Organisms and their Environment
Summary: Ecology is the study of interactions between organisms and their environment. Key terms include population, community, habitat, and ecosystem. Energy flows through food chains and food webs from producers to consumers, with approximately 10% of energy transferred between trophic levels. This can be represented by pyramids of numbers, biomass, and energy. Nutrient cycles (carbon cycle, nitrogen cycle, water cycle) recycle essential elements through the biotic and abiotic components of ecosystems. Population growth is influenced by food availability, predation, disease, and environmental factors. Tags: igcse biology ecology food-chains energy-transfer carbon-cycle nitrogen-cycle population Created: 2026-07-16 Last Updated: 2026-07-16
1. Key Ecological Terms
| Term | Definition |
|---|---|
| Species | A group of organisms that can interbreed to produce fertile offspring |
| Population | All the organisms of a single species living in the same habitat at the same time |
| Community | All the populations of different species living in the same habitat at the same time |
| Habitat | The place where an organism lives (its “address”), including the physical conditions |
| Ecosystem | A community of organisms interacting with each other and with their physical (abiotic) environment |
| Niche | The role of an organism within its ecosystem — what it does, what it eats, where it feeds, when it is active |
2. Food Chains and Food Webs
Food chain: A linear sequence showing the transfer of energy from one organism to the next, starting with a producer.
Example:
Grass (producer) → Rabbit (primary consumer / herbivore) → Fox (secondary consumer / carnivore) → Eagle (tertiary consumer / top carnivore)
Trophic levels:
| Trophic Level | Role | Examples |
|---|---|---|
| Producer (Trophic level 1) | Autotrophs — produce their own food by photosynthesis | Grass, trees, algae, phytoplankton |
| Primary consumer (Trophic level 2) | Herbivores — eat producers | Rabbit, caterpillar, zooplankton, cow |
| Secondary consumer (Trophic level 3) | Carnivores — eat primary consumers | Fox, bird, spider, small fish |
| Tertiary consumer (Trophic level 4) | Top carnivores — eat secondary consumers | Eagle, lion, shark |
| Decomposers | Break down dead organic matter and waste, returning nutrients to the soil | Bacteria, fungi (saprophytes) |
Food web: A network of interconnected food chains showing all the feeding relationships in a community. Most organisms eat more than one type of food, and are eaten by more than one predator.
3. Energy Transfer in Food Chains
Energy enters the ecosystem through photosynthesis (producers convert light energy to chemical energy in glucose).
Energy losses between trophic levels:
- Approximately 90% of energy is lost at each trophic level
- Only about 10% of the energy is transferred to the next level
Reasons for energy loss:
| Reason | Explanation |
|---|---|
| Respiration | All organisms use energy for respiration (movement, growth, maintenance, heat) — this energy is eventually lost as heat |
| Not all of the organism is eaten | Bones, roots, feathers, fur may not be consumed by the next organism |
| Not all food is digested | Some food passes through the gut undigested and is egested as faeces |
| Excretion | Urea and other waste products contain energy that is lost |
Consequences of energy loss:
- Food chains rarely have more than 4-5 trophic levels (there is not enough energy to support another level)
- There are always fewer top carnivores than herbivores (less energy available at higher levels)
- Pyramids of biomass and pyramids of energy always show a decrease at higher trophic levels
4. Pyramids of Numbers, Biomass, and Energy
| Type of Pyramid | What It Shows | Shape | Notes |
|---|---|---|---|
| Pyramid of numbers | Number of organisms at each trophic level | Usually pyramid-shaped, but can be inverted (e.g. one large oak tree supports many caterpillars) | Simple to construct but can be misleading — does not account for organism size |
| Pyramid of biomass | Total dry mass of organisms at each trophic level (g/m2) | Almost always pyramid-shaped | More accurate than numbers, but must be measured at the same time and dried to remove water |
| Pyramid of energy | Energy content at each trophic level (kJ/m2/year) | Always pyramid-shaped | Most informative — accounts for rate of energy flow over time. Hardest to construct |
5. The Carbon Cycle
The carbon cycle describes how carbon atoms are cycled between the atmosphere, organisms, and the Earth.
Key processes:
| Process | Description | Equation |
|---|---|---|
| Photosynthesis | Plants remove CO2 from the atmosphere and convert it to glucose | 6CO2 + 6H2O → C6H12O6 + 6O2 |
| Respiration | All organisms release CO2 back into the atmosphere by breaking down glucose | C6H12O6 + 6O2 → 6CO2 + 6H2O |
| Feeding | Carbon in organic compounds passes from producers to consumers along food chains | — |
| Decomposition | Decomposers (bacteria, fungi) break down dead organisms and waste, releasing CO2 through respiration | — |
| Combustion (burning) | Burning of fossil fuels (coal, oil, natural gas) and wood releases CO2 | Hydrocarbon + O2 → CO2 + H2O |
| Fossilisation | Dead organisms that do not fully decompose may, over millions of years under pressure, form fossil fuels (coal from plants, oil/natural gas from marine organisms) | — |
Carbon reservoirs:
- Atmosphere (as CO2)
- Oceans (dissolved CO2 and carbonates)
- Living organisms (organic compounds)
- Fossil fuels (coal, oil, natural gas)
- Limestone and sedimentary rocks (calcium carbonate)
6. The Nitrogen Cycle
Nitrogen is essential for making proteins and DNA, but most organisms cannot use atmospheric nitrogen (N2) directly.
