1Levels of Organization
Ecologists study life at nested levels, each one containing the one before it.
- Biotic factors are living (plants, animals, fungi, bacteria).
- Abiotic factors are non-living (sunlight, temperature, water, soil, pH).
- A habitat is where an organism lives; its niche is its role: what it eats, when it's active, how it uses resources.
2Energy Flow: Food Chains, Webs & Pyramids
Energy enters most ecosystems as sunlight and flows one way, from producers to consumers. It is not recycled.
Arrows point in the direction energy flows: from the eaten to the eater.
- That's why food chains rarely have more than 4–5 levels, and why there are far fewer hawks than grasshoppers.
- Producers / autotrophs make their own food: photosynthesis (sunlight) or chemosynthesis (chemical energy, e.g. bacteria at deep-sea vents).
- Consumers / heterotrophs: herbivores, carnivores, omnivores, detritivores (eat dead matter, like earthworms) and decomposers.
- A food web is many interconnected food chains, which is more realistic, since most animals eat more than one thing.
- Pyramids can also show biomass (mass of living tissue) or numbers of organisms.
Biomagnification
Toxins that don't break down (like DDT or mercury) become more concentrated at each higher trophic level.
Classic textbook DDT example (bar heights not to scale). Top predators are hit hardest; DDT thinned bird eggshells.
3Cycles of Matter
Unlike energy, matter is recycled. Atoms of carbon, nitrogen, water and phosphorus move between living things and the environment.
Carbon cycle
Decomposers also release CO₂ through respiration as they break things down.
Nitrogen cycle
Water cycle
- Evaporation: the sun heats water and it becomes vapor.
- Transpiration: plants release water vapor through their leaves (stomata).
- Condensation: vapor cools and forms droplets, which make clouds.
- Precipitation: water falls as rain, snow, sleet or hail.
- Runoff flows over land into rivers and oceans. Infiltration is water soaking into the soil to become groundwater.
- Cutting down forests reduces transpiration and increases runoff and erosion.
Phosphorus cycle
- Phosphorus is stored mainly in rocks and ocean sediments. It's the slowest of the major cycles.
- Weathering of rock releases phosphate into soil and water; plants absorb it, and animals get it by eating plants.
- Living things need phosphorus for DNA, RNA, ATP and the phospholipids in cell membranes.
- Because it's often scarce, phosphorus is frequently a limiting nutrient. Extra phosphorus from fertilizer causes eutrophication.
Key ideas for all cycles
- Remember: energy flows one way, but matter cycles. The same atoms get reused again and again.
- Nitrification happens in two steps: soil bacteria turn ammonium (NH₄⁺) into nitrites (NO₂⁻), then other bacteria turn nitrites into nitrates (NO₃⁻), the form plants absorb most easily. Nitrites are toxic to plants, so they rarely build up.
- Organisms can't use N₂ gas directly; they depend on nitrogen-fixing bacteria.
Limiting nutrient: the nutrient whose short supply limits growth.
4Species Interactions
Use + (helped), − (harmed) and 0 (unaffected) to keep them straight.
| Interaction | Effect | Example |
|---|---|---|
| Mutualism (symbiosis) | + / + | Bees get nectar, flowers get pollinated; clownfish & sea anemone |
| Commensalism (symbiosis) | + / 0 | Barnacles riding on a whale; birds nesting in a tree |
| Parasitism (symbiosis) | + / − | Tick on a dog; tapeworm in an intestine (host usually not killed quickly) |
| Predation | + / − | Lion eats zebra (prey is killed) |
| Competition | − / − | Two bird species fighting over the same seeds |
Predator peaks lag behind prey peaks (classic example: lynx and snowshoe hare).
- Competition is closely tied to niches. See Niches & Niche Partitioning below for competitive exclusion and how species avoid competing.
- Keystone species: has a huge effect on its ecosystem compared with its numbers. Sea otters eat urchins; remove otters → urchins explode → kelp forests disappear.
- Invasive species: non-native species that spread quickly, often because they have no natural predators here.
5Habitats, Niches & Niche Partitioning
Niche vs. habitat
These two words get mixed up a lot. The easiest way to remember them: a habitat is where you live; a niche is how you live.
Habitat = the “address”
The physical place where an organism lives, including its conditions (temperature, water, soil, light).Example: a bullfrog's habitat is a freshwater pond or marsh.
Niche = the “job”
The organism's role in its ecosystem: what it eats, where it eats, when it's active, the conditions it can tolerate, and how it interacts with other species.Example: the bullfrog is a nighttime predator of insects at the pond's edge and food for herons.
Many species can share one habitat, but each has its own niche. Think of a school: everyone is in the same building (habitat), but teachers, students, cafeteria staff and the principal all have different jobs (niches).
What makes up an ecological niche?
A niche is the total of how an organism uses its environment: every biotic (living) and abiotic (non-living) factor it depends on or affects. Here's the full niche of the bullfrog:
Tolerance range: the abiotic side of a niche
Every species can survive only within a certain range of each abiotic factor. Its niche includes these ranges.
