Cram sheet
36 points- The one-sentence chapter. Matter cycles — Earth is essentially a closed system for chemicals. Energy flows one way and is never recycled — Earth is an open system for energy, taking in sunlight and radiating heat back to space as infrared. Nearly every question here is a variation on that contrast. Watch Ecosystem Ecology: Links in the Chain - Crash Course Ecology #7 on YouTube, from CrashCourse
- An ecosystem = all the organisms living in a particular place plus the abiotic (nonliving) environment there. A community is the organisms only. The word "abiotic" is the giveaway. Watch Ecosystem Ecology: Links in the Chain - Crash Course Ecology #7 on YouTube, from CrashCourse
- Four cycles to know: carbon, water, nitrogen, phosphorus. Three of the four have a large atmospheric reservoir — phosphorus does not. If a question asks which element does not cycle through the atmosphere, the answer is phosphorus. Watch Biogeochemical cycles | Ecology | Khan Academy on YouTube, from Khan Academy Watch Phosphorous cycle | Ecology | Khan Academy on YouTube, from Khan Academy
- Carbon: CO₂ is 0.03% of atmospheric volume ≈ 750 billion metric tons of carbon, and carbon is almost 20% of human body weight. Carbon fixation = photosynthesis turning gaseous CO₂ into nongaseous organic compounds. In water CO₂ reacts spontaneously to form bicarbonate (HCO₃⁻), and both dissolved CO₂ and HCO₃⁻ feed aquatic photosynthesis. Watch The carbon cycle | Middle school Earth and space science | Khan Academy on YouTube, from Khan Academy
- Two carbon side-doors. Methanogens break organic matter down by anaerobic respiration and release CH₄ instead of CO₂ (biggest source: oxygen-free wetland sediments); CH₄ reaching the atmosphere is oxidized abiotically back to CO₂. Fossil fuels are carbon buried because synthesis outran breakdown over geologic time — burning returns in decades what took millions of years to store. Watch The Hydrologic and Carbon Cycles: Always Recycle! - Crash Course Ecology #8 on YouTube, from CrashCourse
- Water cycle distinctive feature: water is a compound, not an element, so it can be made and destroyed — synthesized in aerobic respiration, split in photosynthesis. Those rates are about equal, so a roughly constant amount of water cycles. Watch The water cycle | Ecology | Khan Academy on YouTube, from Khan Academy
- On land about 90% of the water reaching the atmosphere passes through plants by transpiration. In healthy tropical rainforest over 90% of rainfall is taken up and transpired back — the forest makes its own rain. Watch The water cycle | Ecology | Khan Academy on YouTube, from Khan Academy
- Groundwater sits in aquifers; it is over 95% of US freshwater, supplies 25% of all water used and 50% of drinking water. Upper water table layers feed streams and roots; deeper confined layers need wells. Flow is slow — millimeters to about a meter per day. The Ogallala Aquifer is pumped faster than it recharges. Watch The water cycle | Ecology | Khan Academy on YouTube, from Khan Academy
- Deforestation breaks the local water cycle. Cut the forest and rain drains away in rivers instead of transpiring back into clouds — how large areas of Brazil went from lush forest toward semiarid land. Mountain uplift did the same thing naturally to the Serengeti, turning rainforest into grassland. Watch The water cycle | Ecology | Khan Academy on YouTube, from Khan Academy
- The nitrogen paradox: the atmosphere is 78% N₂ by volume, yet nitrogen is the element in shortest supply in many ecosystems — because no plant or animal can use N₂ directly. Only certain prokaryotes can. Watch The nitrogen cycle | Energy and matter in biological systems | High school biology | Khan Academy on YouTube, from Khan Academy
- Four nitrogen verbs. Fixation: N₂ → NH₃. Nitrification: NH₃ → NO₃⁻. Ammonification: organic nitrogen and urea → NH₃. Denitrification: NO₃⁻ → N₂ (or N₂O). Fixation and denitrification are the two that move nitrogen in and out of the atmosphere. Watch The nitrogen cycle | Energy and matter in biological systems | High school biology | Khan Academy on YouTube, from Khan Academy
- Every one of those nitrogen steps is done by prokaryotes — fixers use the nitrogenase complex (nif genes), denitrifiers include both bacteria and archaea. Most fixers are free-living, but some live symbiotically in the roots of legumes (Fabaceae), alders, and myrtles. Watch The nitrogen cycle | Energy and matter in biological systems | High school biology | Khan Academy on YouTube, from Khan Academy
- Plants and algae take nitrogen up as NH₃/NH₄⁺ or NO₃⁻. Animals get nitrogen only by eating organic nitrogen compounds; most excrete NH₃, mammals excrete urea, and microbes convert that urea back to NH₃ for plants. Proteins are 16% nitrogen by weight. Watch The nitrogen cycle | Energy and matter in biological systems | High school biology | Khan Academy on YouTube, from Khan Academy
