What you'll learn
This topic examines how organisms interact within their environment at the community and ecosystem level. You'll explore feeding relationships, competition for resources, how environmental factors affect distribution, and the importance of biodiversity. These concepts are essential for understanding ecological balance and are frequently tested in OCR GCSE Biology Paper 1.
Key terms and definitions
Community — all the populations of different species living in the same habitat at the same time
Ecosystem — a community of organisms together with the abiotic (non-living) factors in their environment
Interdependence — the way in which organisms depend on other organisms for survival (food, shelter, pollination, seed dispersal)
Abiotic factors — non-living environmental factors that affect the distribution and abundance of organisms (light intensity, temperature, moisture, pH, wind, oxygen availability)
Biotic factors — living factors that affect the distribution and abundance of organisms (availability of food, new predators, new pathogens, competition)
Biodiversity — the variety of all the different species of organisms on Earth, or within an ecosystem
Stable community — a community where all the species and environmental factors are in balance so that population sizes remain relatively constant
Zonation — the gradual change in the distribution of species across a habitat
Core concepts
Feeding relationships and energy transfer
Organisms within a community are organised into trophic levels based on their feeding relationships. Energy flows through ecosystems in one direction only.
Producers (usually green plants and algae) synthesise glucose through photosynthesis. They convert light energy into chemical energy stored in biomass.
Primary consumers are herbivores that feed on producers. Examples include caterpillars, rabbits, and zooplankton.
Secondary consumers are carnivores that feed on primary consumers. Examples include small birds, frogs, and small fish.
Tertiary consumers are carnivores that feed on secondary consumers. These are often apex predators like hawks, sharks, or jaguars.
Decomposers (bacteria and fungi) break down dead organic material and waste products, returning nutrients to the soil.
Food chains show a simple linear feeding relationship:
- Oak tree → Caterpillar → Blue tit → Sparrowhawk
Food webs are interconnected food chains that show the complex feeding relationships in a community. They more accurately represent how energy flows through an ecosystem because most organisms feed on multiple species and are eaten by multiple predators.
When drawing or interpreting food webs:
- Arrows point from the organism being eaten to the organism doing the eating (direction of energy flow)
- Removing one species affects multiple other species in the web
- The wider the base of producers, the more stable the community
Competition and interdependence
Organisms compete for resources to survive and reproduce. Competition can be intraspecific (within the same species) or interspecific (between different species).
Animals compete for:
- Food and water
- Mates for reproduction
- Territory and shelter
Plants compete for:
- Light for photosynthesis
- Water and mineral ions from the soil
- Space for growth
When resources are limited, competition intensifies. The better-adapted organisms survive and reproduce more successfully. This links to natural selection and evolution.
Interdependence means organisms rely on each other for survival. Examples include:
- Pollinators (bees, butterflies) and flowering plants — insects get nectar for food while plants achieve pollination
- Predator-prey relationships — predators control herbivore populations, preventing overgrazing
- Seed dispersers and plants — birds eat fruit and disperse seeds in their droppings
- Nitrogen-fixing bacteria in root nodules provide plants with nitrates while receiving sugars from the plant
In a stable community, these interdependent relationships are balanced. Disrupting one species affects many others through the food web.
Abiotic factors affecting communities
Abiotic factors are non-living environmental variables that influence where organisms can survive.
Light intensity affects the rate of photosynthesis in plants. Shade-tolerant plants like ferns have adaptations for low light conditions, while sun-loving plants like cacti require high light intensity.
Temperature affects enzyme activity and metabolic rates. Most organisms have an optimum temperature range. In the Caribbean, mangrove ecosystems thrive in warm tropical waters (24-28°C), while in UK waters, kelp forests prefer cooler temperatures (6-14°C).
Moisture levels determine which organisms can survive in a habitat. Xerophytes (desert plants) are adapted to low moisture, while hydrophytes (water plants) require aquatic conditions.
Soil pH affects nutrient availability and enzyme function in plant roots. Rhododendrons prefer acidic soils (pH 4.5-6), while clematis prefers alkaline soils (pH 7-8).
