Ecology as a science and how organisms respond
Ecology studies how living things interact with each other and with their non-living surroundings. Ecologists observe in the field, run experiments (for example, fencing out grazers from one plot) and build models. Levels: organism → population → community → ecosystem → biome → biosphere.
Organisms respond to their environment. Some responses are behaviour (birds migrating, animals hiding from heat). Some are changes in the body during life: the same genes can give different phenotypes in different conditions. Examples: hydrangea flowers are blue in acidic soil and pink in alkaline soil; some turtle eggs become male or female depending on temperature; plants grown in shade get longer, thinner stems.
Terrestrial biomes
A biome is a large area with a similar climate and similar kinds of plants and animals. On land, average temperature and yearly rainfall decide it.
- Tundra: very cold, frozen subsoil (permafrost), mosses and lichens.
- Taiga (boreal forest): cold winters, conifers like spruce.
- Temperate forest: four seasons, broad-leaved trees that drop leaves.
- Grassland (prairie, steppe, savanna): grasses, few trees, grazing animals, fires.
- Desert: under about 250 mm rain a year; cacti, deep roots, night-active animals.
- Tropical rainforest: hot and wet all year, over 2,000 mm rain, the highest biodiversity.
Mountains show biomes in layers: going up 1,000 m is a bit like going many hundreds of kilometres towards the poles.
Aquatic and marine ecosystems
Water covers about 71% of Earth. Aquatic biomes are decided by salt, depth, light, temperature and flow.
- Freshwater: ponds and lakes (still) and rivers and streams (flowing); wetlands filter water.
- Estuaries: where rivers meet the sea; mixed salt (brackish); very productive nurseries for fish. Mangroves like the Sundarbans grow here.
- Marine: coral reefs (warm, shallow, clear water), the open ocean and the deep sea.
Zones by light: the photic zone (top ~200 m) has enough light for photosynthesis; the aphotic zone below is dark; the benthic zone is the bottom. Phytoplankton in the photic zone make a large share of Earth's oxygen.
Nutrient cycles: nitrogen and phosphorus
Energy flows through an ecosystem once, but matter is recycled.
Nitrogen cycle
- Fixation: bacteria (e.g. Rhizobium in legume root nodules) and lightning turn N₂ into ammonia/ammonium.
- Nitrification: soil bacteria turn ammonium into nitrite, then nitrate.
- Assimilation: plants take in nitrate to make proteins and DNA; animals eat plants.
- Ammonification: decomposers turn wastes and dead bodies back into ammonium.
- Denitrification: in waterlogged, low-oxygen soil, bacteria turn nitrate back to N₂.
Phosphorus cycle
Phosphorus has no gas form. Weathering of rock releases phosphate into soil and water; plants absorb it for DNA, ATP and bones; it returns via wastes and decay; some settles as sediment and becomes rock over millions of years. It is often the limiting nutrient in lakes.
Aerobic and anaerobic cycling
Decomposers that use oxygen (aerobic) break matter down fully to CO₂, water and minerals. Without oxygen (anaerobic, e.g. in swamps, rice paddies, landfill) breakdown is slower and releases methane and other gases. Denitrification is also anaerobic.
Ecosystems under stress: disruption and change over time
Ecosystems change naturally (succession, fires, floods) and through human action:
- Eutrophication: nitrate and phosphate runoff → algal bloom → algae die → decomposers use up oxygen → fish die.
- Habitat loss: forests cleared for farms and cities.
- Invasive species: e.g. water hyacinth covering lakes.
- Climate change: warmer seas bleach corals; biomes shift up mountains.
- Overharvesting: overfishing collapses fish stocks.
Resistance is how well an ecosystem stays the same when disturbed; resilience is how fast it recovers. Biodiverse systems are usually more resilient.
Change over time and case studies
After the last ice age, many temperate lands went from tundra to woodland; later, people cleared forests for farms, moorland and towns. Study a local ecosystem (a pond, park or wetland): record species, measure light, pH and temperature, and note human pressures. Case studies: a coral reef under warming, a mangrove coast under shrimp farming, a lake recovering after sewage treatment.
Try it: lake doctor
In the last 3D step, find the largest runoff where all 5 fish survive. Then raise it until oxygen falls below 5 mg/L. What could a farmer do to stay below that level? (Hint: less fertiliser, buffer strips of plants along streams, legumes instead of chemical nitrogen.) At home: put pond water in two jars, add a pinch of plant fertiliser to one, keep both in light for a week and compare how green they get.
Key formulas and definitions
- Biome = climate (temperature + rainfall) → plants → animals
- N₂ → (fixation) NH₄⁺ → (nitrification) NO₂⁻ → NO₃⁻ → (assimilation) proteins → (ammonification) NH₄⁺ → (denitrification) N₂
- Phosphorus: rock → soil/water PO₄³⁻ → organisms → sediment → rock (no gas)
- Aerobic decomposition → CO₂ + H₂O + minerals; anaerobic → CH₄ and others
- Eutrophication: nutrients ↑ → algae ↑ → O₂ ↓ → fish ↓
- Resistance = stays the same; resilience = recovers fast
Worked examples
1. A place has an average temperature of 27 °C and 2,500 mm of rain a year. Which biome is it likely to be, and why?
Step 1: Hot (above about 20 °C all year). Step 2: Very wet (over 2,000 mm). Step 3: Hot + very wet = tropical rainforest, with tall layered trees and very high biodiversity.
2. A farmer grows wheat on the same field every year and the yield drops. Then she grows beans for one season and the next wheat crop improves. Explain using the nitrogen cycle.
Wheat removes nitrate from the soil each year (assimilation). Beans are legumes: Rhizobium bacteria in their root nodules fix N₂ from air into ammonium. When the bean roots decay, ammonification and nitrification add nitrate to the soil, so the next wheat crop has more nitrogen.
3. Lake oxygen was 8 mg/L. After heavy fertiliser runoff it falls by 1.5 mg/L each week. Fish need at least 5 mg/L. After how many weeks are fish in danger?
Oxygen after n weeks = 8 − 1.5n. Set 8 − 1.5n < 5 → 1.5n > 3 → n > 2. So after 2 weeks oxygen is exactly 5 mg/L, and during the third week it drops below 5: fish are in danger.
Common mistakes
- Saying plants take nitrogen straight from the air. Most plants need nitrate or ammonium from soil; only fixing bacteria use N₂.
- Thinking the phosphorus cycle has a gas stage like carbon and nitrogen. It does not.
- Mixing up nitrification (ammonium → nitrate) and denitrification (nitrate → N₂).
- Saying fish die in eutrophication because algae are poisonous. Mainly, decomposers rotting dead algae use up the oxygen.