- Humans have already doubled the rate at which N₂ is converted into usable forms entering soils and waters, largely through industrially manufactured ammonium (NH₄⁺) salt fertilizers. Watch Nitrogen & Phosphorus Cycles: Always Recycle! Part 2 - Crash Course Ecology #9 on YouTube, from CrashCourse
- Phosphorus: found in nucleic acids, membrane phospholipids, and ATP; exists in a single oxidation state, phosphate (PO₄³⁻); no gaseous form. Weathering of rock releases it and rivers carry it to the sea — a large one-way flux into deep-sea sediments. Phosphate remineralization is decay microbes releasing inorganic PO₄³⁻ from dead bodies for reuse. Watch Phosphorous cycle | Ecology | Khan Academy on YouTube, from Khan Academy
- Limiting nutrient = the element in shortest supply relative to what plants and algae need — the weak link in the chain. Nitrogen limits about two-thirds of the oceans and many terrestrial systems; iron limits about one-third of the oceans, delivered as wind-borne Saharan dust. Watch Nitrogen & Phosphorus Cycles: Always Recycle! Part 2 - Crash Course Ecology #9 on YouTube, from CrashCourse
- Hubbard Brook (New Hampshire): six gauged tributary valleys; one was clear-cut in 1965–1966 with regrowth blocked. Water runoff rose 40%, calcium loss rose ninefold, and nitrate flipped from gaining ~5 kg/ha/yr to losing ~53 kg/ha/yr, causing algal blooms downstream. Phosphorus did not increase — it stayed locked in insoluble soil compounds. Watch Biogeochemical cycles | Ecology | Khan Academy on YouTube, from Khan Academy
- Autotroph vs heterotroph. Autotrophs build organic compounds from inorganic precursors: photoautotrophs use light (plants, algae, cyanobacteria), chemoautotrophs use inorganic oxidation reactions and are all prokaryotic. Heterotrophs must ingest organic compounds. And no organism of any kind can use heat as an energy source — conversion to heat is one-way (Second Law), which is exactly why energy cannot be recycled the way atoms are. Watch Energy Flow in Ecosystems on YouTube, from Bozeman Science
- Trophic levels in order: 1 primary producers → 2 herbivores → 3 primary carnivores → 4 secondary carnivores, with detritivores (and their subcategory decomposers) feeding on dead matter from every level. Because animals eat at several levels, real systems are food webs, not simple food chains. Watch What Are Trophic Levels? | Ecology & Environment | Biology | FuseSchool on YouTube, from FuseSchool - Global Education Watch Food Webs and Energy Pyramids: Bedrocks of Biodiversity on YouTube, from Amoeba Sisters
- GPP vs NPP. Gross primary productivity is the raw rate at which producers synthesize organic matter. NPP = GPP − producer respiration, and NPP is the part available to herbivores. If a question asks what herbivores can actually eat, the answer is NPP. Heterotroph levels have secondary productivity. Watch Primary productivity in ecosystems| Matter and Energy Flow| AP Environmental Science| Khan Academy on YouTube, from Khan Academy
- The two percentages. Producers capture about 1% of the solar energy hitting an ecosystem (about 99% becomes heat directly); about 10% of the energy at one trophic level reaches the next, occasionally as high as 30%. Watch Energy Flow in Ecosystems on YouTube, from Bozeman Science Watch What Are Trophic Levels? | Ecology & Environment | Biology | FuseSchool on YouTube, from FuseSchool - Global Education
- Where an herbivore's ingested energy actually goes (fig 56.9): 50% leaves in feces, 33% is burned in respiration → heat, and only 17% becomes new insect biomass available to the next level — and not even all of that, since some die before being eaten. Watch What Are Trophic Levels? | Ecology & Environment | Biology | FuseSchool on YouTube, from FuseSchool - Global Education
- Why food chains are short. 1000 units in producers → 100 in herbivores → 10 in primary carnivores → 1 in secondary carnivores. Only about 1/1000 reaches secondary carnivores and 1/10,000 would reach tertiary. Top carnivores are also big with large individual energy demands, so their populations are small — and nothing preys solely on eagles or lions. Watch Impact of changes to trophic pyramids | High school biology | Khan Academy on YouTube, from Khan Academy
- Pyramid rules. Pyramids of energy flow / productivity are always upright — the laws of energy flow require it. Pyramids of biomass and numbers are usually upright but can be inverted: fast-turnover phytoplankton are eaten as fast as they divide, so grazing zooplankton can carry more standing biomass than the algae below them. Watch Impact of changes to trophic pyramids | High school biology | Khan Academy on YouTube, from Khan Academy
