Lesson 4: Ecology and Environment

Ecology is the study of how organisms interact with each other and their environment — from a single garden pond to the entire biosphere. This lesson covers ecosystems and the feeding relationships within them, how materials cycle through nature (the carbon and water cycles), how energy flows and is lost at each stage, the factors affecting biodiversity, and how human activities are reshaping the planet. Fieldwork skills matter here: the required practical on sampling with quadrats and transects is examined, and data-handling questions on food webs and decay are common.

Ecosystems and Feeding Relationships

An ecosystem is the interaction of a community of living organisms (biotic factors) with the non-living (abiotic) parts of their environment — light, temperature, water, soil pH. A habitat is where an organism lives; a population is all the organisms of one species in a habitat; a community is all the populations of different species living together. Interdependence means species rely on each other: remove one, and others are affected — the classic example is bees pollinating crops.

Feeding relationships are shown in food chains and webs. Producers (green plants and algae) photosynthesise; primary consumers (herbivores) eat producers; secondary consumers (carnivores) eat primary consumers; tertiary consumers eat secondary consumers. Decomposers (bacteria and fungi) break down dead matter, returning nutrients to the soil. A food web shows the many interconnected chains in a real ecosystem — always draw arrows pointing in the direction of energy flow, from food to feeder.

Predator-prey cycles show interdependence dynamically: as prey numbers rise, predators have more food and increase; more predators then reduce the prey; prey decline starves predators, whose numbers fall, allowing prey to recover. Exam questions present population graphs and ask you to explain the patterns — describe the trend, then explain it using the predator-prey relationship. Competition also shapes communities: organisms compete for light, water, mates and territory, and the better-adapted outcompete the rest.

Cycles of Materials

The water cycle: water evaporates from seas and lakes, transpires from plants, condenses into clouds, and falls as precipitation, returning via rivers and groundwater. The carbon cycle is heavily examined: carbon dioxide is removed from the air by photosynthesis and locked into biomass; it returns through respiration (by plants, animals and decomposers), combustion of fossil fuels and wood, and decomposition. Fossil fuels formed over millions of years from ancient organisms — burning them releases "old" carbon rapidly.

Learn to draw and annotate the carbon cycle with these processes: photosynthesis, respiration, combustion, decomposition, and feeding (which moves carbon through food chains). A common question asks how deforestation affects atmospheric CO₂: fewer trees means less photosynthesis removing CO₂, and burning or rotting felled trees releases stored carbon — a double blow. Conversely, planting trees (afforestation) removes CO₂.

Decay is carried out by decomposers — bacteria and fungi — which break down dead organisms and waste, releasing nutrients for plants. The rate of decay depends on temperature (faster when warm, up to an optimum), moisture, and oxygen availability — which is why food is refrigerated, dried, vacuum-packed or pickled to slow decay. Compost heaps get hot because respiring decomposers release energy. Required practical work on decay often involves measuring temperature change in milk or the mass loss of leaves.

Energy Flow and Biomass

Energy enters ecosystems as sunlight, captured by producers in photosynthesis. At each trophic level, most energy is lost — only about 10% transfers to the next level. Losses occur through respiration (released as heat and used for movement), waste (faeces, urine), and uneaten parts (bones, roots). This is why food chains rarely have more than five levels: there is simply not enough energy left to support another layer of predators.

Pyramids of biomass illustrate this: each bar represents the mass of living material at a trophic level, and the pyramid narrows sharply upward. Biomass can be measured by drying and weighing samples. Numbers pyramids can be inverted (one oak tree supports thousands of insects), but biomass pyramids almost never are. Exam calculations: if 10,000 kJ of energy is in producers, roughly 1,000 kJ reaches primary consumers and 100 kJ reaches secondary consumers.

This inefficiency explains why eating plants is more energy-efficient than eating meat: a vegetarian diet "skips" a trophic level, so far less land and energy are needed per person fed. Farming reflects this: intensive livestock farming feeds grain to animals at huge energy loss, while also producing methane. When evaluating food production methods in 6-mark questions, weigh efficiency, land use, animal welfare, and environmental costs together.

Biodiversity and Sampling

Biodiversity means the variety of living organisms — species diversity within ecosystems and genetic diversity within species. High biodiversity makes ecosystems more stable and resilient: if one species declines, others fill its role. It also provides resources: new medicines (many drugs derive from rainforest plants), crop varieties for breeding, and ecosystem services like pollination. Human activities reducing biodiversity include deforestation, pollution, and climate change.

The required practical uses quadrats (square frames) to sample plants: place them randomly, count the organisms inside, and estimate population size or percentage cover. Random sampling avoids bias — use random number coordinates, not "interesting-looking" spots. Transects (lines with quadrats at intervals) show how distribution changes across an environmental gradient, such as from a path into woodland or up a rocky shore. Always take repeats and calculate means for reliability.

Data skills are tested with biodiversity measures: species richness (the number of different species) is simple; abundance counts individuals. You may be asked to calculate estimated population size: (mean count per quadrat) × (total area ÷ quadrat area). When evaluating a fieldwork method, discuss random versus systematic sampling, sample size, and control of variables — the "working scientifically" skills from Lesson 13 apply directly here.

