Human Impact on Biodiversity

Human activities are the primary driver of biodiversity loss in the modern era. Scientists have described the current rate of extinction as a "sixth mass extinction" — the first caused by a single species. Understanding how human activities threaten biodiversity, and what can be done to mitigate these threats, is essential for Edexcel A-Level Biology.

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The Scale of Biodiversity Loss

Current extinction rates are estimated to be 100 to 1,000 times higher than the natural background extinction rate. Key statistics:

• Approximately 1 million species are currently threatened with extinction (IPBES, 2019)
• Vertebrate populations have declined by an average of 69% since 1970 (WWF Living Planet Report, 2022)
• 75% of the Earth's land surface has been significantly altered by human activities
• 85% of wetland area has been lost since 1700
• Global forest cover has declined by approximately 32% since pre-industrial times

Exam Tip: When discussing human impacts on biodiversity, use specific data and statistics where possible. Examiners reward answers that demonstrate awareness of the scale and severity of the problem.

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Major Threats to Biodiversity

1. Habitat Destruction and Fragmentation

Habitat destruction is the single greatest threat to biodiversity worldwide. When habitats are destroyed, the organisms that depend on them lose their homes, food sources, and breeding sites.

Causes of habitat destruction:

Activity	Habitat Lost	Scale
Agriculture	Forests, grasslands, wetlands cleared for crops and livestock	Agriculture occupies ~50% of habitable land
Urbanisation	Natural habitats converted to built environments	Urban areas are expanding rapidly worldwide
Deforestation	Tropical rainforests (the most biodiverse terrestrial habitats)	~10 million hectares of forest lost annually
Mining	Surface mining destroys entire landscapes	Widespread in tropical regions for minerals and fossil fuels
Drainage of wetlands	Wetlands converted for agriculture or development	85% of global wetland area lost

Habitat fragmentation occurs when large, continuous habitats are broken into smaller, isolated patches. Fragmentation is particularly harmful because:

• Edge effects — the edges of habitat patches are exposed to wind, light, and temperature changes, altering conditions and favouring edge-adapted species over interior specialists
• Reduced population size — smaller habitat patches support smaller populations, increasing the risk of extinction through genetic drift and inbreeding
• Reduced gene flow — isolation prevents movement between patches, reducing genetic diversity
• Barrier to migration — animals may be unable to cross the gaps between fragments to find food, mates, or suitable breeding sites
• Increased vulnerability — small, isolated populations are more susceptible to stochastic (random) events like disease outbreaks or extreme weather

2. Overexploitation

Overexploitation is the harvesting of species at a rate faster than they can naturally reproduce.

Type	Description	Examples
Overfishing	Fish are caught faster than populations can replenish	Collapse of Atlantic cod stocks in the 1990s; decline of bluefin tuna
Overhunting	Animals are hunted to extinction or near-extinction	Dodo (extinct by 1681); passenger pigeon (extinct by 1914); ongoing poaching of elephants for ivory and rhinos for horn
Overlogging	Timber is harvested faster than forests can regenerate	Mahogany and teak in tropical forests
Overharvesting of plants	Wild plants collected unsustainably for food, medicine, or trade	Wild ginseng; orchid collection

3. Climate Change

Climate change driven by human greenhouse gas emissions is an increasing threat to biodiversity:

Impact	Mechanism	Example
Temperature increase	Many species cannot tolerate higher temperatures	Coral bleaching — when water temperatures rise, corals expel their symbiotic zooxanthellae, turning white and often dying
Shifting ranges	Species move towards the poles or to higher altitudes to track suitable temperatures	Mountain species (e.g., pikas) have nowhere higher to move; Arctic species (e.g., polar bears) lose sea ice habitat
Phenological mismatch	Climate change alters the timing of seasonal events differently for different species	Oak trees leaf out earlier; caterpillars hatch earlier; but migratory birds arrive at the same time, missing the caterpillar peak and failing to feed chicks
Ocean acidification	Increased atmospheric CO₂ dissolves in seawater, lowering pH	Acidic water reduces the ability of corals, molluscs, and other organisms to build calcium carbonate shells and skeletons
Sea level rise	Melting ice and thermal expansion flood low-lying habitats	Coastal wetlands, mangroves, and coral atolls are submerged
Extreme weather events	More frequent and severe storms, droughts, and floods	Droughts cause mass mortality in rainforest trees; hurricanes destroy coral reefs