- Cayuga Lake arithmetic: 1000 cal captured by algae → 150 in herbivores → 30 in smelt → 6 reach humans who eat smelt, but only 1.2 if trout eat the smelt first. Eating lower on the chain feeds at least five times more people. Watch What Are Trophic Levels? | Ecology & Environment | Biology | FuseSchool on YouTube, from FuseSchool - Global Education
- Top-down = trophic cascade: an effect exerted at an upper trophic level flows down through two or more levels. Bottom-up: an effect starting with primary productivity flows up. The direction of the arrow is the whole distinction. Watch How Wolves Change Rivers on YouTube, from Sustainable Human
- Count the levels to predict the sign. Add trout to a 3-level stream: fewer invertebrates, and five times more algae. Add large fish above damselfly nymphs in a 4-level stream: fewer nymphs, more herbivorous insects, and less algae. The effect flips with each level you add. Watch How Wolves Change Rivers on YouTube, from Sustainable Human
- Sea otter → sea urchin → kelp is the classic large-scale cascade: plenty of otters = few urchins = lush kelp forest; few otters = urchin explosion = kelp destroyed. Orcas switching to eating otters pushed the system toward the low-otter state. Watch Community Ecology II: Predators - Crash Course Ecology #5 on YouTube, from CrashCourse
- Human removal of top predators is a cascade engine: Aldo Leopold's wolfless mountains browsed bare by deer; Barro Colorado Island lost its jaguars and mountain lions, mid-size predators exploded, and at least 15 bird species vanished; overfishing cut billfish and cod to about 10% of former numbers, releasing their shrimp and crab prey. Watch How Wolves Change Rivers on YouTube, from Sustainable Human
- Bottom-up model in one line: as productivity rises, herbivores appear and crop the increase, then carnivores appear and crop the herbivores — so herbivore biomass stays roughly flat while producer and carnivore biomass rise. Confirmed in Eel River enclosures with tinted roofs controlling light. Watch Energy Flow in Ecosystems on YouTube, from Bozeman Science
- Cedar Creek (Minnesota): 207 plots monitored 11 years. More plant species meant less year-to-year variation in biomass, smaller declines in drought years, fewer successful invaders, and greater nitrogen uptake and biomass. The critic-proof test is overyielding — a diverse plot must beat the single most productive species grown alone, and Cedar Creek has found it. Watch Biodiversity Research at Cedar Creek on YouTube, from Institute on the Environment, University of Minnesota
- Species richness is hump-shaped against productivity — highest at intermediate productivity, because low productivity lets superior plant competitors win and high productivity lets heavy grazing win. Richness also rises with habitat (spatial) heterogeneity and with climatic stability (mammal richness falls as the temperature range widens). Watch Biodiversity Research at Cedar Creek on YouTube, from Institute on the Environment, University of Minnesota
- Five hypotheses for tropical diversity: longer evolutionary age, higher productivity, climatic stability and narrower niches (the one almost all field tests support), more intense predation that reduces competition, and greater spatial heterogeneity. Watch Biodiversity Research at Cedar Creek on YouTube, from Institute on the Environment, University of Minnesota
- MacArthur and Wilson (1967): island species richness is a dynamic equilibrium where the falling colonization curve crosses the rising extinction curve. Species composition keeps changing at equilibrium even though the number holds steady. Watch 29. Island Biogeography and Invasive Species on YouTube, from YaleCourses
- Area effect and distance effect. Small islands → higher extinction → equilibrium shifts left. Distant islands → lower colonization → equilibrium shifts left. So large + near = most species; small + far = fewest. Asian Pacific bird data fit exactly that. Watch 29. Island Biogeography and Invasive Species on YouTube, from YaleCourses
- Simberloff and Wilson tested the model on small mangrove islands in the Florida Keys: census the island, fumigate to remove all animal life, then follow recolonization with repeated censuses. Two wrinkles later emerged — real islands show less species turnover than predicted (a core of abundant species rarely goes extinct), and speciation matters too, since larger islands allow more allopatric speciation and more niches, while isolated islands diversify more because mainland gene flow is not there to inhibit it. Watch 29. Island Biogeography and Invasive Species on YouTube, from YaleCourses