AP Environmental Science

What people get wrong

1. Confusing GPP and NPP — students forget to subtract respiration and treat GPP as the energy available to consumers. Always compute NPP = GPP − R before doing any consumer-level energy calculations.
2. Treating the 10% rule as an exact law rather than an average — actual transfer efficiency varies by ecosystem and species. State it as 'approximately 10%' or 'on average' rather than an exact universal constant.
3. Mixing up nitrogen fixation, nitrification, and denitrification — students use these terms interchangeably. Keep them straight: fixation converts N2 to NH3/NH4+; nitrification converts NH4+ to NO2−/NO3−; denitrification converts NO3− back to N2 gas.
4. Assuming the phosphorus cycle has a large atmospheric gas phase like carbon and nitrogen — it does not. Remember phosphorus moves mainly through rock weathering, soil, and water, which is exactly why it's so often the limiting nutrient in freshwater systems.

What people get wrong

1. Assuming extinction rate rises with island area — it's the opposite: larger islands support larger, more resilient populations and more habitat diversity, so extinction rate decreases as area increases. Always tie island size to extinction, and distance to immigration.
2. Misclassifying ecosystem services, especially calling pollination or water purification 'provisioning' — these are regulating services because they maintain a process rather than supply a direct product. Ask 'does this give me a good, or does it maintain a condition?' before classifying.
3. Confusing primary and secondary succession by focusing on the disturbance type instead of soil presence. A fire on farmland is secondary succession because soil remains; a lava flow is primary succession because no soil exists. Check for soil, not just 'was there a disaster.'
4. Treating the zone of physiological stress as if organisms cannot survive there at all. Organisms can survive in this zone but show reduced growth or reproduction; only outside the tolerance limits (zone of intolerance) do they die. Read range-of-tolerance graphs by labeling three zones, not two.

What people get wrong

1. Memorizing r-selected and K-selected trait lists without understanding why — instead, reason from the strategy: r-selected species gamble on quantity (many offspring, little care) because their environment is unstable, while K-selected species invest in quality (few offspring, high care) because their environment is stable near carrying capacity.
2. Treating carrying capacity as a fixed number — instead, remember K can rise or fall with resource availability, climate, disease, or human impact, and populations can temporarily overshoot K before crashing.
3. Mixing up Type I and Type III survivorship curves — instead, anchor each to a concrete example: Type I (humans, elephants) loses individuals mostly in old age; Type III (fish, insects) loses most offspring early, with few reaching adulthood.
4. Assuming logistic growth stops abruptly at K — instead, recognize that dN/dt gradually approaches zero as N approaches K because of the (K−N)/K term, not because growth halts instantly.

What people get wrong

1. Assuming Earth's distance from the sun causes the seasons — instead, remember seasons come from the 23.5° axial tilt changing the sun's angle and day length; Earth is actually nearest the sun during Northern Hemisphere winter.
2. Mixing up divergent and convergent boundaries when identifying landforms — instead, ask whether crust is being created (divergent, e.g., mid-ocean ridge) or destroyed (convergent, e.g., trench/subduction) before naming the landform.
3. Applying the Coriolis deflection direction the same way in both hemispheres — instead, always check which hemisphere the question is set in: right-deflection in the Northern Hemisphere, left-deflection in the Southern Hemisphere.
4. Reversing El Niño and La Niña conditions on multiple-choice questions — instead, anchor on trade wind strength first: weak trade winds and warm water in the east = El Niño; strong trade winds and cold upwelling in the east = La Niña.

What people get wrong

1. Treating 'ecological footprint' as if it only measures carbon emissions. Instead, remember it also includes cropland, grazing land, forest land, fishing grounds, and built-up land — it is a total land-use measure, not just a carbon metric.
2. Assuming tragedy of the commons requires greed or bad intent. Instead, recognize it arises from rational self-interest acting on an unregulated shared resource; the fix is regulation, quotas, or property rights, not appeals to individual morality.
3. Assuming all aquaculture reduces pressure on wild fish stocks. Instead, check the trophic level of the farmed species — carnivorous fish like salmon require large amounts of wild-caught fishmeal (a fish-in:fish-out ratio greater than 1), which can worsen wild stock depletion, while herbivorous species like tilapia are far more efficient.
4. Confusing salinization with eutrophication because both involve water and nutrients/chemicals. Instead, keep the mechanisms separate: salinization is salt buildup in soil from evaporating irrigation water, degrading soil fertility; eutrophication is nutrient (nitrogen/phosphorus) enrichment of water bodies causing algal blooms and oxygen depletion.

