Organisms and Their Environment

What you'll learn

This topic examines how organisms interact with each other and their physical surroundings within ecosystems. CIE IGCSE Biology papers regularly test understanding of food chains, energy transfer, population dynamics, nutrient cycling and human activities affecting the environment. Questions range from interpreting ecological pyramids to explaining conservation methods, making this a high-yield revision area.

Key terms and definitions

Ecosystem — a unit containing all the organisms (the community) and the non-living components in a particular area, interacting together.

Population — a group of organisms of the same species living in the same area at the same time.

Community — all the populations of different species living in the same area at the same time.

Habitat — the place where an organism lives.

Trophic level — the position of an organism in a food chain, food web or pyramid of biomass, numbers or energy.

Producer — an organism that makes its own organic nutrients, usually using energy from sunlight through photosynthesis (e.g. green plants, algae).

Consumer — an organism that gets its energy by feeding on other organisms.

Decomposer — an organism that breaks down dead organic material and waste products, returning nutrients to the soil (e.g. bacteria, fungi).

Core concepts

Food chains and food webs

A food chain shows the transfer of energy from one organism to another, beginning with a producer. Each arrow means "is eaten by" or "provides energy for".

Example: grass → grasshopper → frog → snake → hawk

Key features:

• Always begins with a producer that converts light energy into chemical energy through photosynthesis
• Primary consumers (herbivores) eat producers
• Secondary consumers (carnivores) eat primary consumers
• Tertiary consumers eat secondary consumers
• Chains rarely exceed five trophic levels due to energy loss

A food web shows multiple interconnected food chains in an ecosystem, representing the complex feeding relationships more realistically than single chains. Organisms often feed at different trophic levels; for example, humans consume both plants (acting as primary consumers) and meat (acting as secondary or tertiary consumers).

Energy flow through ecosystems

Energy enters ecosystems through producers via photosynthesis and flows through trophic levels, but the transfer is inefficient.

Energy transfer between trophic levels:

• Only approximately 10% of energy is transferred from one trophic level to the next
• About 90% of energy is lost at each level through:
  • Heat from respiration
  • Movement and other life processes
  • Excretion (urine, faeces)
  • Not all parts of organisms are consumed (bones, roots, cellulose)

This explains why:

• Food chains are limited in length (insufficient energy remains to support higher levels)
• Pyramids of biomass typically narrow toward the top
• Large numbers of producers are needed to support smaller numbers of consumers

Pyramids of numbers, biomass and energy:

CIE IGCSE Biology examinations require construction and interpretation of ecological pyramids.

Pyramid of numbers — shows the number of organisms at each trophic level. Can be inverted when one large producer (e.g. oak tree) supports many consumers (e.g. caterpillars).

Pyramid of biomass — shows the dry mass of living material at each trophic level at a particular time. More reliable than pyramids of numbers and rarely inverted. Measured in g/m² or kg/m².

Pyramid of energy — shows the energy flow through each trophic level over a period of time (usually per year). Never inverted because energy is always lost between levels. Measured in kJ/m²/year.

Nutrient cycles

The carbon cycle:

Carbon atoms cycle through ecosystems via biological and chemical processes:

1. Carbon dioxide removed from atmosphere:

   • Photosynthesis by green plants and algae converts CO₂ into organic compounds (glucose, starch, cellulose, proteins, fats)

2. Carbon dioxide returned to atmosphere:

   • Respiration by all living organisms (plants, animals, decomposers) releases CO₂
   • Combustion of fossil fuels (coal, oil, natural gas) releases CO₂ stored over millions of years
   • Decomposition of dead organic matter by bacteria and fungi releases CO₂ through their respiration

3. Carbon transferred through feeding:

   • Herbivores consume plant material
   • Carnivores consume animal material
   • Carbon compounds pass along food chains

The nitrogen cycle:

Nitrogen is essential for making proteins and nucleic acids (DNA/RNA), but atmospheric nitrogen (N₂) cannot be used directly by most organisms.

