What you'll learn
This topic examines how organisms interact within their environments, covering populations, communities, food chains, food webs, and energy transfer between trophic levels. Understanding these ecological relationships is essential for answering questions on energy flow, population dynamics, and the impact of human activity on ecosystems. CIE IGCSE Biology papers regularly test your ability to interpret pyramids of biomass and numbers, analyse food web relationships, and explain how environmental factors affect population size.
Key terms and definitions
Population — a group of organisms of the same species living in the same area at the same time, capable of interbreeding.
Community — all the populations of different species living in the same area at the same time, interacting with each other.
Ecosystem — a unit containing the community of organisms and their non-living environment, interacting together.
Habitat — the place where an organism lives, characterised by physical conditions and the types of other organisms present.
Trophic level — the position of an organism in a food chain, food web or pyramid of biomass or numbers.
Producer — an organism that makes its own organic nutrients, usually through photosynthesis (e.g. green plants, algae).
Consumer — an organism that obtains energy by feeding on other organisms (herbivores, carnivores, omnivores).
Decomposer — an organism that breaks down dead organic matter, releasing nutrients back into the ecosystem (e.g. bacteria, fungi).
Core concepts
Populations and their characteristics
A population consists of individuals of one species sharing a gene pool. Population size fluctuates due to four key processes: birth, death, immigration and emigration. CIE IGCSE Biology exam questions often require you to explain changes in population size using data from graphs or tables.
Factors affecting population size:
- Biotic factors — living factors such as food availability, predation, competition, and disease
- Abiotic factors — non-living factors such as temperature, light intensity, water availability, oxygen concentration, and pH
When resources are unlimited, populations show exponential growth. However, environmental resistance (limiting factors) eventually restricts growth, causing populations to stabilise around the carrying capacity — the maximum population size that an environment can sustainably support.
Communities and ecological relationships
Communities comprise multiple interacting populations. Three key ecological relationships occur within communities:
Competition occurs when organisms require the same limited resources. Intraspecific competition happens between members of the same species (competing for mates, territory, food). Interspecific competition occurs between different species (two plant species competing for light, water and minerals). The competitive exclusion principle states that two species competing for identical resources cannot coexist indefinitely — one will outcompete the other.
Predation involves one organism (predator) killing and consuming another (prey). Predator-prey relationships show cyclical population changes: when prey populations increase, predators have abundant food and their population rises. As predator numbers increase, more prey are consumed, causing prey populations to decline. With less food available, predator populations subsequently fall, allowing prey to recover. This creates characteristic oscillating graphs tested in CIE IGCSE Biology papers.
Interdependence means populations within a community rely on each other for resources such as food, shelter and pollination. Changes affecting one population can have cascading effects throughout the community.
Food chains and food webs
A food chain shows the transfer of energy from one organism to the next, beginning with a producer. Arrows represent energy flow direction, pointing from the organism being eaten to the consumer.
Example food chain: grass → grasshopper → frog → snake → hawk
Each organism occupies a trophic level:
- Producers (first trophic level) — convert light energy into chemical energy through photosynthesis
- Primary consumers (second trophic level) — herbivores feeding on producers
- Secondary consumers (third trophic level) — carnivores feeding on herbivores
- Tertiary consumers (fourth trophic level) — carnivores feeding on other carnivores
A food web consists of interconnected food chains, showing the complex feeding relationships within a community. Food webs more accurately represent ecosystems because most organisms consume multiple food sources and are eaten by various predators.
When interpreting food webs for CIE IGCSE Biology exams, you must be able to:
- Identify producers, primary, secondary and tertiary consumers
- Predict the effects of removing or adding a species
- Trace the flow of energy through multiple pathways
- Explain why changes to one population affect others
Energy transfer and trophic efficiency
Energy enters ecosystems through producers capturing light energy during photosynthesis. Only approximately 1-3% of sunlight reaching producers is converted into chemical energy in organic compounds. Energy transfers between trophic levels are inefficient — typically only 10% of energy from one level passes to the next.
Energy is lost between trophic levels through:
- Respiration — organisms use energy for movement, growth, reproduction and maintaining body temperature (in endotherms), releasing heat energy to the surroundings
- Excretion — waste products like urea contain chemical energy that is not passed to the next trophic level
- Egestion — undigested material in faeces contains energy that cannot be absorbed
- Not all organisms are consumed — parts like bones, roots and cellulose-rich plant material may not be eaten
This energy loss explains why food chains rarely exceed four or five trophic levels — insufficient energy remains to support additional consumers. It also explains why producer biomass far exceeds top consumer biomass in most ecosystems.
Pyramids of numbers and biomass
Pyramids of numbers show the population size at each trophic level in a food chain, with bars representing the number of organisms. Each bar is drawn to scale and stacked vertically, with producers at the base.
Limitations of pyramids of numbers:
- Do not account for organism size (one tree may support thousands of insects)
- Can produce inverted pyramids (e.g. one oak tree → hundreds of aphids → several ladybirds)
- Do not show biomass or energy content
Pyramids of biomass represent the total biological mass of organisms at each trophic level, usually measured as dry mass per unit area. These provide more accurate representations of energy content because biomass correlates with stored chemical energy.
Pyramids of biomass in terrestrial ecosystems are typically pyramid-shaped, with large producer biomass supporting progressively smaller consumer biomass at higher levels. Pyramids of biomass can occasionally be inverted in aquatic ecosystems where phytoplankton (producers) have rapid reproduction rates despite low biomass at any given moment.
