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Unit 1 · The Living World: Ecosystems Unit Hub Flashcards Cheat Sheet Essentials Visual Review MC Practice FRQ Practice

AP Environmental Science Unit 1 Visual Review

An 11-slide visual walkthrough of every Topic 1.1-1.11 concept — energy flow, biomes, and the carbon, nitrogen, phosphorus, and water cycles — built right into the page.

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TOPIC 1.1 Energy Flow Through Ecosystems Producers Primary consumers Secondary Tertiary energy available shrinks at each step up sunlight in First law of thermodynamics Energy can't be created or destroyed, only converted from one form to another — sunlight becomes chemical energy in a leaf, then heat. Second law of thermodynamics Every energy conversion loses some usable energy as heat. This is why energy can't cycle — an ecosystem needs a constant sunlight input. Matter, by contrast, DOES cycle — see 1.4–1.7. Energy flows one direction, from sun to producers to consumers, shrinking at every step. The Review Hub · AP Environmental Science Unit 1 TOPIC 1.2 Terrestrial Biomes: Temperature & Precipitation temperature → precipitation → Tundra Taiga Desert (low precip) Grassland Temperate forest Tropical rainforest cold warm Two variables explain it all Terrestrial biomes are sorted almost entirely by average temperature and annual precipitation. Latitude and elevation both work by controlling temperature — this is why mountaintops near the equator can support tundra-like communities. Quick biome ID checklist Cold + dry → tundra (permafrost, no trees) Cold + moderate → taiga (conifers) Warm + very dry → desert Warm + wet, seasonal → grassland Warm + very wet, year-round → rainforest A biome's identity is set by climate, not by which plants and animals happen to live there. The Review Hub · AP Environmental Science Unit 1 TOPIC 1.3 Aquatic Biomes: Freshwater & Marine FRESHWATER LAKE, TOP VIEW CROSS-SECTION LIMNETIC (open, sunlit) LITTORAL LITTORAL PROFUNDAL (deep, dark) BENTHIC (bottom) Lake zones Littoral — shallow, sunlit edge; rooted plants. Limnetic — open surface water; phytoplankton. Profundal — deep, too dark to photosynthesize. Benthic — bottom; decomposers & detritus. Same photic/aphotic logic applies to the ocean. Marine biomes to know Estuary — river meets ocean; brackish, high productivity, nursery for young fish. Coral reef — warm, shallow, clear water; the most biodiverse marine biome. Open ocean — split into photic and aphotic zones by whether light reaches that depth. Zonation in both lakes and oceans comes down to light penetration and distance from shore. The Review Hub · AP Environmental Science Unit 1 TOPIC 1.4 The Carbon Cycle Atmosphere (CO₂) Biosphere (plants) Ocean (dissolved) Fossil fuels photosynthesis respiration diffusion out absorption burial, millions of yrs combustion (fast!) Fast cycle vs. slow reservoir Photosynthesis and respiration cycle carbon between air and living things in days to years. Fossil fuels formed over millions of years — burning them releases that carbon in decades, far faster than it can be reabsorbed. The ocean as carbon sink The ocean absorbs roughly a quarter of human CO₂ emissions. Dissolved CO₂ forms carbonic acid, lowering ocean pH — ocean acidification. This is a Unit 7/8 connection worth remembering. Carbon cycles fast between air and life, but the fossil-fuel reservoir releases in decades what took eons to form. The Review Hub · AP Environmental Science Unit 1 TOPIC 1.5 The Nitrogen Cycle Atmospheric N₂ (78%) fixation (bacteria, lightning) Ammonium (NH₄⁺) in soil nitrification Nitrate (NO₃⁻) — plant-usable denitrification plant uptake Plants & consumers Why nitrogen needs fixing N₂ gas is 78% of the atmosphere, but its triple bond makes it unusable to most life. "Fixation" breaks that bond — done by soil bacteria (like Rhizobium in legume roots), lightning, or humans making synthetic fertilizer (Haber-Bosch). Human disruption Synthetic fertilizer now fixes more nitrogen than all natural sources combined. Excess nitrate runs off into waterways, driving algal blooms and dead zones — see Topic 1.6. Fixation unlocks unusable N₂ gas; denitrifying bacteria eventually return it to the atmosphere. The Review Hub · AP Environmental Science Unit 1 TOPIC 1.6 The Phosphorus Cycle & Eutrophication Rock (apatite) weathering Soil phosphate (PO₄³⁻) plant uptake Plants → consumers runoff Lakes & rivers ✗ no atmospheric gas phase The odd cycle out Phosphorus has no common gas form, so it never enters the atmosphere. It moves only through rock weathering, soil, water, and living things — making it the slowest of the three