Human Influences on Ecosystems

What you'll learn

This topic examines how human activities affect natural ecosystems and what can be done to reduce negative impacts. CIE IGCSE Biology exams frequently test understanding of pollution types, habitat destruction, conservation methods and sustainable resource management. Questions range from simple recall to data analysis and extended response, making thorough understanding essential for securing marks across all paper types.

Key terms and definitions

Deforestation — the permanent removal of forest or woodland, typically for agriculture, logging or development purposes.

Eutrophication — the excessive enrichment of water bodies with nutrients (particularly nitrates and phosphates), leading to algal blooms and oxygen depletion.

Bioaccumulation — the build-up of persistent or non-biodegradable substances in living organisms over time.

Sustainable development — development that meets present needs without compromising the ability of future generations to meet their own needs.

Conservation — the protection and management of species and habitats to prevent extinction and maintain biodiversity.

Monoculture — the agricultural practice of growing a single crop species over a wide area for consecutive years.

Non-renewable resources — natural resources that cannot be replenished on a human timescale, such as fossil fuels and minerals.

Biomagnification — the increasing concentration of non-biodegradable substances at each successive level in a food chain.

Core concepts

Food supply and agricultural impact

Human population growth has created increasing demand for food production. Modern agriculture has responded through:

Intensive farming methods include:

• Use of chemical fertilisers to increase crop yields
• Application of pesticides to reduce crop damage from pests
• Herbicides to eliminate competing weed species
• Large-scale monoculture systems
• High-density livestock farming with controlled feeding

These methods increase food production but create environmental problems:

Fertiliser runoff leads to eutrophication. When excess nitrates and phosphates enter water bodies:

1. Algae grow rapidly forming dense blooms on the water surface
2. Light penetration decreases, preventing photosynthesis in submerged plants
3. Plants die and decompose
4. Decomposing bacteria multiply and respire aerobically
5. Oxygen concentration drops dramatically
6. Aquatic animals die from oxygen starvation (hypoxia)

Pesticide problems include:

• Killing non-target species including beneficial insects like pollinators
• Bioaccumulation in organisms that cannot break down these chemicals
• Biomagnification through food chains, reaching toxic levels in top predators
• Development of pesticide-resistant pest populations through natural selection

Monoculture disadvantages:

• Reduced biodiversity as single crop supports fewer species
• Soil nutrient depletion from repeated planting of the same crop
• Increased vulnerability to disease spread through genetically similar plants
• Greater dependence on chemical inputs to maintain yields

Deforestation and habitat destruction

Deforestation occurs primarily for:

• Agricultural land (cattle ranching, crop cultivation, palm oil plantations)
• Timber extraction for construction and paper industries
• Urban development and infrastructure projects
• Mining operations

The consequences of large-scale deforestation include:

Impact on carbon dioxide levels:

• Trees remove CO₂ from the atmosphere through photosynthesis
• Cleared forests no longer act as carbon sinks
• Burning or decomposition of felled trees releases stored carbon as CO₂
• Reduced CO₂ absorption contributes to enhanced greenhouse effect and climate change

Soil degradation:

• Tree roots bind soil particles and prevent erosion
• Without tree cover, soil is exposed to wind and rain
• Top soil (richest in nutrients and organic matter) washes away
• Remaining soil has reduced fertility
• Heavy rainfall can cause landslides on deforested slopes

Loss of biodiversity:

• Habitat destruction eliminates niches for countless species
• Forest specialists cannot survive in cleared areas
• Food chains and webs are disrupted
• Species extinction rates increase
• Genetic diversity is permanently lost

Disrupted water cycle:

• Trees absorb water from soil and release it through transpiration
• Deforestation reduces transpiration and local rainfall
• Surface water runoff increases, causing flooding
• Rivers may silt up from eroded soil

Pollution types and effects

Air pollution from burning fossil fuels releases:

Sulfur dioxide and nitrogen oxides cause:

• Formation of acid rain when these gases dissolve in atmospheric moisture
• Damage to aquatic ecosystems as lakes and rivers become acidic
• Fish populations decline due to enzyme denaturation and aluminium ion toxicity
• Tree damage from leaf destruction and mobilisation of toxic metal ions in soil
• Erosion of limestone buildings and statues