Key processes:

1. Nitrogen fixation:

   • Nitrogen-fixing bacteria (e.g. Rhizobium in legume root nodules, Azotobacter in soil) convert atmospheric nitrogen into ammonium compounds
   • Lightning also fixes small amounts of nitrogen

2. Nitrification:

   • Nitrifying bacteria in soil convert ammonium compounds into nitrites, then into nitrates
   • Requires aerobic conditions
   • Plants absorb nitrates through roots and use them to make proteins

3. Feeding and death:

   • Animals obtain nitrogen compounds by eating plants or other animals
   • Death and waste (urine, faeces) add nitrogen compounds to soil

4. Decomposition:

   • Decomposers break down proteins and urea into ammonium compounds, returning them to soil

5. Denitrification:

   • Denitrifying bacteria in waterlogged, anaerobic soil convert nitrates back into nitrogen gas
   • Returns nitrogen to atmosphere, completing the cycle

Population size and dynamics

Population size is determined by the balance between:

• Births and immigration (increase population)
• Deaths and emigration (decrease population)

Factors affecting population size:

Limiting factors restrict population growth:

• Food availability
• Water supply
• Light (for plants)
• Space/territory
• Disease and parasites
• Predation
• Competition (intraspecific within species, interspecific between species)

Populations typically show exponential growth when conditions are ideal, followed by stabilization when limiting factors take effect, creating an S-shaped (sigmoid) growth curve.

Human activities and environmental impact

Deforestation:

Large-scale removal of forests for timber, agriculture, cattle ranching and urban development causes:

• Reduced biodiversity (habitat loss for species)
• Soil erosion (tree roots no longer bind soil; increased flooding)
• Increased atmospheric CO₂ (fewer trees for photosynthesis; burning releases stored carbon)
• Climate change (disruption of water cycle, increased greenhouse effect)

Pollution:

Water pollution:

• Sewage discharge adds nitrates and phosphates, causing eutrophication
• Process: excessive algal growth → blocks light → submerged plants die → decomposers multiply → oxygen depletion → fish and invertebrates die
• Pesticides and heavy metals accumulate in food chains (bioaccumulation)

Air pollution:

• Sulfur dioxide from fossil fuel combustion causes acid rain (damages trees, aquatic ecosystems, buildings)
• Carbon dioxide, methane and other greenhouse gases trap heat, causing global warming

Land pollution:

• Non-biodegradable plastics persist in environment
• Toxic chemicals contaminate soil and groundwater

Overfishing:

Excessive fishing depletes fish stocks faster than natural reproduction, threatening food security and marine ecosystems. Solutions include quotas, net size regulations, and protected breeding areas.

Conservation and sustainable development

Conservation methods tested in CIE IGCSE Biology:

Sustainable resource management:

• Replanting programs (reforestation)
• Fishing quotas and seasonal restrictions
• Recycling to reduce resource extraction
• Alternative energy sources (solar, wind) to reduce fossil fuel use

Protected areas:

• National parks and nature reserves preserve habitats and biodiversity
• Breeding programs for endangered species

Education and legislation:

• International agreements limiting emissions and protecting endangered species
• Public awareness campaigns promoting sustainable practices

Sustainable development meets current human needs without compromising future generations' ability to meet their needs.

Worked examples

Example 1: Energy transfer calculation

Question: A field contains grass with 45,000 kJ/m² of energy. Rabbits feed on the grass and contain 4,200 kJ/m² of energy. Calculate the percentage energy transfer from grass to rabbits.

Answer: Percentage energy transfer = (energy in rabbits ÷ energy in grass) × 100 = (4,200 ÷ 45,000) × 100 = 9.3%

Mark scheme would award:

• 1 mark for correct formula or working
• 1 mark for correct answer (accept 9-10%)

Example 2: Interpreting a food web

Question: In a pond ecosystem: water plants → water fleas → small fish → large fish. Water fleas also eat algae. Predict and explain what would happen to the population of large fish if a disease killed most of the small fish.