Exam Tip: Climate change questions are increasingly common. Be specific about mechanisms — do not just say "climate change kills animals." Explain how temperature rises cause coral bleaching, how phenological mismatches disrupt food chains, or how ocean acidification affects calcifying organisms.

4. Pollution

Pollution introduces harmful substances into ecosystems, directly killing organisms or disrupting ecological processes.

Pollutant	Source	Effect on Biodiversity
Nitrogen and phosphorus (eutrophication)	Agricultural fertilisers, sewage	Excess nutrients cause algal blooms; decomposition of dead algae depletes oxygen, killing aquatic organisms (dead zones)
Pesticides	Agriculture	Kill non-target organisms; bioaccumulation and biomagnification concentrate toxins up food chains (e.g., DDT thinned bird eggshells)
Heavy metals	Industrial waste, mining	Toxic to organisms; accumulate in tissues; contaminate water and soil
Plastic pollution	Consumer waste, fishing gear	Entangles wildlife; ingested by marine animals; microplastics enter food chains
Oil spills	Fossil fuel extraction and transport	Coats and kills seabirds and marine mammals; smothers benthic organisms
Sulfur dioxide and nitrogen oxides	Burning fossil fuels	Acid rain damages forests, acidifies lakes and soils, kills aquatic organisms
Light pollution	Artificial lighting	Disorients migrating birds, disrupts nocturnal insect behaviour, affects sea turtle hatchling navigation

Bioaccumulation and Biomagnification

Bioaccumulation is the build-up of a toxic substance (e.g., a pesticide or heavy metal) within an organism's tissues over its lifetime, because the substance is absorbed faster than it is excreted.

Biomagnification is the increasing concentration of a toxic substance at each successive trophic level in a food chain. Because organisms at higher trophic levels eat many organisms from lower levels, the toxin becomes increasingly concentrated.

Classic example: DDT

• DDT was sprayed on crops to kill insect pests
• Small amounts accumulated in aquatic invertebrates
• Fish ate many invertebrates, accumulating more DDT
• Birds of prey (e.g., peregrine falcons, bald eagles) ate many fish, accumulating very high DDT concentrations
• High DDT levels caused eggshell thinning, crushing eggs and causing population crashes

Trophic Level	Organism	DDT Concentration (ppm)
Producer	Phytoplankton	0.04
Primary consumer	Zooplankton	0.5
Secondary consumer	Small fish	2.0
Tertiary consumer	Large fish	25
Top predator	Osprey/eagle	1,600

Exam Tip: Bioaccumulation and biomagnification are frequently confused. Bioaccumulation occurs within a single organism; biomagnification occurs across trophic levels in a food chain. Be precise in your language.

5. Invasive Species

Invasive species are organisms introduced (deliberately or accidentally) to areas outside their natural range, where they cause harm to native species and ecosystems.

Invasive Species	Location	Impact
Grey squirrel (Sciurus carolinensis)	UK (from North America)	Outcompetes native red squirrel for food and habitat; carries squirrelpox virus (lethal to red squirrels)
Japanese knotweed (Reynoutria japonica)	UK and Europe (from East Asia)	Forms dense monocultures that shade out native plants; damages buildings and infrastructure
Cane toad (Rhinella marina)	Australia (from Central America)	Toxic to native predators that eat them; competes with native amphibians
Nile perch (Lates niloticus)	Lake Victoria, East Africa	Introduced for fishing; caused extinction of ~200 endemic cichlid species through predation
Zebra mussel (Dreissena polymorpha)	North American Great Lakes (from Eastern Europe)	Filters vast quantities of plankton, reducing food for native species; clogs water infrastructure

Invasive species cause harm through:

