Ecosystems & Biomes

You’ll be able to

Distinguish an ecosystem from a biome and describe how the two nest together
Explain how temperature and precipitation determine the global distribution of biomes
Identify major terrestrial and aquatic biomes from their climate and characteristic organisms

From organism to biosphere

Ecology is organized as a set of nested levels. A single organism belongs to a population (same species, same area), which belongs to a community (all interacting populations). Add the nonliving — sunlight, water, soil, air — and you have an ecosystem: a community plus its physical environment, linked by flows of energy and matter. Ecosystems that share a climate and characteristic life form, spread across the globe, make up a biome.

Climate draws the map of biomes

Two variables — average temperature and annual precipitation — predict which biome forms in a given place better than almost anything else. Warm and wet gives tropical rainforest; warm and dry gives desert; cold with a short growing season gives tundra. This is why biomes form broad latitudinal bands: the tropics near the equator, deserts near 30° where dry air descends, and cold biomes toward the poles.

Aquatic biomes run on different variables

Aquatic systems are classified less by temperature and more by salinity, depth, and water flow. Freshwater biomes (lakes, streams, wetlands) hold little dissolved salt; marine biomes (oceans, coral reefs, estuaries) are salty. Estuaries, where rivers meet the sea, mix the two and rank among the most productive systems on Earth because rivers deliver a steady load of nutrients.

What separates biomes

terrestrial biome ≈ f(temperature, precipitation)

Aquatic biomes instead key on salinity, depth, and flow. Same idea: the abiotic setting selects the community.

Worked example

A region receives about 25 cm of rain per year, swings from hot days to cold nights, and supports drought-adapted plants with shallow, wide root systems. Which biome is it, and what is the reasoning?

1.Precipitation is the strongest clue: 25 cm·yr⁻¹ is very low — most biomes need more.
2.Deserts are defined by receiving less than roughly 25–30 cm of precipitation per year.
3.The large day–night temperature swing fits a desert: dry air and sparse cover cannot trap the day’s heat overnight.
4.Shallow, spreading roots let plants grab brief rainfall before it evaporates — a classic desert adaptation.

Answer: A desert — the defining feature is precipitation below ~25–30 cm·yr⁻¹, not the temperature.

Checkpoint

Which pair of abiotic factors is most useful for predicting which terrestrial biome will develop in a given region?

Tip

A quick mental grid helps: put temperature on one axis and precipitation on the other. Warm + wet → rainforest; warm + dry → desert; cold + dry → tundra; cold/temperate + moderate → forest or grassland. Most biome ID questions fall out of this grid.

Checkpoint

Estuaries are among the most biologically productive ecosystems on Earth. What best explains this?

On the exam

AP questions frequently give you a climate description or a climatograph (a bar-and-line graph of monthly rainfall and temperature) and ask for the biome. Read the precipitation first — it eliminates the most options fastest — then use temperature to decide between the remaining candidates.

Answer the 2 checkpoints as you read.