What an ecosystem is
An ecosystem is every living thing in an area plus the nonliving parts it interacts with — sunlight, water, soil, temperature. A rotting log, a coral reef and the whole Amazon all qualify. The boundary is wherever ecologists decide the interactions are mostly internal.
Within any ecosystem, organisms sort into roles: producers make their own food (plants, algae), consumers eat other organisms, and decomposers break dead material back into nutrients. Every energy question in ecology comes back to who eats whom.
- Ecosystem = living community + nonliving environment
- Producers (autotrophs) make food; consumers (heterotrophs) eat it
- Decomposers recycle dead matter into nutrients plants can reuse
- Size doesn't matter — a puddle and a forest both count
Food chains and food webs
A food chain is a simple one-way arrow: grass → grasshopper → frog → snake → hawk. Each arrow points to the eater. A food web is the honest version — the tangled map of who actually eats whom, since most animals eat more than one thing and get eaten by more than one thing.
Trophic levels are the chain's floors: producers at the bottom, then primary consumers (herbivores), secondary consumers, and apex predators on top. Vocabulary to keep straight: herbivores eat plants, carnivores eat animals, omnivores eat both, and decomposers clean up whatever dies. Draw a food web once using animals from your own neighborhood and the vocabulary locks in.
- Arrows in a food chain point toward the eater
- Food web = all the overlapping chains in an ecosystem
- Trophic levels: producer → primary consumer → secondary consumer → apex
- Herbivore: plants · carnivore: animals · omnivore: both
Energy flow and the 10 percent rule
Here's the famous fact: only about 10% of the energy at one trophic level passes to the next — the rest is lost as heat, movement and waste. That's why food chains rarely exceed four or five levels, and why there are millions of grasshoppers but only a few hawks. Energy flows through ecosystems; it does not cycle.
The 10% rule turns into easy math questions. If producers hold 10,000 kcal of energy, primary consumers get about 1,000, secondary consumers about 100, and third-level consumers about 10. That's also why eating lower on the food chain — plants — supports so many more organisms than eating at the top.
- Only about 10% of energy transfers up each trophic level
- Lost energy leaves as heat, movement and waste
- 10,000 kcal of producers → 1,000 → 100 → 10 up the chain
- Short chains exist because energy runs out fast
If a field's plants contain 5,000 kcal and cows eat them, the cows store about 500 kcal (10%). Humans eating the cows access about 50 kcal. If humans ate the plants directly, they'd reach the full 5,000 — that math is exactly why 'eating low on the food chain' can feed more people.
Biomes: Earth's major life zones
A biome is a large region defined by its climate and dominant vegetation — a life-zone template. The big land ones: tropical rainforest (hot, wet, most diverse), desert (very dry), grassland and savanna, temperate forest, taiga (cold conifer forest) and tundra (frozen, treeless). Aquatic biomes split into freshwater and marine, with coral reefs the most diverse water biome.
Two variables predict almost any biome: temperature and rainfall. Hot and wet gives rainforest; hot and dry gives desert; cold and dry gives tundra. When a test question describes a climate, translate it into those two numbers and the biome practically names itself.
- Biome = large region shaped by climate plus dominant plants
- Temperature and rainfall predict the biome almost every time
- Rainforest: most diverse · tundra: coldest land · desert: driest
- Aquatic biomes: freshwater vs marine (ocean)
Key concepts to memorize
🎯 Study tips for this topic
- Memorize one rule about arrows: they point toward the eater — half of all food web errors come from backwards arrows.
- Practice 10% math until dividing by 10 twice is automatic; those are guaranteed test points.
- Build a food web from animals you actually know (squirrels, hawks, grass) — familiar organisms make roles stick.
- Learn biomes as temperature + rainfall pairs, not as lists of species.
- Sketch the energy pyramid (producers wide, predators narrow) once from memory — the shape explains the rules.