• Competition — outcompeting native species for resources
• Predation — preying on native species that have no evolved defences
• Disease — introducing novel pathogens
• Hybridisation — interbreeding with native species and diluting their gene pool
• Habitat modification — altering the physical environment

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Measuring the Impact: Indicator Species and Biodiversity Monitoring

Scientists use various methods to monitor human impacts on biodiversity:

• Indicator species — organisms that are particularly sensitive to environmental changes. Their presence or absence indicates habitat quality (e.g., lichen diversity indicates air quality; mayfly larvae indicate water quality)
• Simpson's Index of Diversity — calculated from field sampling data to quantify and compare biodiversity in different areas or over time
• Population monitoring — tracking population sizes of key species over time using mark-release-recapture, census data, or remote sensing
• Remote sensing — satellite imagery to monitor deforestation, land use change, and habitat fragmentation at large scales

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Mitigating Human Impacts

Reducing Habitat Destruction

• Protected areas — Expanding the global network of nature reserves and marine protected areas
• Sustainable land use — Integrating biodiversity conservation into agriculture, forestry, and urban planning
• Restoration ecology — Actively restoring degraded habitats (e.g., reforestation, wetland restoration, rewilding)

Addressing Climate Change

• Reducing greenhouse gas emissions — Transitioning to renewable energy, improving energy efficiency, reducing deforestation
• Carbon sequestration — Protecting and restoring forests, peatlands, and other carbon sinks
• Assisted migration — Helping species move to suitable habitats as climate zones shift (controversial)

Controlling Pollution

• Regulation — Laws limiting emissions and discharges (e.g., Clean Air Act, Water Framework Directive)
• Waste management — Reducing plastic use, improving recycling, treating sewage before discharge
• Banning harmful substances — The ban on DDT and CFCs led to recovery of affected species and ecosystems

Managing Overexploitation

• Fishing quotas and mesh size regulations — Allowing fish populations to reproduce before being caught
• CITES — Regulating international trade in endangered species
• Anti-poaching enforcement — Rangers, surveillance technology, and penalties

Controlling Invasive Species

• Biosecurity — Border controls to prevent introduction of non-native species
• Eradication programmes — Removing invasive species from sensitive habitats (e.g., rat eradication on seabird islands)
• Biological control — Introducing natural enemies of the invasive species (requires careful risk assessment)

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The Ethics of Conservation

Conservation involves difficult ethical decisions:

Dilemma	Considerations
Development vs conservation	Building homes and infrastructure meets human needs, but destroys habitats. How do we balance these competing demands?
Culling invasive species	Killing grey squirrels or invasive deer may benefit native species, but raises animal welfare concerns
Flagship species	Conservation funding often focuses on charismatic species (pandas, tigers) while less "appealing" species (invertebrates, fungi) may be more ecologically important
Local livelihoods	Establishing protected areas may displace indigenous communities who depend on the land for their livelihoods
Triage	With limited resources, should we focus on species most likely to be saved, or those most at risk?

Exam Tip: Ethics questions require balanced discussion. Present arguments from multiple perspectives and avoid one-sided answers. Acknowledge that conservation decisions involve trade-offs between competing values.

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Required Practical: Investigating the Effect of an Environmental Factor on Species Distribution

This links to the required practical on sampling (covered in Lesson 5). You may be asked to design an investigation into how a human activity (e.g., pollution, mowing regime, grazing) affects species distribution or diversity.

Key Practical Considerations

1. Choose the environmental factor to investigate (independent variable)
2. Select appropriate sampling technique (quadrats for plants, kick sampling for aquatic invertebrates, etc.)
3. Use random sampling within each treatment area
4. Record species and abundance data
5. Calculate Simpson's Index for each treatment area
6. Compare the indices to determine the effect
7. Control variables — time of year, weather conditions, sampling effort, quadrat size
8. Consider ethical implications — minimise disturbance to organisms and habitats