Two factors make a 2-D niche. Add a third (like food size) and it becomes a 3-D shape. Ecologist G. Evelyn Hutchinson described a niche as having many dimensions, one for every factor that matters to the species.
- Tolerance range: the full span of a condition (like temperature) an organism can survive.
- Optimal range: where it grows and reproduces best, so most individuals are found here.
- Zone of physiological stress: it can survive but struggles; few individuals live here.
- Zone of intolerance: beyond the limits; the organism can't survive.
- Where two species' niches overlap, they compete for the same resources. That's what leads to competitive exclusion or niche partitioning.
Generalists, specialists & competition
- Generalists have broad niches (raccoons and coyotes eat almost anything). Specialists have narrow niches (koalas eat almost only eucalyptus). Specialists are hit harder when their environment changes.
Gause grew two Paramecium species separately and both did fine. Grown together on the same food, one species always died out.
Fundamental vs. realized niche
The fundamental niche is the full range a species could occupy with no competitors or predators. The realized niche is the part it actually occupies. Realized ≤ fundamental.
Joseph Connell's barnacle experiment (Scotland): when he removed Balanus, Chthamalus spread down into the lower zone. Its fundamental niche was bigger than its realized niche, and competition was what shrank it.
Niche (resource) partitioning
When similar species live together, they often divide up resources so their niches overlap less. Less overlap means less competition, so they can coexist.
MacArthur's warblers (1958): five similar species all eat insects in the same spruce trees but feed in different zones. Diagram simplified.
- Spatial (where): warblers in different parts of a tree; Anolis lizards perching at different heights.
- Temporal (when): hawks hunt mice by day, owls hunt them at night.
- Dietary (what): Darwin's finches with different beak sizes eat different-sized seeds.
6Populations
A population's size, spacing and growth depend on its resources, its behavior, and births, deaths and movement in or out.
Population dispersion: 3 patterns
Dispersion is how the individuals in a population are spaced out across an area. Population density is how many individuals live in a given area (e.g. 50 trees per hectare).
| Pattern | What it looks like | Why it happens | Examples |
|---|---|---|---|
| Clumped | Individuals in groups | Resources are patchy (water, food, shelter), or animals group for protection, hunting or raising young | Herds of elk, schools of fish, wolf packs, plants growing near a stream |
| Uniform | Evenly spaced | Territorial behavior or competition for resources; some plants release chemicals that stop neighbors from growing nearby | Nesting penguins and seabirds, creosote bushes in the desert, crops in a farm field |
| Random | Unpredictable spacing | Resources are spread evenly and individuals don't strongly attract or repel each other | Dandelions (wind-blown seeds), some forest trees |
Population growth
- Exponential growth (J-curve): unlimited resources, so the population grows faster and faster.
- Logistic growth (S-curve): growth slows as resources run low and levels off at the carrying capacity (K), the largest population the environment can support.
- Growth = (births + immigration) − (deaths + emigration).
Density-dependent (stronger in crowded populations): competition, predation, disease, parasites.
Density-independent (affects any size): hurricanes, fires, droughts, temperature extremes, human activities like dams.
r-selected vs. K-selected species
Species have different “strategies” for reproducing. The letters come from population growth: r = the rate of growth (think J-curve), and K = carrying capacity (think S-curve).
| Trait | r-selected | K-selected |
|---|---|---|
| Number of offspring | Many | Few |
| Size of offspring | Small | Large |
| Parental care | Little or none | A lot |
| Time to maturity | Fast; reproduce early | Slow; reproduce later |
| Lifespan & body size | Short life, usually small | Long life, usually large |
| Survival of young | Most die young | Most survive to adulthood |
| Population growth | Rapid, booms and crashes | Slow and steady, near carrying capacity |
| Best environment | Unstable or unpredictable; quick to colonize disturbed areas | Stable and predictable; good at competing for limited resources |
| Examples | Insects, mice, frogs, most fish, bacteria, dandelions and weeds | Elephants, whales, humans, gorillas, eagles |
Connection: r-selected species are often pioneer species in succession, because they spread fast into disturbed areas. K-selected species are common in mature climax communities.
7Ecological Succession
The gradual, predictable change in the species living in an area over time.
- Primary succession begins where there is no soil: new lava rock, land exposed by a retreating glacier.
- Pioneer species (lichens, mosses) are first; they break down rock and help form soil.
- Secondary succession happens after a disturbance that leaves the soil: forest fire, abandoned farm field, flood. It's faster than primary.
- A climax community is a fairly stable, mature community (though disturbances can reset it).
- Higher biodiversity generally makes an ecosystem more stable and resilient.
8Human Impacts: Agriculture & Climate
How we grow food and use energy changes ecosystems, and it connects to almost every topic above.
Monoculture vs. polyculture
Monoculture means growing one crop over a large area (like miles of only corn). Polyculture means growing several crops together in the same space.
| Monoculture | Polyculture | |
|---|---|---|
| Pros | Easy to plant and harvest with big machines; uniform crop; high yields in the short term; simple to sell in bulk | Higher biodiversity; pests and diseases spread less; healthier soil; less need for fertilizer and pesticides; more habitat for pollinators and predators of pests |
| Cons | Low biodiversity; one pest or disease can wipe out a whole field; drains the same soil nutrients every year; relies heavily on fertilizers and pesticides, whose runoff causes eutrophication and can biomagnify | Harder to use big machines; more planning and labor; any single crop may yield less |
A classic polyculture developed by Indigenous farmers in the Americas. Each crop fills a different niche, and they help each other, which is mutualism in action:
- Corn grows tall and gives beans a pole to climb.
