas — Every One You Need

10% Rule · Energy Transfer

E_level_N = E_base × 0.10^(N−1)

From producers → level N: divide by 10 for each transfer. Count transfers, not level numbers. Level 3 = ÷ 100; Level 4 = ÷ 1,000.

Count transfers, not level numbers. Level 3 = 2 transfers = ÷ 10² = ÷ 100.

GPP / NPP / Respiration

NPP = GPP − R_plants

GPP = gross photosynthesis. NPP = what's available to herbivores. R = plant cellular respiration. GPP is always > NPP. Food chains use NPP.

NPP is always smaller than GPP. If you get NPP > GPP, recheck.

Population Growth Rate

r (%) = (BR − DR) ÷ 10

BR and DR are given per 1,000 (‰). Divide by 10 to convert to percent before using Rule of 70.

BR = 30‰ → 3%, not 0.03%. Dividing by 1,000 gives a nonsense answer.

Rule of 70 · Doubling Time

T₂ = 70 ÷ r (%)

r must be in PERCENT form (e.g., 2), not decimal (0.02). 2% growth → 35 years; 1% → 70 years; 3.5% → 20 years.

Using 0.02 instead of 2 gives T₂ = 3,500 years. Always use the percent number.

IPAT

I = P × A × T

I = impact; P = population; A = affluence (GDP/capita); T = technology (impact per $ GDP). Green tech makes T smaller.

Improved technology → T decreases (not increases). "Reduces by 60%" means T × 0.4, not × 0.6.

Percent Change

% Δ = (New − Old) ÷ Old × 100

Denominator is always the OLD (baseline) value. Example: CO₂ 280→420 ppm = (420−280)÷280×100 = 50%, not 33%.

Using the new value as denominator is a common error — always divide by OLD.

Energy Efficiency

Useful output = Input × efficiency

Waste heat = Input − Useful output. Efficiency as decimal: 38% = 0.38. EROI = Energy out ÷ Energy invested.

Forget to convert % to decimal → answer is 100× too large.

pH / Acidity

Each −0.1 pH unit = +26% more H⁺

Normal rain pH ≈ 5.6 (dissolved CO₂). Acid rain < 5.6. Ocean acidification: pH 8.2 → 8.1 = 26% more acidic. Still alkaline, not acid.

Ocean at pH 8.1 is still alkaline — do not call it "acidic." The term is "more acidic" or "acidified."

Section 02

15 Concept Confusion Pairs — Highest-Tested

❌ Ozone Depletion

CFCs → stratosphere → Cl• destroys O₃ → UV-B increase → skin cancer · Montreal Protocol (1987)

✓ Climate Change

CO₂/CH₄ → troposphere → IR radiation trapped → warming · Paris Agreement (2015)

Different gases · different layers · different effects · different treaties

❌ GPP (Gross)

ALL energy fixed by photosynthesis — includes what plants use themselves. Always larger.

✓ NPP (Net)

GPP − plant respiration. What's actually available to herbivores. Use NPP for food chain calculations.

GPP always > NPP · food chains use NPP not GPP

❌ Nitrification

NH₄⁺ → NO₃⁻ · aerobic · keeps nitrogen in soil · toward "usable" form

✓ Denitrification

NO₃⁻ → N₂ · anaerobic (waterlogged soil) · removes nitrogen from ecosystem

Flooded soil → denitrification increases · nitrification stops

❌ Acid Rain

SO₂ + NOₓ dissolve in water → H₂SO₄ + HNO₃ · from coal combustion/smelters · falls downwind

✓ Ocean Acidification

CO₂ dissolves in seawater → H₂CO₃ → pH drops · carbonate less available → coral/shell damage

Completely different pollutants, reactions, and solutions

❌ Primary Succession

Bare rock/substrate · NO soil · pioneer = lichens · centuries to climax

✓ Secondary Succession

Soil remains (fire, flood, abandoned field) · pioneer = grasses · decades to climax · seed bank intact

Single deciding factor: is soil present?

❌ Bioaccumulation

Toxin builds up within ONE organism over its lifetime

✓ Biomagnification

Toxin concentration increases ACROSS trophic levels. Top predator always has highest concentration.

Across food chain = magnification · within one animal = accumulation

❌ Tundra (not taiga)

NO trees · permafrost (permanently frozen subsoil) · <25 cm precipitation · extreme cold year-round

✓ Taiga (Boreal Forest)

Conifer trees (spruce, fir) · long cold winters · 40–100 cm precipitation · NO permafrost

Permafrost alone = sufficient to identify tundra

❌ Point Source

Single identifiable outlet (factory pipe, sewage plant outfall) · regulated by Clean Water Act NPDES

✓ Non-Point Source

Diffuse origin (farm runoff, urban stormwater) · NOT directly regulated by CWA · managed by BMPs

Agricultural runoff = non-point · not regulated by NPDES

❌ Photochemical Smog

VOCs + NOₓ + sunlight → ground-level O₃ + PANs · sunny afternoons · car-dense cities

✓ Industrial (London) Smog

SO₂ + particulates + fog → sulfuric acid aerosol · cold damp weather · coal heating/industry