For CIE IGCSE Biology exams, you must construct pyramids using provided data, ensuring:
- Bars are drawn to scale using the data values
- Each bar is centred above the one below
- Trophic levels are correctly ordered with producers at the base
- Labels identify each trophic level or organism
Decomposers and nutrient cycling
Decomposers (bacteria and fungi) play essential roles in ecosystems by breaking down dead organic matter through saprobiontic nutrition. They secrete enzymes onto dead material, breaking down complex molecules into simpler soluble substances that are absorbed. This decomposition:
- Returns nutrients like carbon, nitrogen and phosphorus to the soil
- Makes minerals available for plant uptake
- Prevents accumulation of dead organisms
- Completes nutrient cycles
Decomposition rate is affected by:
- Temperature — higher temperatures increase enzyme activity and decomposer metabolism
- Oxygen availability — aerobic decomposition is faster than anaerobic
- Water availability — decomposers require water for enzyme secretion and absorption
- pH — extreme pH values denature enzymes
Worked examples
Example 1: Predator-prey population graph
Question: The graph shows population changes of lynx (predator) and snowshoe hare (prey) over 25 years. Explain the relationship between the two populations. [4 marks]
Answer:
- When hare population increases, more food becomes available for lynx [1]
- Lynx population subsequently increases due to better nutrition and higher survival rates [1]
- Increased lynx predation causes hare population to decrease [1]
- With less food available, lynx population then decreases, allowing hare population to recover [1]
Examiner note: Mark schemes reward explanations showing the cyclical relationship with clear cause-and-effect reasoning. Stating that populations "go up and down" without explaining why would score zero marks.
Example 2: Constructing a pyramid of biomass
Question: A pond ecosystem contains the following biomass values per m²: water plants 800 g, water beetles 90 g, small fish 15 g, pike 2 g. Draw a pyramid of biomass for this food chain. [3 marks]
Answer:
pike (2g)
small fish (15g)
water beetles (90g)
water plants (800g)
Mark scheme:
- Correct order with producers (water plants) at base [1]
- Bars drawn to scale relative to each other [1]
- Bars centred and correctly labelled [1]
Common error: Drawing bars of equal width regardless of biomass values — bars must be proportional to the data provided.
Example 3: Effect of removing a species from a food web
Question: A simple food web shows: grass → rabbit → fox, and grass → grasshopper → mouse → fox. Predict what would happen to the fox population if a disease killed all the rabbits. Explain your answer. [3 marks]
Answer:
- Fox population would decrease [1]
- Because foxes would have less food available / only mice remaining as food source [1]
- The grass → grasshopper → mouse food chain might not provide sufficient energy to support the same number of foxes [1]
Alternative acceptable answer: Fox population might initially remain stable by eating more mice, but long-term decrease likely due to reduced total energy availability.
Common mistakes and how to avoid them
Mistake: Drawing food chain arrows pointing from predator to prey (showing "what eats what" rather than energy flow). Correction: Arrows always point from the organism being eaten toward the organism consuming it, representing the direction of energy transfer.
Mistake: Stating that 90% of energy is transferred between trophic levels, with only 10% lost. Correction: Only approximately 10% of energy is transferred to the next level; 90% is lost through respiration, excretion, egestion and unconsumed biomass.
Mistake: Confusing population (one species) with community (multiple species). Correction: A population comprises individuals of a single species in one area; a community includes all different species populations interacting in that area.
Mistake: Writing that "plants respire at night and photosynthesise during the day." Correction: Plants respire continuously (24 hours) but only photosynthesise in light. This matters when explaining energy loss from producers in food chains.
Mistake: Claiming energy is "destroyed" or "lost from the ecosystem" at each trophic level. Correction: Energy is not destroyed (first law of thermodynamics) but transferred to less useful forms (heat energy) or moves to decomposers rather than the next consumer level.
Mistake: Drawing pyramid of biomass bars of equal thickness regardless of data values. Correction: Bar width or area must be proportional to the biomass value provided — use graph paper and calculate appropriate scaling.
Exam technique for Populations and communities
"Explain" questions require cause-and-effect reasoning, not just description. For population changes, state what happens AND why it happens, linking factors to biological processes (e.g. "increased predation reduces prey population because more individuals are killed before reproducing").
Data analysis questions frequently test food web interpretation. Identify all organisms affected by a change, trace energy pathways, and consider both direct effects (immediate food source removed) and indirect effects (competitor populations change).
Command word "suggest" indicates unfamiliar contexts where you apply knowledge. For unusual pyramid shapes or population graphs, use core principles (energy transfer, competition, predation) to explain novel situations.
Calculations involving pyramids require careful attention to units and scale. Show working clearly: if 1 cm represents 100 g/m², state this explicitly before drawing bars.
Quick revision summary
Populations (one species) form communities (multiple species) within ecosystems. Population size fluctuates due to biotic and abiotic factors affecting births, deaths, immigration and emigration. Food chains show linear energy transfer from producers through consumers; food webs show interconnected feeding relationships. Only approximately 10% of energy transfers between trophic levels due to losses through respiration, excretion, egestion and unconsumed biomass. Pyramids of biomass accurately represent energy content at each trophic level. Decomposers recycle nutrients by breaking down dead organic matter. Predator-prey populations show cyclical changes due to their interdependence.