Key processes:
| Process | Description | Organisms Involved |
|---|---|---|
| Nitrogen fixation | Conversion of atmospheric N2 into ammonia (NH3) or ammonium ions (NH4+) | Nitrogen-fixing bacteria — free-living in soil (e.g. Azotobacter) or in root nodules of leguminous plants (e.g. Rhizobium) |
| Nitrification | Conversion of ammonium ions (NH4+) → nitrites (NO2-) → nitrates (NO3-) | Nitrifying bacteria (e.g. Nitrosomonas, Nitrobacter) — aerobic soil bacteria |
| Assimilation (uptake) | Plants absorb nitrates (NO3-) from the soil through roots and use them to make amino acids and proteins | Plants |
| Decomposition / Ammonification | Decomposers break down proteins and urea from dead organisms and waste → release ammonium ions (NH4+) into soil | Decomposers (bacteria, fungi) |
| Denitrification | Conversion of nitrates (NO3-) back to atmospheric N2 gas | Denitrifying bacteria — anaerobic soil bacteria. Reduces soil fertility |
The role of leguminous plants (peas, beans, clover):
- Have root nodules containing nitrogen-fixing bacteria (Rhizobium)
- The bacteria convert N2 from air into ammonium compounds
- The plant provides glucose (from photosynthesis) to the bacteria; the bacteria provide fixed nitrogen to the plant — this is a mutualistic relationship
- Crop rotation includes legumes to naturally replenish soil nitrates
7. The Water Cycle
The water cycle (hydrological cycle) describes how water moves between the atmosphere, land, and oceans.
| Process | Description |
|---|---|
| Evaporation | Water changes from liquid to water vapour — mainly from oceans, lakes, and rivers. Heat from the sun provides energy |
| Transpiration | Loss of water vapour from plant leaves through stomata |
| Evapotranspiration | Combined evaporation from land and water surfaces plus transpiration from plants |
| Condensation | Water vapour cools and condenses into tiny water droplets, forming clouds |
| Precipitation | Water falls from clouds as rain, snow, hail, or sleet when droplets become too heavy |
| Surface run-off | Water flows over the ground surface into rivers, lakes, and eventually the sea |
| Infiltration / Percolation | Water soaks into the soil and moves down through rocks into groundwater (aquifers) |
| Groundwater flow | Slow movement of water through underground rocks back to the sea |
8. Population Growth
Population size is determined by the balance between births, deaths, immigration, and emigration.
Factors affecting population growth:
| Factor | Effect on Population |
|---|---|
| Food availability | More food → more energy for reproduction → population increases. Food shortage → competition increases → population decreases |
| Predation | Increased predation → prey population decreases. Prey decrease → predator population decreases (less food). This creates predator-prey cycles |
| Disease | Disease outbreak → increased death rate → population decreases |
| Competition | Competition for resources (food, water, territory, mates) limits population size |
| Climate / weather | Extreme conditions (drought, flood, cold) can reduce population |
| Space / territory | Limited space restricts breeding and increases competition |
Population growth curve (sigmoid/S-shaped curve):
- Lag phase: Slow growth — organisms are adapting/acclimatising to the environment
- Exponential (log) phase: Rapid growth — abundant resources, low competition, birth rate exceeds death rate
- Stationary phase: Growth rate slows and levels off — population has reached the carrying capacity of the environment (the maximum population the environment can sustain). Competition, predation, and resource limitation balance births and deaths
Related Notes
- Plant Nutrition — Photosynthesis as the entry point of energy into food chains
- Respiration — Respiration releases energy and CO2 for the carbon cycle
- Characteristics and Classification of Living Organisms — Species definitions, classification
- Human Influences on Ecosystems — Human impacts on nutrient cycles and ecosystems
- Transport in Plants — Transpiration’s role in the water cycle
- IGCSE-Bio-Index — Full IGCSE Biology index
Sources
- BBC Bitesize GCSE Biology — Ecology / Ecosystems and energy transfer, BBC (free educational resource)
- OpenStax Biology 2e — Chapter 46: Ecosystems, Rice University (free, CC BY 4.0)
- Cambridge IGCSE Biology 0610 — Topic 19: Organisms and their Environment, Cambridge Assessment International Education
- CK-12 Biology for High School — Ecology chapter, CK-12 Foundation (free, CC BY-NC 3.0)
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Pyramids of numbers always look like pyramids” | Pyramids of numbers can be inverted — e.g. one large oak tree (producer) can support thousands of insects. Pyramids of biomass and energy are more reliable shapes |
| ”Energy is recycled in ecosystems” | Energy flows through ecosystems (enters as light, exits as heat) — it is NOT recycled. Nutrients (carbon, nitrogen) are recycled |
| ”Decomposers are the last trophic level in a food chain” | Decomposers operate at all trophic levels — they break down dead organisms and waste from producers, consumers at all levels |
| ”Carbon dioxide is a pollutant” | CO2 is a natural and essential component of the carbon cycle — needed for photosynthesis. It becomes a problem when human activities (fossil fuel burning, deforestation) unbalance the cycle |
| ”Nitrogen-fixing bacteria live in all plants” | Nitrogen-fixing bacteria live in root nodules of leguminous plants (peas, beans, clover) — not all plants. Most plants rely on nitrates already in the soil |
| ”Plants absorb nitrogen gas directly from the air” | Plants cannot use N2 gas. They absorb nitrogen as nitrate ions (NO3-) from the soil (or ammonium ions) |