Ozone is the harm in photochemical · SO₂ is the harm in industrial

❌ Climate Mitigation

Reduces GHG emissions = addresses the CAUSE · e.g., renewable energy, efficiency, reforestation

✓ Climate Adaptation

Adjusts to climate impacts = addresses the CONSEQUENCES · e.g., seawalls, drought-resistant crops

Seawalls = adaptation (not mitigation) · Solar panels = mitigation

❌ CITES

International treaty · regulates cross-border TRADE in endangered species only

✓ ESA (Endangered Species Act)

U.S. domestic law · protects listed species HABITAT · prohibits federal projects that "jeopardize" species

Trade between countries = CITES · U.S. habitat/federal projects = ESA

❌ Kyoto Protocol

1997 · binding targets for DEVELOPED nations only · U.S. did not ratify

✓ Paris Agreement

2015 · voluntary NDCs from ALL countries including China/India · 1.5–2°C goal · weak enforcement

Most inclusive = Paris · Most binding targets = Kyoto (developed only)

❌ Renewable Energy

Naturally replenished (solar, wind, hydro, biomass). Biomass burning still emits CO₂.

✓ Clean / Low-Carbon Energy

Little or no pollution during use. Nuclear = clean but NOT renewable (uranium is finite).

Hydro = renewable but ecologically damaging · Nuclear = clean but non-renewable

❌ Temperature ↑

Warm water holds LESS dissolved oxygen (Henry's Law: gas solubility decreases with temperature)

✓ Dissolved Oxygen ↓

Thermal pollution → temperature ↑ → DO ↓ → cold-water fish (trout, salmon) suffocate

In APES contexts, warmer water generally holds less dissolved oxygen (Henry's Law)

❌ Algae cause O₂ crash

Algal blooms do photosynthesis → they actually PRODUCE O₂ during growth phase

✓ Bacteria cause O₂ crash

Dead algae → aerobic BACTERIA decompose them → bacteria consume O₂ → dissolved oxygen crashes → fish kill

The O₂ crash in eutrophication comes from bacterial decomposition, not the algae

Section 03

9 Units · 3 Must-Know Points Each

Unit 1 · Ecosystems (6–8%)

NPP = GPP − R; food chains use NPP; GPP always larger
10% Rule: count transfers not levels; Level 3 = ÷ 100
Denitrification = anaerobic = removes N from ecosystem; nitrification = aerobic = keeps N in soil

Unit 2 · Biodiversity (6–8%)

HIPPO: Habitat loss is #1 cause of biodiversity loss globally
Island biogeography: larger area + closer to mainland = more species; halving area ≈ 10–20% species loss
Invasive species succeed because they lack natural predators/competitors — not because they are "superior"

Unit 3 · Populations (10–15%)

Rule of 70: convert ‰ to % first (÷ 10), then T₂ = 70 ÷ r(%)
DTM Stage 2 = death rate falls before birth rate → fastest growth; Stage 4 = both low = stable
K-selected species: few offspring, slow maturity → highest extinction risk

Unit 4 · Earth Systems (10–15%)

Troposphere = weather, ground-level O₃ (pollutant); Stratosphere = protective O₃ layer, ozone hole
Soil A horizon = topsoil = most important for agriculture; most vulnerable to erosion
El Niño: trade winds weaken → warm E. Pacific → Peru fishing collapses; Australia dries

Unit 5 · Land & Water Use (10–15%)

Salinization: flood irrigation → evaporation → salt accumulates → solution = drip irrigation
Contour plowing = erosion control on slopes; not a solution for salinization
MSY: harvest at 50% of carrying capacity; harvest above MSY → population collapse

Unit 6 · Energy Resources (10–15%)

Coal: highest CO₂ + SO₂; natural gas: ~50% less CO₂ but CH₄ leakage risk
Nuclear: zero operational carbon, non-renewable, radioactive waste
Solar/wind: intermittent (need storage); hydro: renewable but ecologically damaging

Unit 7 · Atmospheric Pollution (7–10%)

Photochemical smog: NOₓ + VOCs + sunlight → O₃ peaks mid-afternoon (not at rush hour)
Acid rain: SO₂ + NOₓ → H₂SO₄ + HNO₃; tall stacks don't reduce total load — they increase acid rain travel distance
Temperature inversion: warm air layer above traps cool polluted air near surface → smog worsens

Unit 8 · Aquatic & Terrestrial Pollution (7–10%)

Eutrophication: N/P → algae → light blocked → SAV dies → bacteria decompose → DO crashes → fish kill
Biomagnification: fat-soluble toxins accumulate up food chain; top predator has highest concentration
Thermal pollution: water temperature ↑ → DO ↓ → cold-water fish die

Unit 9 · Global Change (15–20%)

Greenhouse: shortwave in, longwave out trapped by CO₂/CH₄ → warming; positive feedbacks amplify
Ozone depletion: CFCs → Cl• catalytically destroys O₃ → UV-B → skin cancer; Montreal Protocol
Ice-albedo feedback: warming → ice melts → darker ocean absorbs more heat → more warming

Section 04

FRQ Q1 — Investigation Design Checklist

Q1 always asks you to design a scientific investigation. Answer with experimental design logic — not environmental content alone.