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Summary

Key Concept	Detail
Habitat destruction	Greatest threat to biodiversity; driven by agriculture, urbanisation, deforestation
Fragmentation	Isolates populations, reducing genetic diversity and increasing extinction risk
Overexploitation	Harvesting faster than reproduction — overfishing, overhunting, overlogging
Climate change	Coral bleaching, range shifts, phenological mismatch, ocean acidification
Pollution	Eutrophication, pesticides, plastics — bioaccumulation and biomagnification
Invasive species	Competition, predation, disease, hybridisation with native species
Bioaccumulation	Build-up of toxin within one organism
Biomagnification	Increasing toxin concentration up trophic levels
Mitigation	Protected areas, sustainable practices, pollution control, invasive species management

Exam Tip: Human impact questions are frequently the subject of extended-response questions worth 6+ marks. Structure your answer around specific threats (habitat loss, climate change, pollution, overexploitation, invasive species), give named examples, explain the mechanisms, and discuss possible solutions. Always link back to the effect on biodiversity — both species diversity and genetic diversity.

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A-Level Deep Dive: Human Impact on Biodiversity

Spec mapping

The Edexcel 9BI0 specification places human impact at the close of Topic 4: Biodiversity and Natural Resources, where it functions as the threat-side mirror of lesson 9 (conservation responds to these impacts). Synoptic links saturate the unit. Lesson 4 (Biodiversity defined and measured) supplies the metric being damaged: species richness, evenness, habitat diversity and genetic diversity all decline under sustained anthropogenic pressure, and Simpson's index $D = 1 - \sum (n/N)^2$D=1−∑(n/N)2 is the field-survey measurement that quantifies the loss in concrete terms. Lesson 5 (Sampling methodology) underpins the monitoring that detects trends. Lesson 6 (Natural selection and genetic diversity) is critical because human activity now functions as a dominant selective pressure — pesticide resistance in arthropods, antibiotic resistance in pathogens, harvest-induced selection on body size in fisheries, and urban-adapted morphologies in birds and mammals are all examples of anthropogenic selection acting on contemporary timescales. Lesson 9 (Conservation and biodiversity) is the response side of the same coin and should be cross-referenced in every extended answer. Outside Topic 4, Topic 5 (Ecosystems and energy flow) establishes that biodiversity loss degrades ecosystem services — productivity, decomposition, nutrient cycling, pollination — so impact translates into measurable human-welfare cost. Topic 7 (Modern genetics and gas exchange) matters because climate-change impacts include ocean warming and acidification altering gas exchange and calcification in marine taxa. Topic 6 (Immunity and infection) matters because zoonotic emergence (Ebola, SARS-CoV-2, Nipah) is repeatedly traced to deforestation, bushmeat trade and climate-driven range shifts in vector species. The specification rewards candidates who can quantify the scale (current extinction rates 100-1000$\times$× background; ~1 million species threatened, IPBES 2019; vertebrate population indices declined ~70% since 1970, WWF Living Planet Report), name the five major drivers identified by IPBES (habitat loss; direct exploitation; climate change; pollution; invasive species), and treat conservation (lesson 9) as the integrated response framework (refer to the official Pearson Edexcel 9BI0 specification document for exact wording).

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Worked example with full mark scheme

Question (8 marks):

The IPBES Global Assessment (2019) identified five major drivers of biodiversity loss. A team of conservation scientists is asked to advise the government of a tropical-forest country on which drivers should be prioritised for the next decade.

(a) Identify the five drivers ranked in approximate order of current global contribution, naming a specific empirical signal for each. (5)

(b) Apply the concept of defaunation to evaluate why "extinction-risk lists" alone understate the scale of biodiversity loss. (2)

(c) Justify why an integrated cross-driver strategy outperforms a single-driver focus. (1)

Solution with mark scheme:

(a) M1 (AO1.1) — driver 1: Habitat loss and degradation — the single largest driver globally; signal: ~30% of pre-industrial global forest cover lost, with continuing loss of ~10 million ha tropical forest annually.

M1 (AO1.1) — driver 2: Direct exploitation — overfishing, hunting, logging at rates exceeding replacement; signal: collapse of North Atlantic cod stocks (Grand Banks moratorium, 1992) and ~70% population decline in many large vertebrates.

