How Human Activities Can Disturb Ecosystems
An ecosystem depends on many interconnected relationships. Organisms depend on one another for food, materials are recycled through the environment, and populations are affected by changes in their surroundings. When human activities alter one part of an ecosystem, the effects can therefore spread far beyond the original disturbance.
The most useful way to understand this topic is to follow a cause-and-effect chain:
human activity → environmental change → effect on organisms → change in populations or biodiversity
For example, releasing untreated sewage into a river does not simply make the water dirty. The organic matter is decomposed by microorganisms, their aerobic respiration uses dissolved oxygen, and aquatic organisms may eventually die because too little oxygen remains.
In the same way, burning fossil fuels, removing forests, producing persistent plastic waste and releasing certain insecticides can alter ecosystems in different ways. Conservation aims to reduce these pressures while protecting biodiversity and natural resources for the future.
Try these five questions before reading the article. They test the essential ideas you will build on in this topic.
- 5 diagnostic questions
- Global warming, pollution and conservation
- Instant explanation for every answer option
Global Warming and the Greenhouse Effect
Earth receives energy from the Sun. The surface absorbs some of this energy and later releases energy as infrared radiation.
Greenhouse gases in the atmosphere absorb some of this outgoing infrared radiation and re-radiate energy. This reduces the rate at which energy escapes into space and helps keep Earth warm enough for life.
The greenhouse effect is therefore a natural process. The environmental problem occurs when human activities increase the concentration of greenhouse gases and strengthen this effect. This is known as the enhanced greenhouse effect.
| Term | Meaning |
|---|---|
| Greenhouse effect | A natural process in which greenhouse gases help reduce the rate at which heat energy escapes from Earth. |
| Enhanced greenhouse effect | An increase in the greenhouse effect caused by higher concentrations of greenhouse gases. |
| Global warming | A long-term increase in Earth's average global temperature. |
Burning fossil fuels increases atmospheric carbon dioxide
Fossil fuels such as coal, oil and natural gas contain carbon. When they are burned in power stations, factories and vehicles, carbon dioxide is released into the atmosphere.
Burning fossil fuels → more atmospheric CO₂ → enhanced greenhouse effect → more heat retained → increase in global temperature
Deforestation can also increase atmospheric carbon dioxide
Green plants remove carbon dioxide from the atmosphere during photosynthesis. Large forests therefore play an important role in the carbon cycle.
When forests are cleared, fewer trees remain to carry out photosynthesis. Less carbon dioxide is therefore removed from the atmosphere.
If the removed vegetation is burned, carbon stored in the plant material is also released into the atmosphere as carbon dioxide.
Deforestation → fewer trees → less photosynthesis → less CO₂ removed from the atmosphere
Burning cleared vegetation → stored carbon released as CO₂
What are carbon sinks?
A carbon sink is a system that takes in and stores more carbon than it releases over a period of time.
Forests can act as carbon sinks because plants absorb carbon dioxide during photosynthesis and store carbon in their tissues. Oceans also absorb large amounts of carbon dioxide.
Possible effects of global warming
Global warming can alter environmental conditions and therefore affect both organisms and ecosystems.
| Environmental change | Possible consequence |
|---|---|
| Higher average temperatures | Some organisms may no longer be able to survive in their existing habitats. |
| Melting land ice and thermal expansion of seawater | Sea level can rise and threaten low-lying coastal ecosystems. |
| Changes in rainfall patterns | Water availability can change, affecting plant and animal populations. |
| Changes in habitats | Feeding relationships, breeding conditions and species distributions may change. |
How can the human contribution to global warming be reduced?
One approach is to reduce the amount of fossil fuel being burned. This can involve improving energy efficiency and increasing the use of renewable energy sources.
Protecting forests and planting trees can also increase the amount of carbon dioxide removed from the atmosphere by photosynthesis.
Reduce carbon dioxide release: use less fossil fuel and increase the use of renewable energy.
Increase carbon dioxide removal: protect forests and restore vegetation.
Water Pollution by Sewage
Sewage contains organic material that can be broken down by decomposer microorganisms. If large quantities of untreated sewage enter a river or lake, decomposition can greatly increase.
Why can sewage cause fish to die?
Fish are not necessarily killed directly by the sewage itself. The important biological link is often the effect of decomposition on dissolved oxygen.
More organic matter → more decomposition → more aerobic respiration by decomposers → more dissolved oxygen used → dissolved oxygen falls → aquatic organisms may die
Decomposer bacteria obtain energy by respiration. When large amounts of organic matter become available, bacterial populations and decomposition may increase.
Because aerobic respiration requires oxygen, more dissolved oxygen is removed from the water. If the oxygen concentration falls too far, fish and other aerobic aquatic organisms cannot obtain enough oxygen for respiration.
Biological oxygen demand
The amount of oxygen required by microorganisms while breaking down organic matter is related to the biological oxygen demand.
Water containing a large amount of biodegradable organic material usually has a greater oxygen demand because more decomposition takes place.