Independent variable (IV): the ONE thing you deliberately change/manipulate — state it specifically with levels (e.g., "NaCl concentration: 0, 500, 1,000 mg/L")
Dependent variable (DV): what you measure — state it AND how you will measure it (e.g., "percent egg hatching success, counted at day 30")
Control group: the group receiving zero manipulation of the IV — explain why it is necessary (provides a baseline for comparison)
Controlled variables: list at least 2–3 specific variables held constant (temperature, light, volume, pH...)
Sample size: state a specific number (n ≥ 5 per group) — "several" does not earn the point
Prediction: state direction (DV increases/decreases) AND mechanism (why the IV causes this) — direction alone earns zero
Error source: name ONE specific confounding variable + how you would minimize it

Q1 Most Common Errors

Swapping IV and DV — the IV is what you control; the DV is what responds
No control group — experiment is invalid without it
Sample size n=1 — not replicable; always state a number ≥ 3
Prediction without mechanism — "it will increase" earns zero; "it will increase because [mechanism]" earns the point

Section 05

FRQ Writing Rules — Non-Negotiable

Label every part: Start each answer on a new line with (a), (b), (c)... — graders score parts independently
EXPLAIN ≠ DESCRIBE: "Explain" requires a mechanism with a causal connector (because / therefore / which causes / resulting in). No connector = describing, not explaining.
Specificity is the only currency: "organisms" → name the organism; "chemicals" → name the chemical; "pollution affects the environment" → zero points
Calculations — show every step: formula → labeled variables → substitution → arithmetic → answer WITH UNITS. Partial credit at every step; blank = zero credit
Units on every calculation answer: "5,300" = zero; "5,300 kcal/m²/yr" = full credit. Wrong unit is better than no unit.
PROPOSE = specific + mechanism: "[Named practice] reduces [specific problem] by [mechanism]" — three elements required
EVALUATE = both sides: state a strength AND a limitation — one-sided evaluation earns half credit
Answer exactly the number asked: "ONE" means one; "TWO" means two distinct answers. Extra answers for "ONE" questions: only the first is scored
Never leave a part blank: a partial answer earns more than zero; even a partially correct mechanism can earn 1 of 2 points
FRQ 3 reference sheet: it's on-screen — use it to confirm symbolic notation and how to show work clearly in Bluebook; still show all calculation steps explicitly

Section 06

MCQ Tactics — 90 Minutes, 80 Questions

Pacing: 18 + 30 + 15 + 27 = 90 minutes

~18 min: Questions 1–25 (fast concept questions). ~30 min: Graph/stimulus sets (read stimulus once; answer all attached questions before moving on). ~15 min: Calculation questions (write formula first; never do multi-step arithmetic in your head). ~27 min: Return to flagged questions.

No blank questions: no penalty for wrong answers. Fill every blank — even a random guess has 25% expected value.
Read EXCEPT / NOT / LEAST first: missing these words is the #1 source of incorrect answers on MCQ. Underline them mentally as you read.
Graph questions — use the graph: if your answer would be the same whether or not the graph was there, you're not using it. Cite specific values.
Eliminate two choices first: even one wrong choice eliminated improves your odds. Never just pick the first option that seems right.
"Always / never / all / completely" = usually wrong: environmental systems are complex; absolute statements are almost always distractors.
Novel scenario = same concept: APES wraps familiar concepts in new settings. Strip away the story; identify which unit concept is being tested.
Policy keyword shortcuts: "international treaty / protocol" → Montreal / Kyoto / Paris. "U.S. federal / NPDES / discharge" → Clean Air Act / Clean Water Act / ESA.

Section 07

Day-of Checklist

Bring: a fully charged device with the Bluebook app installed + charging cord/adapter; your College Board login credentials; a government-issued or school-issued photo ID; a College Board-approved calculator (check the approved list the night before). Confirm with your AP coordinator whether you use a school-provided device or your own.
Before the exam: eat a real meal; hydrate. Cognitive performance drops measurably with hunger or dehydration.
Arrive early: arrive at least 20 minutes before your session. Technical setup time in Bluebook adds to the clock before questions appear.
MCQ — first action: read the exam instructions quickly; note the timer in Bluebook; flag your first uncertain question instead of stalling
FRQ — first action: skim all 3 questions before writing anything. Identify question types (Q1 = investigation; Q2 = problem+solution; Q3 = problem+calculate). Start with the question you know best.
FRQ 3: open the on-screen reference sheet at the start; review it for symbolic notation conventions so you know how to show calculation work clearly in Bluebook; do not rely on memory for how to enter mathematical expressions
Final 2 minutes of MCQ: fill any remaining blank questions with a single letter — do not leave anything unanswered
Final 2 minutes of FRQ: scan for blank sub-parts; add units to all calculations; add "because [mechanism]" to any answer that looks like a mere description

You Are Ready

Every student who reaches this file has already done the work. The exam does not reward last-minute cramming — it rewards the habits you have built over weeks of practice. Trust your preparation. Read command words carefully. Write mechanisms, not observations. Show all calculation work. Answer every question.