Big picture
Our environment includes living organisms, physical surroundings and the interactions connecting them. A pond is not just water containing separate plants and animals. Producers capture energy, consumers feed, decomposers recycle materials, and light, temperature and nutrients influence the whole system.
Everyday decisions about food, packaging and waste can affect these connections. This chapter explains ecosystems, food relationships, energy transfer, pollutant concentration, ozone protection and waste management. The aim is to trace causes through a system rather than assume that removing one species or throwing something away ends its effects.
Learning goals
Identify biotic and abiotic components of ecosystems.
Distinguish producers, consumers and decomposers.
Construct food chains and explain trophic levels.
Calculate energy transfer using the approximate ten-percent model.
Explain food webs, biological magnification and ozone depletion.
Compare biodegradable and persistent waste and practical management options.
Core ideas
1. Ecosystems are interacting systems
An ecosystem is a functional unit containing organisms and their non-living surroundings linked through interactions. Biotic components include plants, animals and microorganisms. Abiotic components include water, air, sunlight, soil, temperature and minerals.
Natural examples include forests, ponds and grasslands. Human-managed or artificial ecosystems include crop fields, gardens and aquariums. Artificial does not mean lifeless or free of ecological interactions. An aquarium still needs appropriate light, dissolved oxygen, food and removal or processing of waste.
For example, fertiliser reaching a pond changes nutrient availability. Plant or microbial growth may increase, affecting oxygen and animals. The outcome cannot be predicted by considering only the fish. Ecosystem questions often ask you to connect a change in an abiotic factor with changes in several living components.
2. Producers, consumers and decomposers
Producers, or autotrophs, make organic food using energy, most commonly sunlight through photosynthesis. Green plants and many algae are examples. Consumers obtain organic food by eating organisms or their products. Herbivores feed on producers, carnivores on other animals, and omnivores can feed at different positions.
Decomposers, especially many bacteria and fungi, chemically break down dead organic matter and wastes. They return simpler substances to the surroundings, making nutrients available again. A vulture feeding on a carcass is a scavenger consumer, not the same as a bacterium chemically decomposing material.
Role | Example | System contribution |
|---|---|---|
Producer | Grass | Converts sunlight into chemical energy in food |
Primary consumer | Grasshopper | Obtains energy from a producer |
Secondary consumer | Frog | Feeds on a primary consumer |
Higher consumer | Snake | Obtains energy through other consumers |
Decomposer | Saprotrophic fungus | Breaks down organic remains and recycles nutrients |
If decomposers were absent, dead material would accumulate and nutrient recycling would be disrupted. They act on material from several trophic levels, so they should not be drawn as though they eat only the final predator in every food chain.
3. Food chains and trophic levels
A food chain shows one feeding pathway. Write grass → grasshopper → frog → snake. The arrow points from food to feeder, indicating the direction of energy transfer. It does not mean the grass eats the grasshopper.
A trophic level is a feeding position. Producers form the first level, herbivores the second, and consumers eating those herbivores the third. In the example, snake is at the fourth level. Trophic level describes a position in a pathway, not a permanent moral ranking or always one fixed place for a species.
An omnivorous bird eating grain acts as a primary consumer in that pathway, but the same bird eating an insect that fed on grain acts at a higher level. Read the actual feeding information before assigning a level. The number of organisms at a level and the energy available there are different measurements.
4. Energy transfer and the ten-percent model
Energy enters most ecosystems as sunlight and becomes chemical energy in producers. At each transfer, organisms use much of the available energy in respiration, movement, growth processes and maintenance; energy is dissipated as heat and some material is not eaten or assimilated. Therefore only a fraction becomes available to the next trophic level.
The ten-percent law is a useful approximate school model: about 10% of energy at one trophic level is available at the next. It is not a universal exact efficiency for every species and ecosystem. Use it when the problem specifies this model.
Worked example. Producers contain 18,000 J of energy available in a model chain. Primary consumers receive 18,000 × 0.10 = 1,800 J. Secondary consumers receive 1,800 × 0.10 = 180 J. Tertiary consumers receive 180 × 0.10 = 18 J. Four trophic levels involve three transfers, not four.
Inverse example. If secondary consumers receive 75 J, the previous level needs 75 ÷ 0.10 = 750 J. Producers need 750 ÷ 0.10 = 7,500 J. Count transfers before choosing multiplication or division.
Energy flow is unidirectional: it is not recycled back into sunlight by decomposers. Nutrients, by contrast, can cycle between living organisms and physical surroundings. Limited available energy helps explain why food chains usually contain relatively few trophic levels and why higher-level consumer populations depend on substantial producer support.
5. Food webs and disturbance
A food web joins several feeding pathways. Grass may feed grasshoppers and rabbits; grasshoppers may feed frogs and birds; birds may also eat seeds. This is more realistic than assuming each organism has exactly one food source and one predator.