Human Impact on the Environment

Humans reshape ecosystems on a vast scale. Deforestation for timber and farmland destroys habitats, reduces biodiversity, increases CO₂ (less photosynthesis plus release from burning), and causes soil erosion. Peat bogs, which store huge carbon reserves, are destroyed for fuel and compost. Pollution takes many forms: fertiliser runoff causes eutrophication (algal blooms block light, then decay uses up oxygen, killing fish); untreated sewage spreads disease; toxic chemicals accumulate up food chains (bioaccumulation) — top predators like birds of prey receive the highest doses.

The lesson of the large blue butterfly shows conservation can work: it went extinct in Britain in 1979 but was reintroduced by understanding its complex lifecycle (it depends on specific ants and thyme plants). Conservation strategies include: protecting habitats (national parks, marine reserves), breeding programmes for endangered species, seed banks (like Kew's Millennium Seed Bank), fishing quotas and net-size limits, and reducing deforestation through sustainable forestry certification.

Global warming, covered in depth in Lesson 14, is the overarching human impact: rising CO₂ and methane from fossil fuels, farming and deforestation enhance the greenhouse effect. Effects include sea-level rise, extreme weather, and shifting species ranges — UK examples include new insect pests and seabirds failing to breed as fish move north. In evaluation questions, balance human needs (food, development, energy) against environmental protection, using specific examples and data where given.

Conclusion — Why This Matters

Ecology teaches a humbling lesson: everything is connected. The carbon in your body has cycled through the atmosphere, ancient forests and dinosaurs; the oxygen you breathe is made by organisms you will never see. For the exam, master the carbon and water cycles as annotated diagrams, the 10% energy transfer rule with its calculations, and the quadrat/transect methods with their maths. Food-web questions reward careful arrow direction (energy flow, from food to feeder) and step-by-step explanations of population changes. But the deeper message matters more: human activities are now the dominant force in most ecosystems, from the CO2 in the atmosphere to the plastic in the oceans. The same ecological principles that explain a pond's food web explain why overfishing collapses fisheries and why rewilding can restore them. When you evaluate conservation strategies — whether protecting the large blue butterfly or debating palm oil — you are applying this lesson. Ecology is not just a GCSE topic; it is the science of our survival on this planet, and understanding it makes you a more informed citizen of it.

Worked example — estimating a population with quadrats

A student samples a 20 m × 30 m field (area 600 m²) using a 0.5 m × 0.5 m quadrat (area 0.25 m²) placed at 10 random coordinates. Her daisy counts give a mean of 3.2 plants per quadrat. Estimated population = mean per quadrat × (total area ÷ quadrat area) = 3.2 × (600 ÷ 0.25) = 3.2 × 2,400 = 7,680 daisies.

Evaluation points that earn marks: random coordinates (from random numbers, not chosen "good" spots) remove bias; 10 quadrats is a small sample — more repeats improve reliability; counting plants works for daisies, but for grass, percentage cover is the better measure; and the estimate assumes the sampled area is representative of the whole field.

Common mistakes

• Food-chain arrows reversed. Arrows show energy flow, from the organism being eaten to the eater: grass → rabbit → fox.
• The 10% rule misapplied. About 10% of energy transfers to the next trophic level; the other 90% is lost in respiration, waste and uneaten parts — it is not "used for growth".
• Numbers versus biomass pyramids. A pyramid of numbers can be inverted (one tree, many insects); a pyramid of biomass (dry mass) is almost always pyramid-shaped.

Exam Experience

Question 1. Describe how the carbon in a plant's glucose eventually becomes part of the carbon dioxide in the atmosphere again. [3 marks]

Question 2. In a woodland food chain, producers store 20,000 kJ/m²/year in biomass and primary consumers store 1,700 kJ/m²/year. Calculate the percentage efficiency of energy transfer between these trophic levels. [2 marks]

Question 3. A graph shows that as the population of a prey species rises, the population of its predator rises later, then the prey population crashes, followed by the predator. Explain these changes. [6 marks]

Mark scheme

Q1: any three linked steps — glucose is used in respiration by the plant, releasing CO₂; the plant is eaten and the consumer respires; decomposers respire while breaking down dead material and waste. One mark per correct link in sequence.

Q2: efficiency = (1,700 ÷ 20,000) × 100 = 8.5% (1 mark substitution, 1 mark answer).

Q3: Level 3 (5–6 marks): a fully linked causal chain — more prey means more food for predators, so more predators survive and reproduce (predator population rises, with a time lag); heavier predation then reduces the prey population; with less food, predator numbers fall (starvation, emigration, less reproduction); reduced predation lets the prey population recover, and the cycle repeats. Correctly explains the lag between the two curves. Level 2 (3–4 marks): the chain is present but incomplete — for example, explains the rises but not the falls, or ignores the time lag. Level 1 (1–2 marks): generic statements ("predators eat prey so prey numbers go down") without a sequence.