Human influences on ecosystems and biodiversity

What you'll learn

This topic examines how human activities affect ecosystems and biodiversity, including habitat destruction, pollution, and species extinction. Understanding these impacts and conservation strategies is essential for Paper 2 extended-response questions and Paper 4 alternative-to-practical scenarios. You'll need to explain specific mechanisms, evaluate conservation methods, and apply knowledge to unfamiliar contexts.

Key terms and definitions

Biodiversity — the variety of different species living in a habitat or ecosystem, including genetic diversity within species and ecosystem diversity.

Deforestation — the permanent removal of forests and conversion of land to non-forest uses, typically for agriculture, logging, or urban development.

Eutrophication — excessive nutrient enrichment of water bodies, usually from fertiliser runoff, causing algal blooms that deplete oxygen and kill aquatic organisms.

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

Sustainable development — meeting current human needs without compromising the ability of future generations to meet their own needs.

Invasive species — non-native organisms introduced to new habitats where they outcompete native species and disrupt ecosystems.

Monoculture — the cultivation of a single crop species over large areas, reducing biodiversity and increasing vulnerability to pests and diseases.

Core concepts

Habitat destruction and its consequences

Deforestation represents the most significant form of habitat destruction globally. When forests are cleared for agriculture, cattle ranching, logging, or urban expansion, the impacts extend beyond simple tree removal:

• Loss of habitats for thousands of species, many found nowhere else (high endemism in tropical rainforests)
• Soil erosion increases as tree roots no longer bind soil, leading to nutrient loss and reduced agricultural productivity
• Disruption of the water cycle — reduced transpiration decreases rainfall, and increased surface runoff causes flooding
• Climate change acceleration through release of stored carbon dioxide from burning and decomposition of vegetation
• Loss of potential medicines, foods, and genetic resources before species are even discovered

Agricultural expansion reduces biodiversity through habitat conversion. Large-scale monoculture farming replaces diverse ecosystems with single-crop fields. This creates:

• Reduced food web complexity with fewer niches available
• Increased pest vulnerability, requiring more pesticide use
• Soil depletion from continuous cultivation of one crop type
• Loss of hedgerows and field margins that serve as wildlife corridors

Urban development fragments habitats into isolated patches. Small, disconnected populations face:

• Reduced genetic diversity through inbreeding
• Higher extinction risk from random environmental events
• Barriers to migration and dispersal
• Edge effects that alter conditions within remaining habitat fragments

Pollution and ecosystem damage

Air pollution from industrial emissions, vehicle exhausts, and burning fossil fuels affects ecosystems through:

• Acid rain formation when sulfur dioxide and nitrogen oxides dissolve in atmospheric water, forming sulfuric and nitric acids
• Acidification of lakes and rivers, killing fish and aquatic invertebrates sensitive to low pH
• Damage to tree leaves and mobilisation of toxic aluminium ions in soil
• Lichen death — lichens are indicator species particularly sensitive to sulfur dioxide

Water pollution takes multiple forms with distinct ecological consequences:

Eutrophication occurs through this sequence:

1. Excess fertilisers containing nitrates and phosphates wash from farmland into rivers and lakes
2. Algae rapidly multiply using these nutrients, forming dense surface blooms
3. Algae block light penetration to plants below, preventing photosynthesis
4. Plants die and decompose
5. Aerobic decomposing bacteria multiply, consuming dissolved oxygen
6. Oxygen depletion kills fish and other aerobic organisms
7. Anaerobic bacteria take over, producing toxic hydrogen sulfide

Pesticide accumulation demonstrates bioaccumulation and biomagnification:

• Persistent pesticides like DDT accumulate in organisms faster than they can be broken down
• Concentration increases at each trophic level as predators consume many contaminated prey
• Top predators (eagles, otters) experience highest concentrations, causing reproductive failure and death
• Non-target species suffer unintended harm

