Biology (5090)
Habitat: Where an organism lives (e.g., a pond, a desert) Population: All organisms of one species in a habitat Community: All different species in a habitat Ecosystem: A community plus the non-living environment Biodiversity: Variety of different species in an area
Producer → Primary Consumer → Secondary Consumer → Tertiary Consumer Producers: Plants (make food by photosynthesis) Consumers: Animals (eat other organisms) Decomposers: Bacteria and fungi (break down dead matter)
Energy decreases at each trophic level (only ~10% passed on) Rest is lost as heat from respiration, movement, and waste That's why food chains rarely have more than 4-5 levels
Photosynthesis (CO₂ removed from air) Respiration (CO₂ returned to air) Combustion of fossil fuels (CO₂ released) Decomposition (CO₂ released)
Deforestation, pollution, overfishing Greenhouse effect: CO₂ and methane trap heat → global warming Solutions: renewable energy, recycling, conservation
Topic 3 of 4Cambridge O Levels
Ecology & Environment
Ecosystems, food chains, and environmental impact
Ecology is the study of how living things interact with each other and their environment.
Key Terms:
Food Chains & Webs:
Energy Flow:
Carbon Cycle: Carbon moves between atmosphere, organisms, and earth through:
Human Impact:
Key Points to Remember
- 1Food chain: Producer → Primary → Secondary → Tertiary consumer
- 2Only ~10% of energy transfers between trophic levels
- 3Carbon cycle: photosynthesis removes CO₂, respiration/combustion releases it
- 4Biodiversity is threatened by human activities
Pakistan Example
The Indus River Ecosystem
The Indus River is Pakistan's lifeline. A simple food chain: Algae (producer) → Small fish (primary consumer) → Palla fish (secondary consumer) → Indus dolphin (tertiary consumer). The blind Indus dolphin is endangered — only about 2,000 remain! Pollution from factory waste, overfishing, and dam construction have disrupted this ecosystem. The Indus also supports Pakistan's agriculture — rice paddies in Sindh depend on this water. Protecting the Indus ecosystem means protecting Pakistan's food security.