Wind intensity and direction affects water loss through transpiration in plants and influences seed dispersal patterns.
Oxygen availability is crucial for aquatic organisms. Fast-flowing, cool streams have high dissolved oxygen supporting mayfly larvae and trout. Stagnant, warm water has low oxygen, supporting only adapted species like bloodworms.
Carbon dioxide levels affect the rate of photosynthesis in plants and algae. Higher CO₂ concentrations generally increase photosynthesis rates (until other factors become limiting).
Biotic factors affecting communities
Biotic factors are living influences on community structure.
Availability of food directly affects population sizes. When food is abundant, populations increase. Food scarcity limits population growth through starvation and reduced reproduction.
New predators entering an ecosystem can devastate prey populations that haven't evolved defences. The introduction of mongooses to Caribbean islands decimated native ground-nesting bird populations.
New pathogens can cause disease outbreaks. Dutch elm disease (caused by a fungus) killed millions of elm trees across the UK. Ash dieback is currently affecting UK ash tree populations.
New competitors can outcompete native species for resources. Grey squirrels introduced to the UK outcompete native red squirrels for food and habitat, and carry squirrelpox virus which kills red squirrels but doesn't affect greys.
Adaptations and survival
Organisms possess adaptations that enable them to survive in their environment. These can be:
Structural adaptations — physical features like thick fur for insulation, large ears for heat loss, or waxy cuticles to reduce water loss
Behavioural adaptations — actions like migration, hibernation, or nocturnal activity patterns
Functional (physiological) adaptations — internal processes like producing venom, antifreeze proteins in arctic fish, or concentrated urine in desert mammals
Organisms in extreme environments show remarkable adaptations. Arctic foxes have white fur for camouflage, small ears to reduce heat loss, and thick fur for insulation. Cacti have spines instead of leaves to reduce water loss, extensive shallow roots to capture rainfall, and thick stems for water storage.
Measuring and maintaining biodiversity
Biodiversity refers to the variety of living organisms. High biodiversity indicates a healthy, stable ecosystem. Low biodiversity makes ecosystems vulnerable to change.
Biodiversity is important because:
- It ensures ecosystem stability
- Different species provide resources (food, medicines, materials)
- It maintains functioning ecosystems that provide services (pollination, water purification, nutrient cycling)
- It has cultural and aesthetic value
Measuring biodiversity involves:
- Quadrat sampling — placing a square frame (usually 0.25 m² or 1 m²) randomly in a habitat and counting organisms inside
- Transects — placing a line across a habitat and sampling at regular intervals to study zonation
- Species richness counts — recording the number of different species present
- Percentage cover estimates — estimating what percentage of the quadrat is covered by each species
Calculating population size:
- Place quadrats randomly (avoid bias)
- Count organisms in each quadrat
- Calculate mean number per quadrat
- Multiply by total number of quadrats that would fit in the habitat
For example, if the mean is 12 daisies per 1 m² quadrat and the field is 500 m², the estimated population is 12 × 500 = 6000 daisies.
Threats to biodiversity include:
- Habitat destruction (deforestation, urbanisation)
- Climate change altering temperature and rainfall patterns
- Pollution (air, water, soil)
- Overharvesting of species
- Introduction of invasive species
Conservation efforts to maintain biodiversity:
- Establishing nature reserves and marine protected areas
- Breeding programmes for endangered species
- Reintroduction programmes (otters in UK rivers, parrots in Caribbean islands)
- Legal protection of endangered species
- Reducing pollution and carbon emissions
- Sustainable fishing and forestry practices
Worked examples
Example 1: Food web analysis (4 marks)
Question: The diagram shows a food web in a UK woodland.
Oak tree → Caterpillar → Blue tit → Sparrowhawk Oak tree → Aphid → Ladybird → Blue tit Oak tree → Squirrel
(a) What would happen to the caterpillar population if all the aphids died from disease? Explain your answer. (2 marks)
(b) Explain why removing oak trees would affect the entire community. (2 marks)
Mark scheme answers:
(a) The caterpillar population would decrease (1 mark). This is because ladybirds would have no aphids to eat, so they would eat more caterpillars instead / blue tits would have no ladybirds to eat, so they would eat more caterpillars instead (1 mark).