Carbon dioxide emissions:

• Enhanced greenhouse effect trapping more infrared radiation
• Global temperature increases (climate change)
• Ice cap melting and sea level rise
• Changes to weather patterns and ecosystems
• Ocean acidification affecting marine organisms

Carbon monoxide from incomplete combustion:

• Binds irreversibly to haemoglobin
• Reduces oxygen transport capacity of blood
• Can cause death through oxygen starvation of tissues

Water pollution sources include:

Sewage discharge:

• Increases bacterial populations and disease risk
• Adds organic matter requiring oxygen for decomposition
• Reduces dissolved oxygen levels through bacterial respiration
• Causes eutrophication through nutrient enrichment

Chemical pollutants:

• Heavy metals (lead, mercury) bioaccumulate in organisms
• Industrial chemicals can be toxic even at low concentrations
• Oil spills coat organisms, preventing gas exchange and waterproofing
• Plastics persist in environment, ingested by wildlife causing physical damage

Thermal pollution:

• Power stations discharge heated cooling water
• Raises water temperature reducing oxygen solubility
• Alters metabolism rates of aquatic organisms
• Changes species distribution

Conservation and sustainable resource management

Conservation programs aim to protect biodiversity through:

Habitat protection:

• Establishing nature reserves and national parks with legal protection
• Creating wildlife corridors connecting fragmented habitats
• Restricting development and resource extraction in sensitive areas
• Restoring degraded ecosystems through replanting and species reintroduction

Species protection measures:

• Legal protection for endangered species (ban on hunting/trading)
• Captive breeding programs in zoos and botanical gardens
• Reintroduction of captive-bred individuals to wild populations
• Seed banks preserving genetic diversity of plant species
• International agreements like CITES (Convention on International Trade in Endangered Species)

Sustainable development strategies:

Sustainable forest management:

• Selective logging — removing only mature trees, leaving ecosystem largely intact
• Replanting programs — replacing harvested trees maintaining forest cover
• Longer rotation periods — allowing forests time to recover between harvests
• Certification schemes — ensuring timber from sustainably managed sources

Sustainable fishing practices:

• Fishing quotas limiting catch sizes to allow population recovery
• Net regulations (minimum mesh size) allowing young fish to escape
• Exclusion zones protecting breeding grounds and nursery areas
• Banning destructive fishing methods like bottom trawling in sensitive areas
• Aquaculture (fish farming) reducing pressure on wild stocks

Renewable energy adoption:

• Solar, wind and hydroelectric power reducing fossil fuel dependence
• Biofuels from crops absorbing CO₂ during growth (carbon neutral cycle)
• Nuclear energy producing no greenhouse gases (though creates radioactive waste)

Waste management improvements:

• Recycling materials like paper, glass, metals and plastics reducing resource extraction
• Composting organic waste returning nutrients to soil
• Reducing landfill use preventing methane release and groundwater contamination
• Waste-to-energy incineration with pollution controls

Agricultural sustainability:

• Crop rotation maintaining soil fertility and breaking pest cycles
• Organic farming avoiding synthetic chemicals
• Integrated pest management combining biological control with minimal chemical use
• Precision agriculture applying inputs only where needed

Worked examples

Example 1: Eutrophication process

Question: A farmer applies fertiliser to fields near a lake. After heavy rain, the lake develops an algal bloom and fish are found dead. Explain how fertiliser application led to fish deaths. [5 marks]

Model answer:

• Fertiliser (containing nitrates/phosphates) washed into lake by rain/surface runoff [1]
• Algae grow rapidly using excess nutrients, forming bloom on water surface [1]
• Light blocked from reaching underwater plants, which die [1]
• Bacteria decompose dead plants/algae, respiring aerobically [1]
• Oxygen concentration in water decreases, fish die from lack of oxygen for respiration [1]

Examiner note: Each step in the sequence must be clearly stated. Simply saying "eutrophication occurs" without explanation gains no marks.

Example 2: Bioaccumulation in food chains

Question: The table shows DDT pesticide concentration in organisms from a lake ecosystem.