Plastic pollution creates both physical and chemical hazards:

• Entanglement and ingestion by marine animals
• Microplastics enter food chains and accumulate in tissues
• Leaching of toxic additives into surrounding water
• Habitat smothering in marine and coastal ecosystems

Overharvesting and species extinction

Overfishing depletes marine populations through:

• Capture rates exceeding natural reproduction rates
• Bycatch killing of non-target species including turtles, dolphins, and seabirds
• Destructive fishing methods (bottom trawling) destroying seabed habitats
• Cascade effects when key species are removed from food webs

Hunting and poaching drive species toward extinction:

• Elephants and rhinos killed for ivory and horn
• Tigers and pangolins targeted for traditional medicine markets
• Slow-breeding species (whales, great apes) cannot recover from population losses

Introduced species become invasive when brought to new environments without natural predators or competitors:

• Grey squirrels in Britain outcompete native red squirrels for food and habitat
• Cane toads in Australia poison native predators and compete with native species
• Zebra mussels in North American lakes outcompete native molluscs and clog water infrastructure

Climate change impacts

Rising global temperatures from increased atmospheric carbon dioxide cause:

• Habitat shifts — species distributions move poleward and to higher altitudes as suitable climate zones shift
• Coral bleaching when warm water stresses corals, causing them to expel symbiotic algae
• Phenological mismatches — timing of events like migration or flowering becomes unsynchronised with food availability
• Melting polar ice reducing habitat for species like polar bears and penguins
• Ocean acidification from dissolved CO₂ affecting shell-forming organisms

Conservation and sustainability strategies

Protected areas preserve biodiversity through:

• National parks and nature reserves preventing development and exploitation
• Marine protected areas restricting fishing and extraction
• Wildlife corridors connecting fragmented habitats to allow migration

Sustainable resource management includes:

• Fishing quotas limiting catch to sustainable levels based on population monitoring
• Selective logging removing only mature trees while maintaining forest structure
• Crop rotation and mixed farming maintaining soil fertility without excessive inputs
• Sustainable tourism (ecotourism) generating income while minimising environmental damage

Species-focused conservation employs:

• Captive breeding programmes maintaining populations of endangered species in zoos
• Reintroduction of bred animals to restored wild habitats
• Legal protection through international agreements (CITES restricting trade in endangered species)
• Habitat restoration recreating wetlands, forests, or coral reefs

Reducing pollution through:

• Catalytic converters in vehicles reducing nitrogen oxide and carbon monoxide emissions
• Sewage treatment removing nutrients before water enters rivers
• Legislation limiting industrial emissions
• Alternatives to harmful pesticides and fertilisers

Education and awareness programmes help:

• Local communities benefit from conservation through employment and sustainable resource use
• Consumer choices (sustainable fish, FSC timber, reduced plastic) driving market changes
• Understanding ecosystem services (pollination, water purification, climate regulation) provided by biodiverse ecosystems

Worked examples

Example 1: Eutrophication analysis

Question: A farmer applies fertiliser to fields near a lake. After heavy rain, large numbers of fish are found dead in the lake. Explain how fertiliser runoff could cause fish deaths. [6 marks]

Model answer:

• Fertiliser contains nitrates/phosphates [1]
• Which are washed into the lake by rain [1]
• Algae grow rapidly using these nutrients/algal bloom forms [1]
• Algae block light reaching plants below [1]
• Plants die and are decomposed by aerobic bacteria [1]
• Bacteria use oxygen in the water [1]
• Oxygen concentration decreases below level needed by fish [1]
• Fish die from lack of oxygen/suffocation [1]

Award any 6 marks. Note how each biological step is stated clearly — exam answers require precise explanations of the mechanism, not just "pollution kills fish".

