What you'll learn
This topic examines how organisms interact with each other and their environment, focusing on the organisation of living things from individual organisms through to entire ecosystems. You'll explore the relationships between populations, communities and their physical surroundings, and understand the factors that affect species distribution and abundance. This forms a foundation for understanding biodiversity, conservation and environmental change.
Key terms and definitions
Habitat — the environment in which a species normally lives, characterised by specific physical conditions (e.g. a pond, coral reef or woodland)
Population — all the organisms of one species living in a habitat at a particular time
Community — all the organisms of different species living in a habitat at the same time; the populations of different species that interact with each other
Ecosystem — the interaction of a community of living organisms (biotic factors) with the non-living (abiotic) parts of their environment
Interdependence — the relationship between organisms in a community where each species depends on other species for food, shelter, pollination, seed dispersal or other resources
Producer — an organism that makes its own organic nutrients, usually through photosynthesis (e.g. plants, algae)
Consumer — an organism that obtains its energy by feeding on other organisms
Decomposer — an organism that breaks down dead material and waste products, releasing nutrients back into the ecosystem (e.g. bacteria, fungi)
Core concepts
Organisation in ecology
Ecology follows a hierarchical organisation from individual organisms to the entire biosphere. Understanding this hierarchy is essential for describing ecological relationships accurately.
The levels of organisation are:
- Individual organism — a single living thing
- Population — all members of one species in a habitat
- Community — all different species living together in a habitat
- Ecosystem — the community plus abiotic factors
- Biome — a large-scale ecosystem (e.g. tropical rainforest, tundra)
- Biosphere — all life on Earth
At GCSE level, you need to distinguish clearly between population, community and ecosystem. A common error is confusing these terms, particularly population (one species) and community (multiple species).
Abiotic and biotic factors
Abiotic factors are non-living physical and chemical conditions that affect the distribution and abundance of organisms:
- Light intensity — affects the rate of photosynthesis in plants; plants require different light levels (shade-tolerant vs sun-loving species)
- Temperature — affects enzyme activity and metabolic rates; each species has an optimum temperature range
- Moisture levels — essential for all life processes; determines whether species can survive in a habitat
- Soil pH — affects nutrient availability and enzyme function in soil organisms; different plants prefer acidic or alkaline soils
- Soil mineral content — nutrients like nitrates, phosphates and potassium affect plant growth
- Wind intensity and direction — affects transpiration rates, seed dispersal and temperature
- Carbon dioxide levels — affects photosynthesis rate in plants
- Oxygen levels — essential for aerobic respiration; particularly important in aquatic ecosystems
Biotic factors are living components that affect organisms:
- Availability of food — determines population size of consumers
- New predators — can reduce prey populations
- New pathogens — cause disease, reducing population size
- Competition — for resources like food, water, territory and mates
These factors interact to determine where organisms can survive and how large their populations can grow.
Adaptations to environments
Organisms possess features that enable them to survive in their specific habitats. These adaptations can be structural, behavioural or functional.
Structural adaptations are physical features:
- Thick fur in Arctic foxes for insulation
- Large surface area to volume ratio in desert animals (e.g. large ears in fennec foxes) for heat loss
- Waxy cuticle on leaves in xerophytic plants to reduce water loss
- Streamlined body shape in fish for efficient swimming
Behavioural adaptations are ways organisms act:
- Migration to warmer climates in winter
- Nocturnal hunting to avoid daytime heat in deserts
- Hibernation during cold periods when food is scarce
Functional adaptations are internal processes:
- Production of antifreeze proteins in Arctic fish
- Concentrated urine production in desert mammals to conserve water
- Ability to tolerate high salt concentrations in mangrove plants
Caribbean coral reefs provide excellent examples of adaptation. Coral polyps have a mutualistic relationship with zooxanthellae algae — the algae photosynthesise and provide nutrients to the coral, while the coral provides protection and access to light. This interdependence explains why coral bleaching (loss of algae due to temperature stress) is so damaging.
Competition and interdependence
Organisms compete for limited resources. Intraspecific competition occurs between members of the same species, while interspecific competition occurs between different species.
Plants commonly compete for:
- Light for photosynthesis
- Water and mineral ions from the soil
- Space for root growth and reproduction
Animals compete for:
- Food and water
- Territory for breeding and hunting
- Mates for reproduction
Competition affects population size. When resources are limited, only the best-adapted individuals survive and reproduce — this is the basis of natural selection.
Interdependence means that species rely on each other for survival. If one species is removed from a community, it affects many others. Examples include:
- Pollinators and flowering plants — bees depend on nectar for food; plants depend on bees for pollination
- Predator-prey relationships — lions depend on zebras for food; zebra populations are controlled by lion predation
- Mutualistic relationships — cleaner fish remove parasites from larger fish, gaining food while the host fish benefits from parasite removal
Removing a species can cause a cascade effect throughout the ecosystem. For instance, if bees decline, plant reproduction decreases, which reduces food for herbivores, which in turn affects carnivores.
Stable communities
A stable community is one where all the species and environmental factors are in balance so that population sizes remain relatively constant. Tropical rainforests and ancient oak woodlands are examples of stable communities.
Characteristics of stable communities:
- Populations fluctuate around an average size
- Biodiversity is maintained
- Nutrient cycling continues efficiently
- No species becomes extinct or dominates completely
These communities have developed over long periods and contain complex interdependencies. Caribbean mangrove ecosystems represent stable communities adapted to tidal zones, providing nursery habitats for fish and protection against coastal erosion.
Unstable communities show rapid changes in population sizes and species composition, often due to disturbance (natural disasters, human activity) or because they are still developing (new volcanic islands, abandoned farmland).
