Biodiversity and Ecosystems

What you'll learn

This revision guide covers everything you need to know about biodiversity and ecosystems for WJEC GCSE Biology. You'll learn how organisms interact within communities, how scientists measure biodiversity, and the factors that affect population sizes. Understanding these concepts is essential for answering exam questions on ecological relationships and environmental change.

Key terms and definitions

Biodiversity — the variety of different species of organisms living in an area, including all plants, animals and microorganisms

Ecosystem — a community of organisms interacting with each other and their non-living environment

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 the same habitat at the same time

Quadrat — a square frame used to sample organisms in a habitat, typically measuring 0.5 m × 0.5 m or 1 m × 1 m

Transect — a line along which ecological sampling is carried out to investigate how species distribution changes across an area

Abiotic factors — non-living physical and chemical features of an environment that affect organisms, such as temperature, light intensity and pH

Biotic factors — living components of an environment that affect other organisms, such as predators, prey, competitors and pathogens

Core concepts

Levels of organisation in ecosystems

Ecosystems operate at different scales of organisation. Understanding these levels helps you analyse ecological relationships systematically.

The hierarchy runs from smallest to largest:

• Individual organism
• Population
• Community
• Ecosystem
• Biome (large-scale ecosystems like tropical rainforests or coral reefs)

Within any ecosystem, organisms interact through feeding relationships, competition and interdependence. A change at one level affects all other levels. For example, if a disease reduces the rabbit population (population level), foxes will have less food (affecting another population), which impacts the entire community structure.

Abiotic and biotic factors affecting communities

Communities are shaped by both abiotic and biotic factors. You must be able to identify these factors and explain how they influence distribution and abundance of organisms.

Key abiotic factors:

• Light intensity — affects rate of photosynthesis in plants; plants in shaded areas often have larger leaves to capture more light
• Temperature — affects enzyme activity and metabolic rates; most organisms have an optimum temperature range
• Moisture levels — essential for life; desert plants have adaptations to conserve water
• Soil pH and mineral content — determines which plants can grow; some plants (acidophiles) prefer acidic soil, others prefer alkaline
• Wind intensity and direction — affects water loss through transpiration and seed dispersal
• Carbon dioxide levels — affects photosynthesis rate in plants; higher CO₂ (to a point) increases growth
• Oxygen levels — aquatic organisms are particularly sensitive; pollution can cause oxygen depletion

Key biotic factors:

• Food availability — determines population sizes of consumers; more food supports larger populations
• Competition — organisms compete for resources (food, territory, mates, light, water, minerals)
• Predation — predator-prey relationships control population sizes
• Disease — pathogens can rapidly reduce populations, especially in dense populations

Adaptations for survival

Organisms possess adaptations that help them survive in their specific habitats. These adaptations can be structural, behavioural or functional.

Examples of adaptations:

Arctic fox:

• Thick white fur (insulation and camouflage in winter)
• Small ears (reduced surface area minimises heat loss)
• Furry paws (insulation from ice)

Cactus:

• Thick waxy cuticle (reduces water loss)
• Spines instead of leaves (reduces surface area for water loss and deters herbivores)
• Widespread shallow roots (absorbs water quickly after rare rainfall)
• Stores water in stem

These adaptations result from natural selection over many generations. Organisms better adapted to their environment are more likely to survive and reproduce.

Measuring biodiversity using sampling techniques

Scientists use standardised methods to measure biodiversity. You need to know how to use quadrats and transects, and how to calculate population estimates.

Using quadrats:

Quadrats are used to sample organisms that don't move or move very slowly (typically plants). The method involves:

1. Place quadrats randomly across the study area (using random number generators for coordinates)
2. Count the number of individuals of each species inside each quadrat
3. Record percentage cover for species that are hard to count individually
4. Repeat multiple times (minimum 10 quadrats for reliability)
5. Calculate mean number per quadrat
6. Estimate total population

Formula for population size estimation:

Population size = mean number per quadrat × (total area ÷ quadrat area)

Using transects:

Transects investigate how distribution changes across an environmental gradient (e.g., from sea to shore, or across a path). Two main types:

• Line transect — record organisms touching the line at regular intervals
• Belt transect — place quadrats at regular intervals along the line

Transects are particularly useful for studying zonation patterns, such as seaweed distribution down a rocky shore or plant species across a sand dune system.

Measuring abiotic factors

Quantifying environmental conditions helps explain distribution patterns. You should be familiar with equipment used to measure abiotic factors:

• Light intensity — light meter (lux meter)
• Temperature — thermometer or temperature probe
• Soil moisture — moisture meter
• Soil pH — pH probe or universal indicator
• Wind speed — anemometer
• Oxygen concentration in water — oxygen probe

When collecting data, take multiple readings and calculate a mean to improve accuracy and identify anomalies.

Population changes and interdependence

Populations rarely remain constant. They fluctuate due to changes in biotic and biotic factors.

Predator-prey cycles:

Classic population cycles show regular fluctuations:

1. Prey population increases (more food available)
2. This supports a larger predator population (more food for predators)
3. Increased predation reduces prey population
4. Less prey means predator population decreases (starvation)
5. With fewer predators, prey population recovers
6. Cycle repeats

The predator population change lags behind prey population change.