- Beans have nitrogen-fixing bacteria in their root nodules that add nitrogen to the soil, a natural fertilizer (see the nitrogen cycle).
- Squash spreads big leaves over the ground, shading out weeds and keeping the soil moist.
Other sustainable practices: crop rotation (planting different crops in a field from year to year to restore nutrients and break pest cycles), cover crops, no-till farming and integrated pest management (using natural predators of pests).
The greenhouse effect
Greenhouse gases (water vapor, carbon dioxide, methane and nitrous oxide) act like a blanket. The natural greenhouse effect keeps Earth warm enough for life (without it, Earth's average would be about −18 °C / 0 °F). Adding more gas traps more heat, which causes global warming.
| Gas | Main human sources | Heat-trapping power |
|---|---|---|
| Carbon dioxide (CO₂) | Burning fossil fuels (cars, power plants, factories); cutting and burning forests | 1× (the baseline). The most abundant human-added gas, so it has the biggest total effect. |
| Methane (CH₄) | Cattle and sheep digestion (burps!), manure, flooded rice paddies, landfills, natural gas leaks | About 30× CO₂ per molecule over 100 years |
| Nitrous oxide (N₂O) | Nitrogen fertilizer on farm fields, manure | About 270× CO₂ over 100 years |
| Water vapor (H₂O) | Mostly natural: evaporation from oceans, lakes and plants (transpiration). Humans don't add much directly. | The largest contributor to the natural greenhouse effect. Also amplifies warming (see below). |
Water vapor feedback loop: CO₂ and methane warm the planet → more water evaporates → more water vapor in the air → even more heat trapped → more warming. So water vapor doesn't start climate change, but it amplifies (strengthens) the warming caused by other gases.
Agriculture is a major source of methane and nitrous oxide. Counting land clearing, farming, processing and transport, the food system produces roughly a quarter to a third of global greenhouse gas emissions (IPCC estimate: 21–37%).
Data: annual averages from NOAA's Mauna Loa Observatory (the “Keeling Curve”). The monthly average hit a record of about 432 ppm in May 2026.
Effects on ecosystems
- Ocean acidification: oceans absorb CO₂, becoming more acidic, which makes it harder for corals and shellfish to build shells. (More below)
- Coral bleaching from warmer water.
- Species' ranges shift toward the poles and uphill; some can't move fast enough.
- Timing mismatches: flowers bloom earlier, but pollinators or migrating birds may not arrive in time.
9Ocean Acidification & the pH Scale
Burning fossil fuels doesn't just warm the planet. It also changes the chemistry of the ocean. To understand how, you need the pH scale.
The pH scale
pH measures how acidic or basic a solution is, based on how many hydrogen ions (H⁺) it has. It runs from 0 to 14: below 7 is acidic (more H⁺), 7 is neutral, and above 7 is basic or alkaline (fewer H⁺).
- Lower pH = more H⁺ = more acidic.
- Higher pH = fewer H⁺ = more basic.
- pH is an abiotic factor. Every organism has a tolerance range for it (see Niches). Most fish and aquatic life do best around pH 6.5–8.5.
- Acid rain (pH below about 5.6) forms when sulfur dioxide and nitrogen oxides from burning coal and gasoline mix with water in the air. It harms lakes, forests and soils.
How the ocean is becoming more acidic
The ocean absorbs a large share of the CO₂ humans release. When CO₂ dissolves in seawater, a chain of chemical reactions adds hydrogen ions and takes away the carbonate that shell-building animals need.
Effects on ocean ecosystems
- Shell builders struggle: corals, clams, oysters, sea urchins and tiny plankton use calcium carbonate (CaCO₃) for shells and skeletons. With less carbonate, building shells takes more energy, and shells can even dissolve.
- Coral reefs weaken and grow more slowly. Combined with bleaching from warmer water, this threatens one of Earth's most biodiverse ecosystems.
- Food webs are affected: pteropods (“sea butterflies”) are tiny swimming snails eaten by salmon, herring and whales. If they decline, effects ripple up the food chain.
- Behavior changes: some fish, like clownfish, are worse at detecting predators in more acidic water.
- Some winners: seagrasses and some algae may benefit, since they use CO₂ for photosynthesis.
- People are affected too: oyster hatcheries in the Pacific Northwest have lost huge numbers of baby oysters to more acidic water.
Solutions: the main fix is the same as for climate change: cut CO₂ emissions. Protecting seagrass and kelp, which absorb CO₂, can help locally.
10Quick Vocab
Want to memorize these? Study them as flashcards →
Click the card or press Space to flip · ← → to move · Cards you mark “Got it” are skipped in “Still learning only.”
Each round pulls questions you haven't seen yet, so you'll cycle through the whole bank before any repeat. Answer choices are shuffled every time.