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🌿 Biology · Ecology

Memory tricks for DNA, heredity & mutations

From food chains to biogeochemical cycles — ecology connects every living thing to its environment. These memory tricks lock in the energy flow, population dynamics, and biome characteristics your exam will test.

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🌿 Ecology

Memory Tricks

Proven mnemonics — fast to learn, hard to forget.

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Energy Flow
10% Rule — only 10% of energy transfers to the next trophic level
Producer → Primary Consumer → Secondary Consumer — 90% lost as heat
At each trophic level, only about 10% of energy is passed up to the next level. The other 90% is lost as heat through cellular respiration, waste, and movement. This is why food chains rarely exceed 4-5 levels and why ecosystems can support far more herbivores than carnivores.
📖 Full Lesson →Difficulty: Beginner
Ecological pyramids
Energy pyramid always narrows upward — less energy at each level. Biomass pyramid usually also narrows. Numbers pyramid can be inverted (one tree supporting many insects).
Trophic levels
Level 1 = Producers (plants, algae). Level 2 = Primary consumers (herbivores). Level 3 = Secondary consumers. Level 4 = Tertiary consumers. Decomposers work at all levels.
GPP vs NPP
Gross Primary Productivity = total energy fixed by photosynthesis. Net Primary Productivity = GPP minus plant respiration. NPP is the energy available to consumers.
Why so little energy?
Heat loss from metabolism, energy in uneaten body parts, energy in feces. Only energy actually assimilated and stored as biomass transfers upward.
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Energy Flow: Why Nobody Farms Lions. Alex explains the ten percent rule, producers, consumers, and decomposers, live from the quad. (4:13)
Flashcard
🃏 Energy Flow
The 10% rule — what is it?
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🃏 Answer
10% Rule — only 10% of energy transfers to the next trophic level
Ecological pyramids — Energy pyramid always narrows upward — less energy at each level. Biomass pyramid usually also narrows. Numbers pyramid can be inverted (one tree supporting many insects).
Trophic levels — Level 1 = Producers (plants, algae). Level 2 = Primary consumers (herbivores). Level 3 = Secondary consumers. Level 4 = Tertiary consumers. Decomposers work at all levels.
GPP vs NPP — Gross Primary Productivity = total energy fixed by photosynthesis. Net Primary Productivity = GPP minus plant respiration. NPP is the energy available to consumers.
Why so little energy? — Heat loss from metabolism, energy in uneaten body parts, energy in feces. Only energy actually assimilated and stored as biomass transfers upward.
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Biomes
Temperature, precipitation and latitude determine biomes
Tropical rainforest → Temperate forest → Taiga → Tundra going poleward
Biomes are classified primarily by temperature and precipitation patterns. Moving from equator to poles: Tropical rainforest (hot/wet) → Tropical savanna → Desert → Temperate grassland → Temperate deciduous forest → Taiga/Boreal forest → Tundra → Polar ice. Altitude mirrors latitude — going up a mountain recreates this sequence.
📖 Full Lesson →Difficulty: Beginner
Tropical rainforest
Highest biodiversity on Earth. Hot and wet year-round. Poor soil (nutrients locked in vegetation). Stratified canopy: emergent layer, canopy, understory, forest floor.
Taiga (Boreal forest)
Largest terrestrial biome by area. Coniferous trees (pines, spruce, fir). Long cold winters, short summers. Acidic, nutrient-poor soil. Moose, wolves, bears.
Tundra
Permafrost prevents deep root growth. Very low precipitation (technically a desert). Short growing season. Low biodiversity. Arctic tundra vs alpine tundra (high altitude).
Desert
Defined by low precipitation (<25cm/year), not necessarily heat. Hot deserts (Sahara) and cold deserts (Gobi). Organisms adapted for water conservation.
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🃏 Biomes
Biomes — what determines them?
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🃏 Answer
Temperature, precipitation and latitude determine biomes
Tropical rainforest — Highest biodiversity on Earth. Hot and wet year-round. Poor soil (nutrients locked in vegetation). Stratified canopy: emergent layer, canopy, understory, forest floor.
Taiga (Boreal forest) — Largest terrestrial biome by area. Coniferous trees (pines, spruce, fir). Long cold winters, short summers. Acidic, nutrient-poor soil. Moose, wolves, bears.
Tundra — Permafrost prevents deep root growth. Very low precipitation (technically a desert). Short growing season. Low biodiversity. Arctic tundra vs alpine tundra (high altitude).