M1 (AO1.1) — driver 3: Climate change — temperature rise, range shifts, phenological mismatch, ocean acidification; signal: mass coral-bleaching events in 1998, 2010 and 2016 on the Great Barrier Reef.

M1 (AO1.1) — driver 4: Pollution — eutrophication, pesticides, microplastics, heavy metals, light/noise pollution; signal: hundreds of coastal "dead zones" worldwide driven by agricultural nitrogen runoff (e.g., Gulf of Mexico hypoxic zone).

M1 (AO1.1) — driver 5: Invasive species — competitive displacement, predation, disease introduction, hybridisation; signal: cane toad expansion across northern Australia and grey squirrel displacement of native red squirrel in the UK.

(b) M1 (AO1.2) — defaunation describes the catastrophic decline in abundance of populations within still-existing species; the WWF Living Planet Index records ~70% mean decline in monitored vertebrate populations since 1970 without a corresponding 70% extinction figure.

A1 (AO3.1a) — therefore species-extinction lists capture only the endpoint of population collapse; many lineages are ecologically extinct (functionally absent from ecosystems) long before they are formally extinct, so risk classifications systematically understate ecosystem-service damage.

(c) A1 (AO3.2a) — drivers interact synergistically: habitat fragmentation blocks the climate-driven range shifts species need to track suitable conditions, so climate change becomes lethal where it would otherwise be tolerable; nitrogen pollution reduces the resilience of habitats already stressed by warming. A single-driver focus misses these synergies — only an integrated cross-driver strategy matches the mechanism of loss.

Total: 8 marks.

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Specimen question modelled on the Edexcel 9BI0 paper format

Question (6 marks): Discuss whether the term "sixth mass extinction" is biologically defensible. Use specific extinction-rate evidence and refer to at least two of the IPBES-identified drivers.

Mark scheme decomposition by AO:

Marking point	AO	Credit-worthy content
1	AO1.1	Defines a mass extinction as an episode in which a substantial fraction of global species (~75% canonical threshold) is lost over a geologically short interval; the five recognised events are end-Ordovician, late Devonian, end-Permian (the largest), end-Triassic, and end-Cretaceous (K-Pg).
2	AO1.2	States that the background extinction rate is approximately 1 species per million species-years; current rates are estimated at 100-1000$\times$× background, comparable in pace to past mass extinctions if sustained.
3	AO2.1	Applies driver evidence: habitat loss (~30% global forest cover lost; ~75% of land surface significantly altered) and climate change (mass coral bleaching 1998/2010/2016; range shifts blocked by fragmentation) are both contributing simultaneously, with direct exploitation (cod collapse 1992) and invasive species (cane toads, Nile perch in Lake Victoria) compounding the loss.
4	AO2.1	Notes the IPBES (2019) finding that ~1 million species are threatened with extinction, alongside the WWF Living Planet Index (~70% mean decline in monitored vertebrate populations since 1970) — the defaunation signal.
5	AO3.1a	Evaluates: the term is biologically defensible on rate grounds (current rates are at or above the historical pace of mass extinctions); it is not yet defensible on cumulative-loss grounds (we are not yet at the 75% canonical threshold); it is uniquely diagnostic in being driven by a single species, distinguishing it from all five prior events in cause if not in form.
6	AO3.2a	Concludes that the term is best read as a trajectory warning: continuation of present trends would deliver a sixth mass extinction within centuries, but the future is policy-conditional — the IPBES "1 million threatened" estimate measures the gap between current trajectory and conservation effort (lesson 9), not an inevitable outcome.

Total: 6 marks split AO1 = 2, AO2 = 2, AO3 = 2. Section B "discuss and evaluate": Edexcel rewards candidates who quantify the rate evidence (AO2), contrast current loss with the canonical 75%-threshold definition of past mass extinctions (AO3), and land an integrated trajectory thesis (AO3) — a recognisable Topic-4 examiner cue.

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Synoptic links