Ecosystems and Biodiversity
What is an Ecosystem?
An ecosystem is a community of living organisms (biotic) interacting with each other and with the non-living environment (abiotic factors) such as climate, soil, water, and light. Ecosystems can be small (a pond, a hedgerow) or large (a tropical rainforest, an ocean).
Key Terms
- Biotic factors — living components: plants, animals, fungi, bacteria
- Abiotic factors — non-living components: temperature, rainfall, sunlight, soil type, water availability, wind
- Producer — an organism that makes its own food through photosynthesis (green plants, algae)
- Consumer — an organism that eats other organisms (primary = herbivore, secondary = carnivore, tertiary = top predator)
- Decomposer — organisms (bacteria, fungi) that break down dead organic matter, returning nutrients to the soil
- Food chain — a linear sequence showing energy transfer: grass → rabbit → fox → eagle
- Food web — interconnected food chains showing the complexity of feeding relationships
Nutrient Cycling
Nutrients are continuously recycled between the soil, biomass (living organisms), and litter (dead organic matter) in a closed loop:
1. Plants absorb nutrients from the soil through their roots
2. Nutrients are stored in the biomass (living plant material)
3. When leaves fall or organisms die, nutrients enter the litter layer
4. Decomposers break down the litter, returning nutrients to the soil
5. The cycle repeats
The speed of this cycle depends on temperature and moisture — decomposition is fastest in warm, wet conditions (tropical rainforests) and slowest in cold or dry environments.
Global Ecosystems (Biomes)
The world's major biomes are determined primarily by climate (temperature and precipitation):
| Biome | Climate | Location | Key Features |
|---|---|---|---|
| Tropical rainforest | Hot (25–30°C), wet (>2,000 mm/yr) | Equatorial regions (0–5°N/S) | Greatest biodiversity; dense canopy layers; nutrient-poor soils |
| Hot desert | Hot days (40°C+), cold nights, very dry (<250 mm/yr) | ~30°N/S (high pressure belts) | Sparse vegetation; adapted organisms; sandy/rocky terrain |
| Tropical grassland (savanna) | Hot, distinct wet/dry seasons | 5–15°N/S | Tall grasses; scattered trees; grazing animals; seasonal fires |
| Temperate deciduous forest | Mild, moderate rainfall (500–1,500 mm) | 40–60°N/S (including UK) | Deciduous trees; four distinct seasons; fertile brown earth soils |
| Temperate grassland | Hot summers, cold winters, moderate rain | Continental interiors | Deep fertile soils; few trees; large grazing mammals |
| Boreal forest (taiga) | Long cold winters, short cool summers | 50–70°N | Coniferous (evergreen) trees; acidic podzol soils |
| Tundra | Very cold (<−10°C average), very dry | Above ~70°N and high mountains | Permafrost; mosses, lichens; short growing season |
| Polar | Permanently below freezing | Arctic and Antarctic | Ice caps; almost no vegetation |
Small-Scale UK Ecosystem: A Freshwater Pond
A pond demonstrates all ecosystem principles at a small scale:
- Producers: algae, duckweed, water lilies, reeds
- Primary consumers: tadpoles, water snails, mayfly larvae
- Secondary consumers: dragonfly larvae, sticklebacks, frogs
- Tertiary consumers: herons, grass snakes
- Decomposers: bacteria in the mud at the bottom
Interdependence
All parts of an ecosystem are linked. If one component changes, it creates a ripple effect:
- Removing pond weed reduces oxygen → fish die → herons lose food source
- Adding fertiliser causes algal bloom → blocks light → plants die → organisms that depend on them decline (eutrophication)
Exam tip: For any ecosystem question, always consider both biotic and abiotic factors and explain how they are interconnected (interdependent).