Desert — Defined by low precipitation (<25cm/year), not necessarily heat. Hot deserts (Sahara) and cold deserts (Gobi). Organisms adapted for water conservation.
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Population Ecology
J-curve = exponential · S-curve = logistic (carrying capacity)
Exponential growth unlimited · Logistic growth levels off at K
Two models of population growth: Exponential (J-shaped) assumes unlimited resources — population grows faster as it gets bigger. Logistic (S-shaped) includes a carrying capacity (K) — growth slows as population approaches K due to limiting factors. Most real populations show logistic growth with fluctuations around K.
📖 Full Lesson →Difficulty: Intermediate
Carrying capacity (K)
Maximum population size an environment can sustainably support. Determined by food, water, space, and other resources. Population oscillates around K in logistic growth.
r vs K selection
r-selected species: many offspring, little care, short lifespan (insects, rodents). K-selected species: few offspring, intensive care, long lifespan (elephants, humans). r = intrinsic growth rate.
Density-dependent factors
Effects increase with population density: competition, predation, disease, parasitism. Provide negative feedback that limits growth.
Density-independent factors
Affect population regardless of density: temperature extremes, floods, fires, drought. Can cause sudden crashes independent of population size.
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🃏 Population Ecology
Population growth — J-curve vs S-curve?
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🃏 Answer
J-curve = exponential · S-curve = logistic (carrying capacity)
Carrying capacity (K) — Maximum population size an environment can sustainably support. Determined by food, water, space, and other resources. Population oscillates around K in logistic growth.
r vs K selection — r-selected species: many offspring, little care, short lifespan (insects, rodents). K-selected species: few offspring, intensive care, long lifespan (elephants, humans). r = intrinsic growth rate.
Density-dependent factors — Effects increase with population density: competition, predation, disease, parasitism. Provide negative feedback that limits growth.
Density-independent factors — Affect population regardless of density: temperature extremes, floods, fires, drought. Can cause sudden crashes independent of population size.
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Community Ecology
CCMM — Competition, Commensalism, Mutualism, and More
Symbiosis types: +/+ mutualism · +/0 commensalism · +/- parasitism
Species interactions defined by effects on each: Mutualism (+/+): both benefit (bees/flowers). Commensalism (+/0): one benefits, other unaffected (barnacles on whales). Parasitism (+/-): one benefits, other harmed. Predation (+/-). Competition (-/-): both harmed. Competitive exclusion: two species cannot occupy the same niche indefinitely.
📖 Full Lesson →Difficulty: Intermediate
Competitive exclusion principle
Two species competing for identical resources cannot coexist — one will outcompete and exclude the other. Led to concept of ecological niche: fundamental vs realized niche.
Keystone species
Species with disproportionately large effect on ecosystem relative to its abundance. Sea otters control sea urchin populations, protecting kelp forests. Removal causes trophic cascade.
Succession
Primary succession: bare rock → pioneer species → climax community. Secondary succession: after disturbance on existing soil — faster. Climax community = stable endpoint.
Character displacement
Competing species evolve to use different resources, reducing overlap. Classic example: Darwin's finches with different beak sizes on same island evolved to eat different seeds.
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🃏 Community Ecology
CCMM
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🃏 Answer
CCMM — Competition, Commensalism, Mutualism, and More
Competitive exclusion principle — Two species competing for identical resources cannot coexist — one will outcompete and exclude the other. Led to concept of ecological niche: fundamental vs realized niche.
Keystone species — Species with disproportionately large effect on ecosystem relative to its abundance. Sea otters control sea urchin populations, protecting kelp forests. Removal causes trophic cascade.
Succession — Primary succession: bare rock → pioneer species → climax community. Secondary succession: after disturbance on existing soil — faster. Climax community = stable endpoint.
Character displacement — Competing species evolve to use different resources, reducing overlap. Classic example: Darwin's finches with different beak sizes on same island evolved to eat different seeds.
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Biogeochemical Cycles
CHNOPS — Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, Sulfur
The 6 elements that cycle through all living systems
CHNOPS are the essential elements that cycle between living organisms and the environment. The Carbon cycle (photosynthesis/respiration/decomposition), Nitrogen cycle (fixation/nitrification/denitrification), and Water cycle are the most tested. Phosphorus cycle has no atmospheric component — moves through rocks, soil, and water only.
📖 Full Lesson →Difficulty: Intermediate
Carbon cycle
CO₂ fixed by photosynthesis → organic carbon in organisms → released by respiration, decomposition, combustion. Fossil fuels = ancient stored carbon. Ocean is largest active carbon reservoir.