What people get wrong

1. Assuming 'renewable' automatically means 'environmentally harmless.' Biomass combustion releases CO₂ and particulates, and large hydroelectric dams flood habitats and block fish migration — always evaluate impact separately from renewability.
2. Confusing capacity factor with efficiency. Efficiency compares energy output to energy input at the device level; capacity factor compares actual output over time to theoretical maximum output, which matters for intermittent sources like wind and solar.
3. Treating nuclear power as a major direct greenhouse gas source. Reactor operation emits very little CO₂; the real environmental concerns are high-level radioactive waste storage, thermal pollution of water sources, and catastrophic accident risk, not routine air pollution.
4. Forgetting that fossil fuels power more than electricity generation. Oil is used heavily for transportation and natural gas for heating and industrial processes, so 'switching the grid to renewables' does not eliminate fossil fuel demand by itself.

What people get wrong

1. Assuming ozone is emitted directly from tailpipes or smokestacks. Correct approach: ozone is always a secondary pollutant, formed when NOx and VOCs react in sunlight — never list it as a primary pollutant.
2. Treating CO2 as one of the EPA's six criteria air pollutants. Correct approach: the criteria pollutants are CO, NO2, SO2, O3, particulate matter, and lead; CO2 is a greenhouse gas regulated separately, not under NAAQS.
3. Describing a thermal inversion as 'cold air rising' or 'warm air rising through cold air.' Correct approach: an inversion is warm air sitting ABOVE cool surface air, which suppresses the normal upward mixing rather than promoting it.
4. Calculating pH or decibel differences linearly (e.g., saying pH 4 is 'twice as acidic' as pH 6). Correct approach: treat each unit as a factor of 10, so a 2-unit pH difference is a 100-fold difference in acidity.

What people get wrong

1. Assuming a lower LD50 means a substance is safer. It is the opposite — a lower LD50 means a smaller dose is lethal, so the substance is MORE toxic. Always state the direction of the LD50-toxicity relationship in your answer.
2. Misclassifying diffuse pollution as point source because it originates within one city or region. Check whether there is a single identifiable outlet (point source) or many scattered inputs like runoff from streets and lawns (nonpoint source), regardless of how localized the overall area is.
3. Treating a high BOD reading as a sign of clean water. High BOD means more organic matter is being decomposed, which consumes more dissolved oxygen — this signals pollution and risk of hypoxia, not water quality.
4. Describing thermal pollution as only 'warmer water is bad for fish' without the mechanism. Explain that warmer water holds less dissolved oxygen AND raises organisms' metabolic rate, so oxygen demand rises just as oxygen supply falls — that combination causes fish kills.

What people get wrong

1. Confusing stratospheric ozone depletion with the greenhouse effect/climate change — they are different problems (UV shielding vs. heat trapping) with different primary chemicals (CFCs vs. CO₂/CH₄); state which system a question is asking about before answering.
2. Assuming the Montreal Protocol addressed climate change — it specifically phased out ozone-depleting substances (CFCs); the treaties addressing greenhouse gas emissions are the Kyoto Protocol and Paris Agreement.
3. Thinking one chlorine atom destroys only one ozone molecule — the cycle is catalytic, so a single Cl atom is regenerated and can destroy many thousands of O₃ molecules before it leaves the stratosphere.
4. Describing ocean acidification as 'CO₂ making water acidic' without the mechanism — always mention that CO₂ forms carbonic acid, which releases H⁺ and lowers carbonate ion availability, since that carbonate loss is what actually harms calcifying organisms.