(b) Oak trees are producers that provide food/energy for multiple species (1 mark). Without oak trees, primary consumers (caterpillars, aphids, squirrels) would die, which would then affect all secondary and tertiary consumers in the food web / they provide the energy source for the entire ecosystem (1 mark).
Example 2: Investigating abiotic factors (6 marks)
Question: A student investigates how light intensity affects the distribution of daisies in a school field. She uses a light meter and quadrats.
(a) Describe a method the student could use to investigate this. (4 marks)
(b) Suggest one way to make the results more reliable. (1 mark)
(c) Predict the student's results and explain your prediction. (1 mark)
Mark scheme answers:
(a)
- Place a transect/tape measure from a shaded area to a sunny area (1 mark)
- Place quadrats at regular intervals along the transect (e.g., every 2 metres) (1 mark)
- Measure light intensity using the light meter at each position (1 mark)
- Count the number of daisies in each quadrat / calculate percentage cover of daisies (1 mark)
(b) Repeat the investigation and calculate a mean / use multiple transects in different locations (1 mark)
(c) More daisies will be found where light intensity is higher (1 mark) because they need light for photosynthesis / to produce glucose for growth and reproduction (1 mark).
Example 3: Biodiversity calculation (3 marks)
Question: A student uses a 1 m² quadrat to estimate the population of buttercups in a meadow measuring 200 m². She places 10 quadrats randomly and counts the buttercups in each. Her results are:
4, 7, 3, 6, 5, 4, 8, 3, 5, 5 buttercups per quadrat
Calculate the estimated total population of buttercups in the meadow. Show your working. (3 marks)
Mark scheme answer:
Mean = (4 + 7 + 3 + 6 + 5 + 4 + 8 + 3 + 5 + 5) ÷ 10 = 50 ÷ 10 = 5 buttercups per m² (1 mark)
Estimated total population = mean × total area (1 mark)
= 5 × 200 = 1000 buttercups (1 mark)
Common mistakes and how to avoid them
Confusing food chain arrows — remember arrows show energy flow FROM the organism being eaten TO the consumer eating it, not the other way around. Think "energy goes to..."
Mixing up abiotic and biotic factors — abiotic factors are non-living (temperature, light, pH), while biotic factors involve living organisms (predators, pathogens, competitors). Check whether the factor is alive.
Saying "plants compete for food" — plants make their own food through photosynthesis. They compete for light, water, minerals, and space, NOT food.
Forgetting to show working in calculations — always write out your calculation steps. If your final answer is wrong but your method is correct, you can still gain marks.
Using vague explanations — avoid phrases like "it helps them survive." Be specific: "thick fur provides insulation, reducing heat loss, maintaining a constant body temperature in cold climates."
Not considering the whole food web — when one species is affected, think about ALL the organisms connected to it, both as predators and prey.
Exam technique for "B4: Community Level Systems"
Command word awareness — "Describe" requires stating what happens; "Explain" requires giving reasons using because/therefore/so. "Suggest" means applying your knowledge to unfamiliar contexts (often worth more marks).
Quantitative questions — show all working, include units, and use data from graphs or tables in your answer. For 3-mark calculations, expect: method (1), substitution (1), answer with unit (1).
Extended response questions — structure answers logically, use scientific terminology correctly, and make sure you address all parts of the question. For 6-mark questions, aim for 6 distinct scientific points.
Practical questions — know how to use quadrats and transects, identify variables (independent, dependent, control), and suggest improvements to reliability (repeats, larger sample size) and validity (control variables, random sampling).
Quick revision summary
Communities consist of interdependent populations affected by abiotic and biotic factors. Energy flows through trophic levels from producers to consumers. Competition for resources drives population changes. Organisms show structural, behavioural, and functional adaptations to their environment. Biodiversity is measured using quadrats and transects; it indicates ecosystem health and stability. Conservation protects biodiversity through habitat protection, breeding programmes, and sustainable practices. Understanding food webs allows prediction of how changes affect communities.