Nitrogen cycle
N₂ → NH₃ (nitrogen fixation by Rhizobium) → NO₃⁻ (nitrification) → N₂ (denitrification). Plants take up NH₄⁺ or NO₃⁻. Nitrogen is often the limiting nutrient in ecosystems.
Phosphorus cycle
No atmospheric phase — cycles through rock weathering → soil → plants → animals → decomposers → soil. Slow cycle. Phosphorus often limiting in aquatic systems. Eutrophication from P runoff.
Eutrophication
Excess nutrients (N,P) cause algal bloom → algae die → bacteria decompose them consuming O₂ → hypoxic dead zone. Common in agricultural runoff areas. Gulf of Mexico dead zone.
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🃏 Biogeochemical Cycles
CHNOPS
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🃏 Answer
CHNOPS — Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, Sulfur
Carbon cycle — CO₂ fixed by photosynthesis → organic carbon in organisms → released by respiration, decomposition, combustion. Fossil fuels = ancient stored carbon. Ocean is largest active carbon reservoir.
Nitrogen cycle — N₂ → NH₃ (nitrogen fixation by Rhizobium) → NO₃⁻ (nitrification) → N₂ (denitrification). Plants take up NH₄⁺ or NO₃⁻. Nitrogen is often the limiting nutrient in ecosystems.
Phosphorus cycle — No atmospheric phase — cycles through rock weathering → soil → plants → animals → decomposers → soil. Slow cycle. Phosphorus often limiting in aquatic systems. Eutrophication from P runoff.
Eutrophication — Excess nutrients (N,P) cause algal bloom → algae die → bacteria decompose them consuming O₂ → hypoxic dead zone. Common in agricultural runoff areas. Gulf of Mexico dead zone.
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Biodiversity
HIPPO — Habitat loss, Invasive species, Pollution, Population growth, Overharvesting
Five main causes of biodiversity loss — habitat loss is #1
HIPPO identifies the five major threats to biodiversity. Habitat loss and fragmentation is by far the leading cause. Invasive species outcompete natives. Pollution degrades ecosystems. Human population growth amplifies all other threats. Overharvesting depletes populations faster than they can reproduce. The current extinction rate is 100-1000× the natural background rate.
📖 Full Lesson →Difficulty: Beginner
Habitat fragmentation
Breaking continuous habitat into isolated patches. Reduces gene flow, increases edge effects, limits migration. Island biogeography theory: species richness depends on island size and distance from mainland.
Invasive species
Non-native species released from natural predators/competitors. Examples: kudzu vine, cane toad, zebra mussels. Can devastate native communities. Second leading cause of extinction.
Biodiversity hotspots
Regions with exceptional biodiversity AND high threat. Criteria: >1500 endemic plant species AND lost >70% of original habitat. 36 hotspots cover <3% of land but hold 50%+ of plant species.
Ecosystem services
Benefits humans get from ecosystems: provisioning (food, water), regulating (climate, floods), cultural (recreation), supporting (nutrient cycling, soil formation). Biodiversity underpins all services.
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🃏 Biodiversity
HIPPO
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🃏 Answer
HIPPO — Habitat loss, Invasive species, Pollution, Population growth, Overharvesting
Habitat fragmentation — Breaking continuous habitat into isolated patches. Reduces gene flow, increases edge effects, limits migration. Island biogeography theory: species richness depends on island size and distance from mainland.
Invasive species — Non-native species released from natural predators/competitors. Examples: kudzu vine, cane toad, zebra mussels. Can devastate native communities. Second leading cause of extinction.
Biodiversity hotspots — Regions with exceptional biodiversity AND high threat. Criteria: >1500 endemic plant species AND lost >70% of original habitat. 36 hotspots cover <3% of land but hold 50%+ of plant species.
Ecosystem services — Benefits humans get from ecosystems: provisioning (food, water), regulating (climate, floods), cultural (recreation), supporting (nutrient cycling, soil formation). Biodiversity underpins all services.
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Aquatic Ecosystems
Littoral → Limnetic → Profundal — lake zones from shore to deep
Littoral (shore) · Limnetic (open water) · Profundal (deep/dark)
Lake zones: Littoral zone = shallow, rooted plants, most productive. Limnetic zone = open water, phytoplankton and zooplankton. Profundal zone = deep, dark, cold, decomposers. Thermocline separates warm surface from cold deep water. Marine zones: Intertidal, Neritic (continental shelf), Oceanic, Abyssal.