Example 2: Conservation strategy evaluation

Question: A country wants to protect an endangered tiger population. Suggest and explain two conservation methods that could be used. [4 marks]

Model answer:

• Create protected reserves/national parks [1] where hunting is banned and habitat is preserved [1]
• Captive breeding programme in zoos [1] to increase population numbers before reintroduction to wild [1]

Alternative valid answers: anti-poaching patrols/enforcement, education programmes, corridors between habitats, ecotourism, compensation for livestock losses. Each method needs both identification (1 mark) and explanation (1 mark).

Example 3: Deforestation consequences

Question: Large areas of tropical rainforest are cleared for cattle farming. Describe the effects this will have on: (a) carbon dioxide in the atmosphere [2 marks] (b) soil quality [2 marks]

Model answer: (a)

• Trees contain stored carbon/are removed/burned [1]
• Releasing carbon dioxide into atmosphere/increasing atmospheric CO₂ [1]
• Less photosynthesis occurs/less CO₂ absorbed [1]

Award any 2 marks

(b)

• Tree roots no longer bind/hold soil [1]
• Soil is eroded/washed away by rain [1]
• Nutrients are lost/leached from soil [1]

Award any 2 marks

Common mistakes and how to avoid them

• Mistake: Describing eutrophication as "fertilisers poison the fish directly." Correction: Eutrophication is an indirect process — fertilisers stimulate algal growth, decomposition depletes oxygen, and fish die from lack of oxygen (suffocation), not poisoning.

• Mistake: Stating "pollution kills all organisms equally." Correction: Different species have different tolerances. Lichens are highly sensitive to sulfur dioxide, making them useful indicator species. Some bacteria thrive in polluted conditions.

• Mistake: Writing "biodiversity loss reduces the number of organisms." Correction: Biodiversity refers to variety of species, not total number of individuals. A field of a million wheat plants has lower biodiversity than a meadow with hundreds of individuals from dozens of species.

• Mistake: Confusing bioaccumulation and biomagnification. Correction: Bioaccumulation is buildup in individual organisms over time; biomagnification is increasing concentration at successive trophic levels in food chains.

• Mistake: Claiming "conservation means leaving habitats completely untouched." Correction: Sustainable management often requires active intervention — controlled burning, selective culling, or removal of invasive species to maintain ecosystem health.

• Mistake: Only describing negative consequences without mechanisms in exam answers. Correction: CIE mark schemes reward explanations of how and why. For example, don't just state "deforestation causes flooding" — explain that tree roots normally absorb water and without them, surface runoff increases.

Exam technique for Human influences on ecosystems and biodiversity

• Command word awareness: "Explain" questions (common in this topic) require reasons or mechanisms, not just descriptions. "Suggest" allows reasonable answers not directly taught. "Describe" needs factual detail without explanation. Each marking point typically carries 1 mark, so a 4-mark explain question needs four distinct biological points.

• Application to unfamiliar contexts: Paper 2 extended response questions often present novel scenarios (e.g., a specific endangered species or pollution incident you haven't studied). Apply principles from the syllabus to the new context — markers reward correct biological reasoning more than memorised examples.

• Data interpretation in Paper 4: Questions may present graphs showing population changes, pollution levels over time, or biodiversity indices. Practice extracting specific values, describing trends, and explaining patterns using biological knowledge. Always quote data from graphs to support statements.

• QWC (Quality of Written Communication) marks: Extended 6-mark questions assess logical structure and scientific terminology. Plan your answer's sequence before writing. Use linking words (therefore, because, this causes, as a result) to show causation clearly.

Quick revision summary

Human activities reduce biodiversity through habitat destruction (especially deforestation), pollution (air, water, plastic), overharvesting, and climate change. Eutrophication occurs when fertiliser runoff causes algal blooms that deplete oxygen. Pesticides bioaccumulate through food chains. Conservation strategies include protected areas, captive breeding, sustainable resource management, and pollution reduction. Understanding mechanisms (not just consequences) is essential for exam success — explain how each human activity produces its ecological effects through specific biological processes.