Measuring population size and distribution
Ecologists use sampling techniques to estimate population sizes and investigate distribution patterns. You need to understand these methods for practical investigations.
Quadrats are square frames used to sample plant populations or slow-moving animals:
- Random placement ensures representative samples
- Multiple quadrats are used (typically 10+) to increase reliability
- Calculate mean number of organisms per quadrat, then multiply by total area to estimate total population
Transects investigate how distribution changes across an environmental gradient:
- A tape measure is laid across the area
- Samples are taken at regular intervals along the line
- Used to show zonation (e.g. seaweed species at different heights on a rocky shore)
Key principles for sampling:
- Random sampling removes bias (use random number generators for coordinates)
- Large sample size increases reliability and reduces effect of anomalies
- Repeats allow calculation of mean and identification of outliers
The equation for estimating population size using quadrats:
Population size = (number of organisms in all quadrats ÷ number of quadrats) × (total area ÷ area of one quadrat)
Food chains and webs
Energy flows through ecosystems via feeding relationships.
Food chains show the transfer of energy from one organism to the next, beginning with a producer:
grass → grasshopper → lizard → hawk
Each stage is called a trophic level:
- Producers (first trophic level)
- Primary consumers/herbivores (second trophic level)
- Secondary consumers/carnivores (third trophic level)
- Tertiary consumers/top carnivores (fourth trophic level)
Food webs show multiple interconnected food chains, representing the complex feeding relationships in a real ecosystem. They demonstrate interdependence — one species usually eats several others and is eaten by several predators.
Decomposers break down dead organisms and waste at every trophic level, recycling nutrients.
In Caribbean ecosystems, food webs might include:
- Phytoplankton → zooplankton → small fish → barracuda
- Seagrass → parrotfish → grouper
- Decomposing matter → detritus feeders → crabs → octopus
Worked examples
Example 1: Interpreting ecosystem changes
Question: A coral reef ecosystem contains coral, algae, parrotfish, octopus and sharks. Disease kills most of the parrotfish. Explain the likely effects on the coral and the shark populations. [4 marks]
Answer:
- Algae population will increase [1 mark] because fewer parrotfish are eating the algae / less competition for resources [1 mark]
- Shark population will decrease [1 mark] because less food is available / fewer parrotfish to eat [1 mark]
Examiner note: This tests understanding of interdependence and food webs. You must explain the mechanism (reduced predation, less food) not just state the change.
Example 2: Abiotic factors
Question: A student investigates the distribution of daisies in a school field. She finds more daisies growing in areas where the grass is cut short. Suggest two abiotic factors that might explain this distribution. [2 marks]
Answer: Any two from:
- More light reaches the daisies when grass is short / less shading [1 mark]
- Higher soil temperature when grass is short / less insulation [1 mark]
- Better air circulation / higher wind speeds affecting moisture or temperature [1 mark]
Examiner note: The question asks for abiotic (non-living) factors. "Less competition" is a biotic factor and would score zero marks.
Example 3: Population calculation
Question: A student uses 10 quadrats of size 0.5 m² to sample dandelions in a field measuring 200 m². The total count across all quadrats is 80 dandelions. Estimate the total dandelion population in the field. Show your working. [3 marks]
Answer:
- Mean per quadrat = 80 ÷ 10 = 8 dandelions [1 mark]
- Number of quadrats that would fit = 200 ÷ 0.5 = 400 [1 mark]
- Total population = 8 × 400 = 3200 dandelions [1 mark]
Examiner note: Always show your working in calculation questions. Marks are awarded for method even if your final answer is incorrect.
Common mistakes and how to avoid them
Confusing population and community — Remember: population = one species, community = all species together. Always check which term the question requires.
Listing biotic factors when asked for abiotic — Abiotic means non-living (temperature, light, pH). Biotic means living (predators, competition, disease). Read the question carefully.
Describing competition vaguely — Don't just write "competition for resources." Specify what organisms are competing for (light, water, minerals, food, territory, mates).
Forgetting producers in food chains — All food chains must start with a producer (plant or algae). Writing "caterpillar → bird → fox" is incomplete and incorrect.
Writing circular food chains — Energy flows in one direction only. Arrows show energy transfer from food to feeder. Never draw arrows forming a circle.
Poor quadrat calculations — When estimating populations, multiply the mean per quadrat by the total number of quadrats that would fit in the whole area, not by the total area. Check your units match.
Exam technique for "Ecosystems, habitats and communities"
Command word precision — "Describe" means state what happens; "Explain" requires reasons using scientific knowledge. For "Explain" questions, use linking words like "because," "therefore," "this causes" to connect observations to mechanisms.
Use scientific terminology accurately — Terms like habitat, population, community, and ecosystem have precise meanings. Using them correctly demonstrates understanding and secures marks.
Quantitative questions — Always show your working in calculations. Even if your final answer is wrong, you can gain method marks. Include units in your final answer.
Application to unfamiliar contexts — Questions may use ecosystems you haven't studied (Arctic tundra, Australian outback). Apply the principles you've learned to the new context. The biology is the same even if the organisms differ.
Quick revision summary
Ecosystems consist of communities (all species in a habitat) interacting with abiotic factors. Populations of different species show interdependence through feeding relationships, competition and mutualism. Both abiotic factors (light, temperature, moisture, pH, minerals) and biotic factors (predation, disease, competition) affect distribution and population size. Energy flows through food chains from producers to consumers and decomposers. Sampling techniques like quadrats and transects allow scientists to estimate population sizes and investigate distribution patterns. Stable communities maintain balance through complex interdependencies developed over time.