cycles. Eutrophication, step by step Fertilizer runoff floods a lake with phosphorus → algae bloom explosively → algae die and are decomposed by bacteria → decomposition consumes dissolved oxygen → fish and other aerobic organisms suffocate — a dead zone. No atmospheric step means phosphorus is limiting in most ecosystems — which is exactly why fertilizer runoff causes so much damage. The Review Hub · AP Environmental Science Unit 1 TOPIC 1.7 The Hydrologic (Water) Cycle Ocean / surface water evaporation clouds (condensation) precipitation plant transpiration infiltration → groundwater runoff Five moving parts Evaporation — liquid to vapor, powered by the sun. Transpiration — water vapor released by plants. Condensation — vapor cools into cloud droplets. Precipitation — rain, snow, sleet, or hail falls. Runoff / infiltration — flows to rivers, or soaks in. Watershed vocabulary A watershed is the land area that drains into a given body of water. Impervious surfaces (pavement, roofs) increase runoff and reduce infiltration — a recurring theme in Unit 5. Evaporation and transpiration lift water up; condensation and precipitation bring it back down. The Review Hub · AP Environmental Science Unit 1 TOPIC 1.8 Primary Productivity: GPP vs. NPP GPP total energy captured R used for the plant's own life NPP available to consumers NPP = GPP − R Two ways to count "growth" GPP (gross primary productivity) is ALL the chemical energy producers capture through photosynthesis, before any is spent. Producers use some of that energy (R) just to stay alive — growth, repair, respiration. Why NPP is what matters NPP is the energy left over — the biomass a producer actually adds, which is what's available to feed primary consumers. It's the number that sets the base of the energy pyramid (Topic 1.10). Tropical rainforests & estuaries have the highest NPP. GPP is everything a plant captures; NPP is what's left after the plant pays its own energy bill. The Review Hub · AP Environmental Science Unit 1 TOPIC 1.9 Trophic Levels Producers grasses, algae, trees Primary consumers herbivores — deer, grasshoppers Secondary carnivores — frogs, foxes Tertiary top carnivores — hawks Decomposers & detritivores act on ALL levels Producers, consumers, decomposers Producers (autotrophs) make their own food via photosynthesis or chemosynthesis. Consumers (heterotrophs) eat other organisms; classified by what they eat. Decomposers are the recyclers Fungi and bacteria break down dead matter at every trophic level, returning nutrients to the soil. Without them, matter would stay locked in dead bodies instead of cycling — see 1.4–1.7. Every ecosystem sorts into the same few functional roles, no matter which species fill them. The Review Hub · AP Environmental Science Unit 1 TOPIC 1.10 Energy Flow and the 10% Rule Producers — 10,000 kcal Primary consumers — 1,000 kcal Secondary — 100 kcal Tertiary — 10 ↑ ~90% lost as heat at each transfer (respiration, movement, waste) The rule & why it holds On average, only about 10% of the energy at one trophic level makes it into the next. The rest is lost as heat (2nd law), used for the organism's own life, or left undigested. Applying it on the exam Multiply by 0.10 to move UP a level; divide by 0.10 to move DOWN. If secondary consumers have 500 kcal, how much energy did producers need? 500 ÷ 0.10 ÷ 0.10 = 50,000 kcal This is why food chains rarely run past 4–5 links — there's simply not enough energy left to support another level. The Review Hub · AP Environmental Science Unit 1 TOPIC 1.11 Food Chains and Food Webs Grass Mouse Seeds Sparrow Snake Hawk arrows point in the direction energy flows Chain vs. web A food chain is one straight-line path of who eats whom. Real ecosystems have many overlapping chains — a food web — because most organisms eat, and are eaten by, several species. Why webs matter more than chains Removing a single species from a chain breaks it entirely. In a web, other species can partly absorb the loss — more connections mean more resilience. Removing a keystone species, though, can still collapse the whole web. A food web is really just many food chains layered on top of each other, sharing species between them. The Review Hub · AP Environmental Science Unit 1
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How to use the visual review

Spend 30 seconds per slide before clicking next. Look at the diagram, then ask yourself: "Could I draw this from memory and explain it?"

Use the fullscreen button () on desktop for the best experience. Use arrow keys to navigate. Tap "Show all slides" to jump around.

This covers every one of Unit 1's 11 CED topics, from energy flow through the biogeochemical cycles.