Pollution by Plastic
Plastic is useful because it is strong, lightweight and durable. However, this durability also creates an environmental problem.
Most conventional plastics are non-biodegradable. They are not readily broken down by decomposer microorganisms and can therefore remain in the environment for many years.
| Problem | Effect on organisms |
|---|---|
| Entanglement | Animals may become trapped in plastic bags, loops, ropes or discarded fishing equipment. |
| Ingestion | Animals may mistake plastic for food, which can interfere with feeding and digestion. |
| Persistence | Plastic waste can remain in habitats for long periods because it is not readily biodegradable. |
| Fragmentation | Larger pieces can break into smaller fragments without actually being biologically decomposed. |
A plastic object becoming smaller does not necessarily mean that it has biodegraded. It may simply have fragmented into many smaller pieces.
Insecticides and Biomagnification
Insecticides are chemicals used to kill insect pests. They can help protect crops and control insects that transmit disease, but some insecticides may remain in the environment for a long time.
A persistent chemical that enters an organism may not be broken down or removed quickly. The chemical can therefore accumulate in body tissues.
Bioaccumulation
Bioaccumulation is the gradual build-up of a substance within an individual organism over time.
Biomagnification
Biomagnification refers to an increase in the concentration of a persistent substance at successive trophic levels of a food chain.
Small organisms contain pollutant → consumers eat many contaminated organisms → predators eat many contaminated prey → pollutant concentration increases at higher trophic levels
Consider the food chain shown above:
phytoplankton → small fish → large fish → bird
If phytoplankton contain a persistent insecticide, a small fish may consume many contaminated phytoplankton. A large fish then consumes many contaminated small fish, and a bird may consume many contaminated large fish.
If the chemical is not readily broken down or excreted, organisms at higher trophic levels may therefore contain progressively greater concentrations.
| Term | Meaning |
|---|---|
| Bioaccumulation | A substance builds up within one organism over time. |
| Biomagnification | The concentration of a substance increases at successive trophic levels. |
Conservation and Biodiversity
Human beings can damage ecosystems, but they can also take action to protect them. This is the purpose of conservation.
Conservation involves protecting species, habitats and natural resources so that they remain available in the future.
What is biodiversity?
Biodiversity refers to the variety of living organisms found in an area.
A habitat with many different species contains many ecological relationships. Organisms may be connected through feeding relationships, pollination, decomposition, competition and other interactions.
Loss of one species may therefore affect other organisms in the ecosystem.
Why is biodiversity important?
| Reason | Explanation |
|---|---|
| Ecological importance | Species interact through food webs and contribute to the functioning of ecosystems. |
| Genetic resources | Different species contain genetic information that may be useful in agriculture, medicine and research. |
| Economic importance | Natural ecosystems can support fisheries, agriculture, forestry and tourism. |
| Ethical and cultural value | Species and natural environments may also be valued in their own right. |
Why protect entire habitats?
Protecting an endangered species without protecting its habitat may not be enough.
An organism depends on its environment for food, shelter, breeding sites and interactions with other species. Protecting a habitat therefore helps conserve many species at the same time.
| Ecosystem | Importance |
|---|---|
| Tropical rainforests | Support high biodiversity and store large quantities of carbon. |
| Mangroves | Provide habitats and nursery areas for many organisms and help protect coastlines. |
| Coral reefs | Provide complex habitats, feeding areas and breeding grounds for many marine organisms. |
What does sustainable use mean?
Conservation does not necessarily mean that humans can never use natural resources.
Sustainable use means using resources in a way that allows them to remain available in the future.
For example, if fish are harvested faster than they can reproduce, the population may decline. A sustainable fishery controls harvesting so that the population has enough time to replace individuals that are removed.
Methods of conservation
Habitat protection: protect important ecosystems and reduce habitat destruction.
Reforestation: restore forests where large numbers of trees have been removed.
Pollution control: treat sewage and reduce the release of harmful waste and chemicals.
Regulation: control activities such as hunting, fishing and logging.
Species conservation: protect threatened populations and their habitats.
Common Mistakes
| Common mistake | Correct idea |
|---|---|
| Saying that the greenhouse effect itself is caused by humans. | The greenhouse effect is natural. Human activities can enhance it by increasing greenhouse gas concentrations. |
| Saying that deforestation only increases CO₂ because trees are burned. | Deforestation also leaves fewer trees available to remove CO₂ by photosynthesis. |
| Saying untreated sewage always kills fish by poisoning them directly. | A major biological effect is increased decomposition, which consumes dissolved oxygen. |
| Saying decomposer bacteria reduce oxygen by photosynthesis. | Decomposer bacteria use oxygen during aerobic respiration. |
| Saying that plastic has biodegraded when it breaks into smaller pieces. | Fragmentation is not the same as biodegradation. |
| Treating bioaccumulation and biomagnification as identical. | Bioaccumulation occurs within one organism. Biomagnification occurs across trophic levels. |
| Saying conservation means humans must never use natural resources. | Conservation can include sustainable use of resources. |
What You Should Be Able to Do
Explain the enhanced greenhouse effect
Link increasing greenhouse gas concentrations to greater heat retention and global warming.