📖 Full Lesson →Difficulty: Intermediate
Photic vs aphotic zone
Photic zone = sunlight penetrates, photosynthesis possible (top ~200m). Aphotic zone = no light, only chemosynthesis or decomposition. Most ocean biomass in photic zone.
Estuary
Where freshwater meets saltwater. Highly productive ecosystem — nutrients from land AND sea. Nursery habitat for many marine species. Sensitive to pollution and development.
Upwelling
Cold, nutrient-rich deep water rises to surface. Fuels massive phytoplankton blooms → rich fisheries. Pacific coast of South America (Peru Current) supports world's largest fisheries. El Niño disrupts upwelling.
Coral reefs
Built by coral polyps with symbiotic algae (zooxanthellae). Most biodiverse marine ecosystem. Bleaching occurs when stressed corals expel zooxanthellae — mass bleaching from ocean warming and acidification.
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🃏 Aquatic Ecosystems
Lake zones — from shore to deep?
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🃏 Answer
Littoral → Limnetic → Profundal — lake zones from shore to deep
Photic vs aphotic zone — Photic zone = sunlight penetrates, photosynthesis possible (top ~200m). Aphotic zone = no light, only chemosynthesis or decomposition. Most ocean biomass in photic zone.
Estuary — Where freshwater meets saltwater. Highly productive ecosystem — nutrients from land AND sea. Nursery habitat for many marine species. Sensitive to pollution and development.
Upwelling — Cold, nutrient-rich deep water rises to surface. Fuels massive phytoplankton blooms → rich fisheries. Pacific coast of South America (Peru Current) supports world's largest fisheries. El Niño disrupts upwelling.
Coral reefs — Built by coral polyps with symbiotic algae (zooxanthellae). Most biodiverse marine ecosystem. Bleaching occurs when stressed corals expel zooxanthellae — mass bleaching from ocean warming and acidification.
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Evolution & Ecology
Natural selection: VISH — Variation, Inheritance, Selection, Heritable change
Variation exists · Traits inherited · Some traits improve survival · Those traits increase in population
Darwin's four postulates for natural selection: Variation exists in the population. Variation is heritable. More offspring are produced than survive (struggle for existence). Individuals with favorable traits survive and reproduce more. Result: heritable favorable traits increase in frequency over generations — evolution by natural selection.
📖 Full Lesson →Difficulty: Intermediate
Types of selection
Directional: one extreme phenotype favored (antibiotic resistance). Stabilizing: intermediate favored (human birth weight). Disruptive: both extremes favored, middle eliminated (can lead to speciation).
Genetic drift
Random changes in allele frequency due to chance, not selection. Stronger in small populations. Bottleneck effect (population crash) and founder effect (small group colonizes new area) are types of drift.
Coevolution
Two species evolve in response to each other. Predator-prey arms race. Flower-pollinator matching. Host-parasite coevolution. Mimicry — Batesian (harmless mimics harmful) and Müllerian (two harmful species resemble each other).
Speciation
Allopatric: geographic isolation prevents gene flow → populations diverge. Sympatric: speciation without geographic isolation (polyploidy in plants, habitat differentiation). Reproductive isolation is the key criterion.
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🃏 Evolution & Ecology
Natural selection: VISH
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🃏 Answer
Natural selection: VISH — Variation, Inheritance, Selection, Heritable change
Types of selection — Directional: one extreme phenotype favored (antibiotic resistance). Stabilizing: intermediate favored (human birth weight). Disruptive: both extremes favored, middle eliminated (can lead to speciation).
Genetic drift — Random changes in allele frequency due to chance, not selection. Stronger in small populations. Bottleneck effect (population crash) and founder effect (small group colonizes new area) are types of drift.
Coevolution — Two species evolve in response to each other. Predator-prey arms race. Flower-pollinator matching. Host-parasite coevolution. Mimicry — Batesian (harmless mimics harmful) and Müllerian (two harmful species resemble each other).
Speciation — Allopatric: geographic isolation prevents gene flow → populations diverge. Sympatric: speciation without geographic isolation (polyploidy in plants, habitat differentiation). Reproductive isolation is the key criterion.
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Human Impact
Climate change: CO₂ up → Temperature up → Ice melts → Sea level rises
Greenhouse effect enhanced by fossil fuel combustion and deforestation
Greenhouse gases (CO₂, CH₄, N₂O, H₂O) trap infrared radiation — the greenhouse effect is natural and essential. Burning fossil fuels and deforestation increase CO₂, amplifying the effect. Consequences: rising temperatures, melting ice caps, sea level rise, ocean acidification (CO₂ + H₂O → H₂CO₃), shifts in species ranges, and more extreme weather events.
