Factors affecting ecosystems and interdependence

What you'll learn

This revision guide covers how organisms interact with each other and their environment, focusing on the factors that affect ecosystems and the interdependence of species within communities. You'll understand the difference between biotic and abiotic factors, how these influence populations, and the crucial concepts of food webs, biodiversity, and human impact on ecosystems. This topic is essential for AQA GCSE Biology Paper 2 and links directly to environmental management and sustainability.

Key terms and definitions

Ecosystem — a community of organisms (biotic factors) interacting with each other and their physical environment (abiotic factors) in a defined area.

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

Population — all the organisms of one species living in a habitat at a particular time.

Biotic factors — living components of an ecosystem that affect other organisms (e.g., predators, competitors, pathogens, food availability).

Abiotic factors — non-living physical or chemical components of an ecosystem that affect organisms (e.g., temperature, light intensity, moisture, pH, wind).

Interdependence — the relationship where organisms in a community depend on each other for resources such as food, shelter, pollination, or seed dispersal.

Biodiversity — the variety of all the different species of organisms on Earth, or within a particular ecosystem.

Trophic level — the position an organism occupies in a food chain, food web, or pyramid (producer, primary consumer, secondary consumer, etc.).

Core concepts

Biotic factors affecting communities

Biotic factors are the living elements within an ecosystem that directly influence population sizes and community structure.

Competition occurs when organisms require the same resources:

• Interspecific competition — competition between different species for the same resources (food, territory, mates, light)
• Intraspecific competition — competition between members of the same species, typically more intense as they need identical resources

Predation directly affects population dynamics:

• Predator populations control prey populations by hunting and consumption
• Prey availability limits predator numbers
• This creates cyclical population fluctuations where predator and prey numbers rise and fall in a pattern

Disease and parasitism reduce population sizes:

• Pathogens spread more easily in dense populations
• Parasites gain nutrition from host organisms, weakening them
• Infected individuals may die or reproduce less successfully

Food availability is fundamental:

• Producers depend on light, carbon dioxide, water, and minerals
• Consumers depend on sufficient prey or plant food
• Scarcity reduces population growth and survival rates

Mutualism benefits both organisms:

• Bees pollinate flowers whilst obtaining nectar
• Cleaner fish remove parasites from larger fish whilst gaining food
• Both species depend on this relationship for survival

Abiotic factors affecting communities

Abiotic factors are non-living environmental conditions that determine which species can survive in a habitat.

Light intensity affects plant growth and distribution:

• Photosynthesis rate increases with light intensity (up to a limit)
• Shade-tolerant plants thrive in woodland floors; sun-loving species dominate open areas
• Animals depend on plants, so light indirectly affects entire food webs

Temperature influences enzyme activity and metabolic rates:

• Each species has an optimum temperature range
• Cold-blooded (ectothermic) animals are particularly sensitive
• Temperature affects distribution (polar bears in Arctic; camels in deserts)
• Seasonal temperature changes trigger migration, hibernation, or dormancy

Moisture levels determine water availability:

• Plants adapted to dry conditions (xerophytes) have features like thick cuticles, reduced leaves
• Aquatic organisms require permanent water
• Soil moisture affects plant distribution and, consequently, herbivore populations

Soil pH and mineral content:

• Most plants prefer pH 6-7; some species adapted to acidic (heathland) or alkaline (chalk grassland) soils
• Nutrient-poor soils support fewer plant species
• Nitrogen, phosphorus, and potassium are essential for plant growth

Wind intensity and direction:

• Strong winds increase water loss (transpiration) from plants
• Shapes tree growth in exposed areas (windswept appearance)
• Affects seed dispersal patterns
• Wind-pollinated plants more successful in open, windy habitats

Carbon dioxide levels (for aquatic ecosystems):

• Essential for photosynthesis by aquatic plants
• Levels vary with temperature, decomposition rates, and respiration
• Low levels limit plant growth; high levels may indicate pollution

Oxygen levels (for aquatic ecosystems):

• Fish and aquatic invertebrates require dissolved oxygen
• Levels decrease with temperature increase and organic pollution
• Indicator species signal water quality (e.g., stonefly larvae indicate clean water)

Interdependence in ecosystems

Organisms within a community are interdependent — changes affecting one species create ripple effects throughout the ecosystem.