Organism	DDT concentration (ppm)
Water	0.02
Algae	0.5
Small fish	12
Pike	85

(a) Describe the trend shown in the data. [2 marks]

(b) Explain why this pattern occurs. [3 marks]

Model answer (a):

• DDT concentration increases at each level in the food chain [1]
• Pike (top predator) has highest concentration, 4250 times higher than in water [1]

Model answer (b):

• DDT is non-biodegradable/persistent, cannot be broken down by organisms [1]
• Accumulates in organisms' bodies over time (bioaccumulation) [1]
• Each consumer eats many prey organisms, concentrating DDT from each one [1]

Example 3: Carbon dioxide and climate change

Question: Explain how deforestation contributes to increased atmospheric carbon dioxide levels. [4 marks]

Model answer:

• Trees remove/absorb carbon dioxide from atmosphere during photosynthesis [1]
• Fewer trees means less CO₂ removed from atmosphere [1]
• Cleared trees may be burned, releasing carbon dioxide from combustion [1]
• Dead wood decomposes, releasing stored carbon as CO₂ through decomposer respiration [1]

Common mistakes and how to avoid them

Mistake: Stating that acid rain is caused by carbon dioxide or that CO₂ directly damages trees.

Correction: Acid rain results from sulfur dioxide and nitrogen oxides dissolving in rain. Carbon dioxide causes climate change through the greenhouse effect, not acid rain.

Mistake: Describing eutrophication as "too much oxygen in water" or skipping the decomposition step.

Correction: Eutrophication causes oxygen depletion. The sequence must include: nutrient addition → algal bloom → plant death → bacterial decomposition → oxygen decrease → animal death.

Mistake: Confusing bioaccumulation and biomagnification, using terms interchangeably.

Correction: Bioaccumulation is build-up within one organism over time. Biomagnification is the increasing concentration at successive trophic levels in a food chain.

Mistake: Claiming conservation simply means "protecting animals" without explaining methods.

Correction: Conservation requires specific strategies: habitat protection, captive breeding, legal restrictions, sustainable management, education programs. Name actual methods for marks.

Mistake: Stating that renewable energy is "unlimited" or has no environmental impact.

Correction: Renewable sources are naturally replenished but still have impacts (hydroelectric dams flood habitats; wind turbines affect bird populations). State that renewables reduce fossil fuel use and greenhouse gas emissions.

Mistake: Writing that fishing quotas or protected areas "stop extinction" immediately.

Correction: These measures allow population recovery over time by reducing pressure and protecting breeding populations, giving species chance to increase numbers.

Exam technique for Human Influences on Ecosystems

Command word recognition:

• "Explain" questions require cause-and-effect reasoning with biological processes named. State what happens AND why it happens for full marks.
• "Describe" questions need trends or patterns stated without explanation. For graphs/tables, quote data values to support descriptions.
• "Suggest" questions assess application to unfamiliar contexts. Use knowledge from familiar examples but adapt to the specific scenario given.

Process sequences: For eutrophication or bioaccumulation questions, write each step on a new line. Examiners award one mark per correct sequential point. Missing steps lose marks even if later stages are correct.

Real examples: Questions may ask for named examples of conservation (e.g., captive breeding of pandas, seed banks at Kew Gardens) or specific pollutants (sulfur dioxide, mercury, nitrates). Learn at least two examples per category.

Extended response structure: For 6-mark questions, plan before writing. Include: (1) definition/statement of issue, (2) explanation of biological process, (3) consequences, (4) solution or management strategy. Use scientific terminology precisely throughout.

Quick revision summary

Human activities cause pollution (air, water, land), habitat destruction through deforestation, and biodiversity loss. Eutrophication occurs when fertiliser runoff causes algal blooms, plant death and oxygen depletion killing aquatic life. Non-biodegradable pesticides bioaccumulate in organisms and biomagnify through food chains. Burning fossil fuels releases greenhouse gases (CO₂) causing climate change, and sulfur/nitrogen oxides causing acid rain. Conservation requires habitat protection, captive breeding, legal restrictions and sustainable resource management including selective logging, fishing quotas, crop rotation and renewable energy adoption to balance human needs with ecosystem preservation.