Explain the effect of burning fossil fuels
State that combustion releases carbon dioxide and relate this to the enhanced greenhouse effect.
Explain the effect of deforestation
Link fewer trees to reduced carbon dioxide uptake by photosynthesis.
Explain sewage pollution
Connect organic matter, decomposers, aerobic respiration, dissolved oxygen and the death of aquatic organisms.
Describe the effects of plastic pollution
Explain why plastic persists and how ingestion or entanglement can harm organisms.
Distinguish bioaccumulation from biomagnification
Identify whether a pollutant is increasing within one organism or through successive trophic levels.
Explain why top consumers may contain high pollutant concentrations
Relate this to consuming many contaminated organisms and the persistence of the pollutant.
Explain the importance of biodiversity
Relate biodiversity to ecological interactions and the continued functioning of ecosystems.
Explain conservation and sustainability
Describe how habitats and resources can be protected and managed for future use.
Construct complete cause-and-effect explanations
Trace an environmental change from the human activity to its biological consequence.
These ten questions test whether you can apply the cause-and-effect relationships from this topic.
- 10 mastery questions
- Application and cause-and-effect reasoning
- Instant explanation for every answer option
Effects of Man on the Ecosystem: Questions Students Often Ask
The hardest questions in this topic usually involve connecting several steps together. These answers reinforce the relationships that are most often confused.
What is the difference between the greenhouse effect and global warming?
The greenhouse effect is a natural process in which greenhouse gases reduce the rate at which heat energy escapes from Earth. Global warming is the long-term increase in Earth's average temperature. Human activities can increase greenhouse gas concentrations, strengthening the greenhouse effect and contributing to global warming.
Is the greenhouse effect itself harmful?
No. The natural greenhouse effect helps keep Earth warm enough for life. The concern is the enhanced greenhouse effect, in which increasing greenhouse gas concentrations cause additional warming.
Why does burning fossil fuels contribute to global warming?
Burning fossil fuels releases carbon dioxide. Increasing atmospheric carbon dioxide strengthens the enhanced greenhouse effect, causing more heat to be retained and contributing to an increase in global average temperature.
Why does deforestation increase atmospheric carbon dioxide?
Removing trees reduces the amount of carbon dioxide taken in during photosynthesis. If the removed vegetation is burned, additional carbon dioxide is released from the carbon stored in the plant material.
What is a carbon sink?
A carbon sink is a system that absorbs and stores more carbon than it releases over a period of time. Forests can act as carbon sinks because plants take in carbon dioxide during photosynthesis and store carbon in their tissues.
Does untreated sewage poison fish directly?
Sewage may contain harmful substances, but a major biological reason that fish die is oxygen depletion. Decomposer microorganisms break down the organic material and respire aerobically. Their respiration uses dissolved oxygen, leaving less oxygen available for fish.
Why does more decomposition mean less dissolved oxygen?
Aerobic decomposers require oxygen for respiration. If more organic matter is available, decomposition and respiration increase, so more dissolved oxygen is consumed.
Why is plastic harmful even if it is not poisonous?
Plastic can harm organisms physically. Animals may become entangled in it or mistake it for food. Because many plastics are non-biodegradable, they can remain in ecosystems for long periods.
What is the difference between bioaccumulation and biomagnification?
Bioaccumulation is the build-up of a substance within one organism over time. Biomagnification is the increase in concentration of a persistent substance at successive trophic levels in a food chain.
Why do organisms near the top of a food chain often contain the most persistent pollutant?
A predator consumes many organisms from the trophic level below it. If the pollutant is not readily broken down or excreted, the predator receives the accumulated pollutant from its prey. The concentration can therefore become greater at higher trophic levels.
What does biodiversity mean?
Biodiversity refers to the variety of living organisms in an area. High biodiversity means that many different species are present and interacting within the ecosystem.
Why is protecting a habitat often better than protecting only one species?
A species depends on its habitat for food, shelter, breeding sites and interactions with other organisms. Protecting the whole habitat therefore helps preserve many species and the ecological relationships between them.
Does conservation mean that humans cannot use natural resources?
No. Conservation can include sustainable use. Resources can be used provided they are managed so that they are not depleted faster than they can recover.
How should I answer an examination question about environmental effects?
Avoid jumping directly from the human activity to the final effect. Write the biological mechanism as a complete chain. For example: sewage adds organic matter → decomposer activity increases → aerobic respiration increases → dissolved oxygen decreases → fish may die.
The different examples in this topic all follow the same principle: human activity changes an environmental condition, and that change affects organisms. Burning fossil fuels alters the atmosphere, sewage changes oxygen availability in water, plastics persist in habitats, persistent insecticides can become concentrated through food chains, and habitat destruction reduces biodiversity.
When answering examination questions, concentrate on the biological link between the cause and the final effect. Once you can explain that chain clearly, this topic becomes far more logical than a collection of facts to memorise.