Food webs demonstrate interdependence:

• Organisms occupy different trophic levels (producer → primary consumer → secondary consumer → tertiary consumer)
• Most organisms consume multiple food sources and are eaten by multiple predators
• Removal of one species affects populations throughout the web

Stability and balance:

• Stable communities have balanced populations where numbers fluctuate around an equilibrium
• Predator-prey cycles maintain this balance
• Disruption (disease outbreak, extreme weather, human interference) destabilises the community

Examples of interdependence:

• Red squirrels, grey squirrels, and pine martens in UK forests — grey squirrels outcompete reds, but pine martens preferentially predate greys
• Tropical rainforests — fig trees depend on fig wasps for pollination; wasps depend on figs for reproduction
• Coral reefs — corals provide shelter for fish; fish remove algae that would smother corals; both depend on zooxanthellae (photosynthetic algae)

Disrupting interdependence:

• Introduction of non-native species can outcompete natives or lack natural predators
• Loss of keystone species (e.g., sea otters controlling sea urchin populations) dramatically alters entire ecosystems
• Climate change shifts species distributions, breaking established relationships

Measuring environmental factors

Scientists measure abiotic and biotic factors to understand ecosystem dynamics.

Measuring abiotic factors:

• Light intensity — light meter (lux meter)
• Temperature — thermometer or electronic temperature probe
• Soil pH — pH probe or indicator solution
• Soil moisture — moisture meter
• Wind speed — anemometer
• Oxygen levels — oxygen probe or dissolved oxygen test kit

Measuring biotic factors (abundance and distribution):

• Quadrats — square frames (often 0.25 m² or 1 m²) placed randomly to sample plants or slow-moving animals
• Calculate percentage cover or count organisms within quadrat
• Multiple samples improve reliability and allow mean calculation
• Transects — tape measure laid across habitat; quadrats placed at regular intervals
• Useful for showing how distribution changes across environmental gradient (e.g., from woodland edge to centre)
• Belt transect — continuous recording along line; line transect — recording only organisms touching line

Population size estimation:

• Capture-mark-release-recapture for mobile animals
• First sample: capture, mark harmlessly, release
• Second sample after time for mixing: count total captured and how many are marked
• Formula: population size = (number in 1st sample × number in 2nd sample) ÷ number marked in 2nd sample
• Assumptions: no immigration/emigration, marks don't affect survival, population hasn't changed between samples

Biodiversity and human impact

Biodiversity encompasses genetic diversity (variation within species), species diversity (number of different species), and ecosystem diversity (range of habitats).

Importance of biodiversity:

• Ecosystem stability — diverse communities more resilient to change
• Food security — diverse crops and wild relatives for breeding
• Medicine — many drugs derived from plants and microorganisms
• Economic value — tourism, resources, ecosystem services
• Ethical responsibility — preventing species extinction

Threats to biodiversity:

• Habitat destruction (deforestation, urbanisation, agricultural expansion)
• Climate change altering temperature and rainfall patterns
• Pollution (air, water, soil contamination)
• Overexploitation (overfishing, hunting, collection)
• Invasive species outcompeting natives

Human impacts on ecosystems:

• Eutrophication — fertiliser runoff increases algal growth, blocking light; algae die and decompose; bacteria use oxygen, causing fish death
• Pesticides accumulate in food chains (bioaccumulation), harming top predators
• Deforestation reduces habitats, increases atmospheric CO₂, reduces oxygen production
• Introduced species lack natural predators (grey squirrels, Japanese knotweed in UK)

Conservation strategies:

• Protected areas (national parks, nature reserves, marine protected areas)
• Captive breeding programmes for endangered species
• Legal protection for threatened species
• Habitat restoration and rewilding projects
• Sustainable resource management

Adaptations and ecosystems

Organisms possess adaptations enabling survival in specific ecosystems.

Types of adaptation:

• Structural — physical features (thick fur, large ears for heat loss, streamlined body shape)
• Behavioural — actions increasing survival (migration, hibernation, nocturnal activity)
• Functional — physiological processes (producing venom, antifreeze proteins, water conservation mechanisms)

Extreme environments:

• Polar regions — thick insulation, white camouflage, behavioural adaptations like huddling (emperor penguins)
• Hot deserts — water storage (cacti), nocturnal behaviour, large surface area for heat loss (fennec fox ears)
• Deep ocean — bioluminescence, pressure resistance, slow metabolism

Tropical rainforests (important globally diverse ecosystem):

• High biodiversity due to constant warmth, high rainfall, year-round growing season
• Layered structure (emergent, canopy, understorey, forest floor) creates multiple niches
• Nutrient cycling rapid; most nutrients in biomass not soil

Worked examples

Example 1: Interpreting a predator-prey graph

Question: The graph shows populations of lynx (predator) and snowshoe hare (prey) over time. Explain the changes in the lynx population between years 10 and 30. [4 marks]

Model answer:

• Between years 10-15, lynx population increases because snowshoe hare population is high, providing abundant food [1 mark]
• This allows lynx to survive and reproduce successfully [1 mark]
• Between years 15-25, lynx population decreases because they have eaten many hares, reducing prey availability [1 mark]
• Reduced food means lynx cannot survive or reproduce as successfully, so their population falls [1 mark]

Examiner insight: Notice the answer links cause to effect clearly. Each statement explains why the change occurs, not just describes what happens. Reference specific time periods from the graph.