If frogs decline, insects they consumed may increase, potentially affecting plants. However, a web containing other insect predators may modify that outcome. Avoid saying a change must cause one precise numerical result when the data show only possible relationships.
A food web can offer alternative feeding routes, but it does not make an ecosystem immune to disturbance. Habitat loss, pollution or removal of an important species can affect many pathways together. In a diagram, label the organisms, preserve arrow direction and explain at least one connected consequence.
6. Biological magnification
Biological magnification, or biomagnification, is the increase in concentration of certain persistent pollutants at successive trophic levels. The substance must be sufficiently resistant to breakdown and capable of accumulating in organisms. A predator consumes many contaminated prey, so its body can build up a higher concentration.
A school example uses persistent pesticide residues passing from water into small aquatic organisms, then small fish, larger fish and fish-eating birds. The largest concentration may occur at the highest consuming level in that chain. This is an increase in concentration, not an increase in usable food energy.
Biomagnification does not apply equally to every pollutant or every natural substance. Substances readily metabolised or eliminated may not behave in this way. Bioaccumulation refers to buildup within an organism; biomagnification specifically compares concentrations through a feeding pathway. Questions often contrast falling energy with rising persistent-pollutant concentration.
7. Ozone protects living systems
Ozone, O₃, forms a protective region in the stratosphere by absorbing much harmful ultraviolet radiation. High-energy radiation can split an oxygen molecule into oxygen atoms; an atom can combine with O₂ to form O₃. A simplified sequence is O₂ → O + O, followed by O + O₂ → O₃.
Some human-made chemicals, notably chlorofluorocarbons, can release reactive substances in the upper atmosphere that damage ozone through repeated reactions. Ozone depletion allows more harmful UV to reach the surface, affecting organisms and increasing risks such as skin damage and eye harm.
The Montreal Protocol, agreed in 1987, established international action to control ozone-depleting substances. Do not confuse this ozone issue with the greenhouse effect: they involve different mechanisms, although some chemicals are relevant to both. Also distinguish beneficial stratospheric ozone from ground-level ozone, which can be an air pollutant.
8. Biodegradable and persistent wastes
Biodegradable waste can be broken down by biological processes under suitable conditions. Food scraps and many plant residues are examples. Non-biodegradable or persistent waste resists such breakdown over relevant timescales, as many conventional plastics do.
Biodegradable does not mean harmless in every situation. Large amounts of organic waste can create odour, attract pests or reduce oxygen in receiving water. Similarly, recyclability is not identical to biodegradability: glass can be recycled but does not compost like a vegetable peel.
Segregation at source helps preserve useful materials. Keep compostable organic waste separate from clean recyclables and from hazardous or sanitary waste according to local arrangements. Mixing batteries or chemical residues into compost contaminates the material. Burning mixed plastic waste can release harmful pollutants rather than solving persistence.
9. Waste choices and an observation activity
Prioritise reducing unnecessary consumption and reusing suitable materials before relying entirely on disposal. A refillable bottle avoids repeated single-use items, while repairing an object can extend its life. Recycling helps recover resources but requires collection, sorting and processing; a recycling symbol alone does not guarantee that a particular local system accepts an item.
Activity: compare safe plant-based materials such as dry leaves and vegetable peel under monitored composting conditions. Record changes over time, maintaining comparable moisture and aeration. Do not handle hazardous waste or infer complete chemical safety merely because material becomes less visible.
A waste audit can record categories produced during a school day and identify avoidable sources. State the period and location; one classroom's observation cannot establish national waste proportions. Use evidence to propose a specific change, such as separating food waste from clean paper, then review whether separation actually improves.
A reliable approach to questions
Identify organisms, physical factors and the system being described.
Draw food arrows from food towards consumer.
Count trophic transfers before calculating energy.
Distinguish nutrient recycling, energy flow and pollutant concentration.
For environmental problems, trace the mechanism from source to consequence.
Propose actions suited to the waste type and evidence available.
Connections that matter
Life processes explains photosynthesis, respiration and decomposition-related metabolism.
Carbon compounds explains persistent materials and the chemistry of fuels and packaging.
Geography connects ecosystems with land use, resources and environmental management.
Common mistakes
Reversing food arrows: energy moves from food to feeder.
Recycling energy in a diagram: nutrients cycle, but usable energy is dissipated.
Treating biodegradability as automatic harmlessness: amount and conditions matter.
Equating ozone depletion with global warming: distinguish the atmospheric mechanisms.
Revision checklist
I can identify ecosystem components and their interactions.
I can build food chains and distinguish trophic positions.
I can apply the ten-percent model in either direction.
I can explain biomagnification without confusing it with energy transfer.
I can describe ozone protection and depletion.
I can justify waste-management choices using material properties.