📖 Full Lesson →Difficulty: Beginner
Ocean acidification
CO₂ dissolves in seawater → carbonic acid → lowers pH. Threatens coral reefs and shell-forming organisms (mollusks, echinoderms) that need carbonate ions. pH has dropped from 8.2 to 8.1 since industrialization.
Ozone depletion
CFCs (chlorofluorocarbons) catalytically destroy ozone (O₃) in the stratosphere. UV radiation increases → skin cancer, cataracts, reduced photosynthesis. Montreal Protocol (1987) phased out CFCs — ozone layer recovering.
Deforestation effects
Releases stored carbon, reduces carbon fixation, destroys habitat, disrupts water cycle, increases soil erosion. Tropical deforestation = 10-15% of global carbon emissions.
Biomagnification
Toxins (DDT (dichlorodiphenyltrichloroethane — a pesticide), mercury, PCBs (polychlorinated biphenyls — industrial chemicals)) accumulate and concentrate as they move up food chains. Top predators have highest concentrations. DDT caused eggshell thinning in eagles and falcons — drove bald eagle near extinction.
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🃏 Human Impact
Climate change — the chain of effects?
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🃏 Answer
Climate change: CO₂ up → Temperature up → Ice melts → Sea level rises
Ocean acidification — CO₂ dissolves in seawater → carbonic acid → lowers pH. Threatens coral reefs and shell-forming organisms (mollusks, echinoderms) that need carbonate ions. pH has dropped from 8.2 to 8.1 since industrialization.
Ozone depletion — CFCs (chlorofluorocarbons) catalytically destroy ozone (O₃) in the stratosphere. UV radiation increases → skin cancer, cataracts, reduced photosynthesis. Montreal Protocol (1987) phased out CFCs — ozone layer recovering.
Deforestation effects — Releases stored carbon, reduces carbon fixation, destroys habitat, disrupts water cycle, increases soil erosion. Tropical deforestation = 10-15% of global carbon emissions.
Biomagnification — Toxins (DDT (dichlorodiphenyltrichloroethane — a pesticide), mercury, PCBs (polychlorinated biphenyls — industrial chemicals)) accumulate and concentrate as they move up food chains. Top predators have highest concentrations. DDT caused eggshell thinning in eagles and falcons — drove bald eagle near extinction.
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Geologic Periods
Cows Often Sit Down Carelessly, Perhaps Their Joints Crack
Cambrian · Ordovician · Silurian · Devonian · Carboniferous · Permian · Triassic · Jurassic · Cretaceous
The nine periods of the Paleozoic and Mesozoic eras, oldest to youngest. The Cenozoic, our current era, follows. Five mass extinctions punctuate this history; the worst, the End-Permian "Great Dying," wiped out about 96% of marine species.
📖 Full Lesson →Difficulty: Intermediate
Paleozoic (541–252 mya)
Cambrian to Permian. Cambrian explosion, first land plants, the Devonian "Age of Fish," coal-swamp forests. Ends with the End-Permian Great Dying.
Mesozoic (252–66 mya)
Triassic, Jurassic, Cretaceous — the Age of Reptiles. First dinosaurs and mammals, first bird, flowering plants. Ends with the K-Pg asteroid impact.
Cenozoic (66 mya–now)
The Age of Mammals. Mammals fill the niches dinosaurs left; hominids appear about 7 mya.
Five mass extinctions
End-Ordovician, Late Devonian, End-Permian (worst), End-Triassic, K-Pg. Today's extinction rate is 100–1,000× background.
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🃏 Geologic Periods
Cows Often Sit Down Carelessly, Perhaps Their Joints Crack
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🃏 Answer
Paleozoic (541–252 mya) — Cambrian to Permian. Cambrian explosion, first land plants, the Devonian "Age of Fish," coal-swamp forests. Ends with the End-Permian Great Dying.
Mesozoic (252–66 mya) — Triassic, Jurassic, Cretaceous — the Age of Reptiles. First dinosaurs and mammals, first bird, flowering plants. Ends with the K-Pg asteroid impact.
Cenozoic (66 mya–now) — The Age of Mammals. Mammals fill the niches dinosaurs left; hominids appear about 7 mya.
Five mass extinctions — End-Ordovician, Late Devonian, End-Permian (worst), End-Triassic, K-Pg. Today's extinction rate is 100–1,000× background.
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