Example 2: Quadrat sampling calculation

Question: A student investigates dandelion distribution in a school field measuring 50 m × 20 m. She uses ten randomly placed 0.5 m × 0.5 m quadrats and counts the dandelions in each. Her total count is 35 dandelions. Estimate the total dandelion population in the field. [3 marks]

Model answer:

• Area of one quadrat = 0.5 × 0.5 = 0.25 m² [1 mark]
• Mean number per quadrat = 35 ÷ 10 = 3.5 dandelions per 0.25 m² [1 mark]
• Field area = 50 × 20 = 1000 m², containing 1000 ÷ 0.25 = 4000 quadrats; estimated population = 3.5 × 4000 = 14,000 dandelions [1 mark]

Examiner insight: Show your working clearly. Even if your final answer is incorrect, you can gain marks for correct method. Remember units and don't round excessively during calculations.

Example 3: Eutrophication sequence

Question: Describe how the use of fertilisers on farmland can lead to the death of fish in nearby rivers. [6 marks]

Model answer:

• Fertilisers containing nitrates are washed into the river by rain (leaching) [1 mark]
• Nitrates cause rapid growth of algae (algal bloom) on the water surface [1 mark]
• The algae block light from reaching plants below the surface [1 mark]
• Submerged plants cannot photosynthesise and die [1 mark]
• Bacteria decompose the dead plants and algae, using oxygen for aerobic respiration [1 mark]
• Oxygen levels in the water decrease, so fish cannot respire and die [1 mark]

Examiner insight: This is a sequence question requiring clear step-by-step logic. Don't jump steps — examiners award marks for each distinct stage. Use scientific terms like "leaching," "algal bloom," and "aerobic respiration."

Common mistakes and how to avoid them

• Confusing community and population — A population is one species; a community contains all species in a habitat. Always check which term the question asks for.

• Mixing up biotic and abiotic factors — Remember: biotic = biological/living (other organisms). Abiotic = non-living (temperature, light, pH). Food availability is biotic because it refers to living organisms; temperature of the environment is abiotic.

• Vague answers about interdependence — Don't just say "organisms depend on each other." Specify exactly which organisms and what they depend on each other for (food, pollination, shelter, etc.).

• Incomplete eutrophication explanations — Many students miss that plants die first (from lack of light), then bacteria decompose them using oxygen. Don't jump straight to "bacteria use oxygen."

• Not calculating means in sampling questions — When using multiple quadrats, you must calculate the mean (average) count before estimating population size. Don't use just one quadrat or the total.

• Forgetting units in measurements — Always include units: light intensity (lux), temperature (°C), area (m²), pH (no unit, just a number 0-14). Missing units loses marks.

Exam technique for "Factors affecting ecosystems and interdependence"

• Command words matter: "State" needs brief facts (1-2 words); "Describe" requires what happens (no explanation); "Explain" needs reasons and mechanisms using "because," "this causes," or "therefore." A 4-mark explain question typically needs 4 distinct linked points.

• Use data from the question: If given a graph, table, or diagram, quote specific values or time periods. Answers that reference data score higher than generic responses. For example: "temperature increased from 15°C to 25°C" rather than "temperature increased."

• Link organism features to environmental factors: When discussing adaptation or distribution, always connect the organism's characteristics to specific abiotic factors. For example: "thick fur provides insulation against low temperatures in Arctic habitats" — don't just list features without explaining their advantage.

• Practise calculations: Quadrat sampling and capture-recapture calculations appear frequently. Learn the formulas, show working, and check your answer makes biological sense (e.g., you can't have 0.7 of an animal).

Quick revision summary

Ecosystems consist of communities of organisms interacting with their environment. Biotic factors (competition, predation, disease) and abiotic factors (temperature, light, pH, moisture) determine population sizes and species distribution. Organisms are interdependent through food webs, with changes to one species affecting others. Scientists measure environmental factors using equipment and sampling techniques including quadrats and transects. Biodiversity is threatened by habitat destruction, pollution, and climate change but can be protected through conservation. Understanding these interactions is essential for managing ecosystems sustainably and predicting the impact of environmental change.