Competition:

Organisms compete when resources are limited. Two types exist:

• Interspecific competition — between different species (e.g., red and grey squirrels competing for food and territory in UK woodlands)
• Intraspecific competition — between members of the same species (e.g., oak trees competing for light)

Competition can limit population growth and influence community structure. The competitive exclusion principle states that two species cannot occupy exactly the same niche indefinitely — one will outcompete the other.

Pollution and its effects on biodiversity

Human activities cause pollution that reduces biodiversity. You need to understand specific examples.

Water pollution:

Eutrophication caused by fertiliser runoff:

1. Excess nitrates/phosphates enter water from agricultural runoff
2. Algae grow rapidly (algal bloom)
3. Algae block light, preventing photosynthesis in underwater plants
4. Plants die, bacteria decompose dead material
5. Bacteria respire aerobically, using up dissolved oxygen
6. Aquatic animals suffocate due to low oxygen levels
7. Biodiversity decreases dramatically

Indicator species:

Some organisms are sensitive to pollution and their presence/absence indicates environmental quality:

• Mayfly larvae, stonefly larvae, freshwater shrimp — indicate clean, well-oxygenated water
• Bloodworms, sludgeworms — tolerate polluted, low-oxygen water
• Lichen — sensitive to air pollution (sulfur dioxide); different species tolerate different pollution levels

Worked examples

Example 1: Quadrat sampling calculation

Question: A student investigates the population of daisies in a school field measuring 50 m × 40 m. She places ten 0.5 m × 0.5 m quadrats randomly and counts the following number of daisies: 12, 8, 15, 10, 9, 14, 11, 13, 8, 10.

(a) Calculate the mean number of daisies per quadrat. [2 marks] (b) Estimate the total daisy population in the field. [3 marks]

Answer:

(a) Total daisies counted = 12 + 8 + 15 + 10 + 9 + 14 + 11 + 13 + 8 + 10 = 110 [1 mark] Mean per quadrat = 110 ÷ 10 = 11 daisies [1 mark]

(b) Quadrat area = 0.5 × 0.5 = 0.25 m² [1 mark] Field area = 50 × 40 = 2000 m² [1 mark] Population estimate = 11 × (2000 ÷ 0.25) = 11 × 8000 = 88,000 daisies [1 mark]

Example 2: Predator-prey relationships

Question: The graph below shows population changes for lynx (predator) and snowshoe hare (prey) over 20 years.

Explain why the lynx population peaks approximately two years after the hare population peaks. [4 marks]

Answer:

When hare population is high, lynx have abundant food [1 mark]. Well-fed lynx are more likely to reproduce successfully [1 mark]. However, there is a time delay because lynx cubs take time to mature [1 mark]. As lynx population increases, they consume more hares, causing the hare population to decline before the lynx population peaks [1 mark].

Example 3: Abiotic factors

Question: A transect was carried out from the edge to the centre of a pond. Oxygen concentration and light intensity were measured at five points.

Explain why oxygen concentration decreased toward the centre of the pond. [3 marks]

Answer:

Light intensity decreases with water depth [1 mark]. Less light means aquatic plants photosynthesise less / photosynthesis rate decreases [1 mark]. Therefore less oxygen is produced / more oxygen is used by respiring organisms than is produced [1 mark].

Common mistakes and how to avoid them

• Confusing population with community — Remember a population is one species; a community contains all species in an area. Always check which term the question asks for.

• Thinking all competition is between different species — Competition within the same species (intraspecific) is often more intense because organisms need exactly the same resources. State which type of competition you're describing.

• Placing quadrats non-randomly — Systematic or convenience sampling introduces bias. Use random number generators to determine coordinates. Don't just place quadrats in easy-to-reach locations.

• Forgetting units in calculations — Always include units in your final answer (e.g., individuals per m², g per dm³). Check if you need to convert units (cm² to m²).

• Mixing up cause and effect in eutrophication — The sequence matters: algae block light → plants can't photosynthesise → plants die → bacteria decompose plants → bacteria use oxygen → animals suffocate. Learn the order.

• Writing vague answers about adaptations — Don't just state the adaptation; explain how it helps survival. For example, "thick fur provides insulation, reducing heat loss in cold environments" not just "thick fur keeps it warm."

Exam technique for "Biodiversity and Ecosystems"

• Command words matter: "State" requires a simple fact (1 mark). "Explain" requires a reason using because/therefore (2+ marks). "Describe" requires characteristics without explanation. "Suggest" means apply knowledge to unfamiliar contexts.

• Show your working in calculations: Even if your final answer is incorrect, you can gain method marks. Write out formulas, substitute values, then calculate. Include units.

• Use data from graphs/tables: When questions provide data, you must reference specific values. Write "The population decreased from 250 to 180" not just "The population decreased."

• Link answers to the context given: If the question asks about oak woodland, mention oak trees specifically. Generic answers about "plants" score fewer marks than context-specific responses.

Quick revision summary

Ecosystems contain communities of organisms interacting with abiotic and biotic factors. Biodiversity is measured using quadrats (random sampling) and transects (distribution along gradients). Populations fluctuate due to factors like predation, competition and disease. Predator populations lag behind prey populations in cycles. Pollution reduces biodiversity through mechanisms like eutrophication. Indicator species reveal environmental quality. Adaptations help organisms survive specific conditions. Always show calculations clearly, reference data provided, and explain cause-and-effect relationships in your exam answers.