Answer: The population of large fish would decrease [1 mark].

Explanation: Large fish feed on small fish, so their food source would be reduced [1 mark]. Some large fish would die from starvation / competition for remaining food would increase [1 mark].

Alternative point: Eventually the population might recover if water flea populations increase (due to reduced predation by small fish), providing more food for any remaining small fish to recover [1 mark for development].

Example 3: Nitrogen cycle application

Question: A farmer plants clover (a leguminous plant with nitrogen-fixing bacteria in root nodules) before planting wheat. Explain how this improves wheat growth.

Answer: Nitrogen-fixing bacteria in clover roots convert atmospheric nitrogen into nitrates/ammonium compounds [1 mark]. When clover is ploughed into soil, decomposers break down the plant material, releasing nitrates into soil [1 mark]. Wheat plants absorb these nitrates through roots [1 mark] and use them to make proteins/amino acids for growth [1 mark].

Common mistakes and how to avoid them

• Mistake: Stating that energy is transferred with 100% efficiency between trophic levels, or that energy is "lost" completely. Correction: Only approximately 10% of energy transfers to the next level; 90% is lost as heat from respiration, through movement, in waste products, and in unconsumed parts.

• Mistake: Drawing food chain arrows pointing from consumer to producer. Correction: Arrows always point in the direction of energy flow—from the organism being eaten to the organism doing the eating (e.g., grass → rabbit, not rabbit → grass).

• Mistake: Confusing decomposers with detritivores. Correction: Decomposers (bacteria, fungi) are microorganisms that break down dead material through extracellular digestion. Detritivores (earthworms, woodlice) are larger organisms that feed on dead material but are consumers, not decomposers.

• Mistake: Stating that plants respire only at night or don't respire at all. Correction: Plants respire continuously, day and night, releasing CO₂. During daylight, photosynthesis rate usually exceeds respiration rate, resulting in net CO₂ uptake.

• Mistake: Thinking biomass increases between trophic levels. Correction: Biomass almost always decreases at higher trophic levels because energy (and therefore mass of living tissue) is lost at each stage through respiration, waste and unconsumed parts.

• Mistake: Describing eutrophication as direct poisoning of fish by fertilizers. Correction: Eutrophication is an indirect process: excess nutrients → algal bloom → light blocked → plants die → decomposers increase → oxygen depleted → fish suffocate.

Exam technique for Organisms and Their Environment

• Command word awareness: "Explain" requires reasons or mechanisms (2-3 marks typically), not just description. "Suggest" indicates an unfamiliar context requiring application of principles. "State" or "Name" needs brief, precise answers (1 mark each).

• Drawing and interpreting pyramids: Use a ruler for neat, proportional bars. Label each trophic level (producer, primary consumer, etc.) and include units (g/m², kJ/m²/year). For interpretation questions, refer to specific data from the diagram and explain trends using energy transfer principles.

• Calculations: Show all working clearly for energy transfer or percentage questions. Include units in final answers. Common calculations include percentage energy transfer between trophic levels and percentage changes in population size.

• Extended response questions: Structure answers logically when explaining cycles or processes. Use bullet points or numbered sequences for the carbon or nitrogen cycles. Connect each step causally ("this leads to..." or "as a result..."). CIE mark schemes award marks for each correct distinct point, so aim for one developed point per mark available.

Quick revision summary

Energy enters ecosystems through producers via photosynthesis and flows through trophic levels with approximately 10% efficiency. Food webs show feeding relationships; pyramids of biomass and energy illustrate energy loss at each level. The carbon cycle involves photosynthesis removing CO₂ and respiration/combustion returning it. The nitrogen cycle requires bacteria for fixation, nitrification and denitrification. Human activities—deforestation, pollution, overfishing—damage ecosystems through habitat loss, eutrophication and climate change. Conservation requires sustainable resource management, protected areas and international cooperation.