IB ESS SL3.2 Human Impact on BiodiversityPaper 1 & 2Core idea~13 min read
What Is Driving Species Loss
Species rarely disappear because of one thing. A population gets squeezed by habitat loss, then hunted, then hit by a warming climate, and the last few are finished off by something that would have been survivable on its own. Understanding that stacking is the whole point of this page.
📚 What you need to know
Biodiversity is affected by both direct and indirect human influences.
Direct threats target the organisms themselves: overharvesting, poaching, the illegal pet trade.
Indirect threats change the conditions species need: habitat loss, climate change, pollution, invasive species.
Overharvesting means taking a species faster than it can reproduce, which causes population decline and can end in extinction.
Invasive alien species often have no natural predators, competitors or pathogens in their new home, so their numbers explode.
Most ecosystems face several threats at once, producing cumulative effects that reduce resilience.
Human impact on biodiversity can be investigated in the field using transect surveys.
Direct and indirect threats
The split is not about how serious a threat is. Habitat loss is indirect and it is the biggest driver of species loss worldwide.
Direct threats
Overharvesting and overexploitation
Harvesting a species faster than it can naturally reproduce, so the population declines.
Overfishing of Atlantic cod in the North Sea drove the population to collapse.
Tropical rainforest timber is cut and harvested much faster than reforestation replaces it, despite the forests’ large ecological and economic value.
Continued overexploitation can drive a species all the way to extinction.
This is the natural capital idea from 1.3 with a species in the box. Harvest within the natural income and the stock holds; harvest above it and every year’s yield is smaller than the last. Saying that out loud in an exam links two topics and costs you one sentence.
Poaching
The illegal hunting or capture of wildlife, usually for trade or consumption.
African elephants are poached for their tusks, which has caused serious declines in elephant populations.
If enough individuals are killed, the population becomes too small to survive and the species may go extinct.
Illegal pet trade
Trafficking live animals for the exotic pet market.
Many animals die during capture and transport, so far more are taken than ever reach a buyer.
It often targets exactly the species that can least afford it — rare, slow-breeding animals whose rarity is what makes them valuable.
Indirect threats
Habitat loss and fragmentation
Destruction or fragmentation of natural habitat through deforestation, urbanisation and agricultural expansion — for example clearing Amazon rainforest for cattle ranching.
Terrestrial causes include deforestation, agriculture, inland dams, desertification and pollution.
Aquatic causes include destructive fishing techniques, dredging of wetlands, damage from ships, tourism and pollution.
Once a habitat is destroyed or degraded, species lose the resources and support systems they need to survive.
Fragmentation is not just loss. Cutting one forest into four small patches leaves less total habitat, but it also isolates the populations in each patch. Small isolated populations lose genetic diversity, cannot recolonise after a local disaster, and have more edge exposed to wind, light and predators. The damage is bigger than the area removed.
Climate change
Changes to global climate patterns caused by greenhouse gas emissions.
Effects include habitat disruption, shifts in species distributions, and more frequent extreme weather events.
Melting polar ice caps threaten species such as polar bears, whose hunting habitat is the ice itself.
Species that cannot move or adapt fast enough are squeezed out, especially those already restricted to small ranges.
Pollution
The introduction of harmful substances or contaminants into air, water and soil.
Plastic pollution in the oceans endangers marine species through entanglement and ingestion.
Nutrient pollution from fertiliser causes eutrophication, which can tip a lake into a low-oxygen state and kill most of what lives there.
Invasive alien species
Non-native species introduced into an ecosystem that disrupt native species and ecosystems.
Humans move species between countries and continents, sometimes deliberately and sometimes by accident in ships, cargo and soil.
In the new location, the species often has no natural competitors, predators or pathogens to limit its population growth.
Without those natural checks, its numbers can increase enormously.
It then harms native species through competition for food and space, and through disease.
Japanese knotweed in the UK outcompetes native plants and damages buildings.
🧩 Case study: grey squirrels in the UK
The introduction. Grey squirrels (Sciurus carolinensis) were brought from North America to the UK in the 19th century as ornamental additions to estates.
Competition. They outcompete the native red squirrel (Sciurus vulgaris) for food and habitat.
Disease. They carry squirrelpox virus, which is fatal to red squirrels but harmless to greys — so the greys spread it without paying any cost.
Culling. Some areas run culling programmes to reduce grey numbers and protect the remaining reds.
Forest management. Selective planting and wildlife corridors create conditions that favour reds, which are more arboreal than greys.
Research and monitoring. Continued study of both populations allows the strategy to be adjusted over time.
The disease point is what makes this case study worth learning. A competitor you can sometimes coexist with. A competitor carrying a disease that kills you and not them is a different problem entirely, and it explains why reds vanished so fast.
Threats do not arrive one at a time
Most ecosystems face multiple human impacts simultaneously, and the result is cumulative effects: the negative effects are amplified when different threats act together, reducing ecosystem resilience.
Bleaching is the pressure that finishes the reef here, but it is not really the cause. It arrived at a system that had already been stripped of its ability to absorb anything.
The reef is the standard example. Overfishing by human populations weakens the resilience of a coral reef to coral bleaching caused by climate change, making ecosystem collapse far more likely than either pressure would on its own.
Investigating human impact in the field
🧩 Using a transect survey
Lay a transect line perpendicular to the site of human interference — a path, a car park, a discharge pipe.
Record species and abundance in quadrats at set intervals along the line.
Calculate species diversity at each distance, using Simpson’s index from 3.1.
Plot diversity against distance from the disturbance to see how it changes as you move away.
Alternatively, sample randomly within transects before and after a human activity to compare directly.
Repeat the transect and take a mean, because one line tells you about one line and nothing else.
Worked examples
WE 1
Distinguish direct and indirect threats
Distinguish between direct and indirect threats to biodiversity, giving two examples of each. (4 marks)
Point 1: direct
Direct threats act on the organisms themselves, usually by removing individuals from the population.
Examples
Overharvesting, such as overfishing of cod, and poaching, such as elephants killed for ivory.
Point 2: indirect
Indirect threats change the conditions a species depends on, rather than killing it outright.
Examples
Habitat loss through deforestation, and climate change shifting species distributions.
Direct removes the animals; indirect removes what they neednamed examples are required — “hunting” and “pollution” alone are too vague to score
WE 2
Explain the impact of an invasive species
Explain why an invasive alien species can cause a rapid decline in native species. (4 marks)
Point 1: no natural checks
In its new location the species usually has no natural predators, competitors or pathogens, so nothing limits its population growth.
Point 2: population explosion
Its numbers can increase enormously, so the pressure it puts on native species rises quickly.
Point 3: competition
It outcompetes native species for food, space and habitat — grey squirrels outcompete native red squirrels in the UK.
Point 4: disease
It may also carry disease. Greys carry squirrelpox, which is fatal to reds but does not affect greys.
No checks on the invader, and two ways for it to harm the nativesthe disease route is often forgotten; include it and the answer is much stronger
WE 3
Explain cumulative effects
Explain why a coral reef facing both overfishing and rising sea temperatures is more likely to collapse than one facing only one of these. (3 marks)
Point 1: the first pressure
Overfishing removes species and simplifies the food web, which reduces the resilience of the reef.
Point 2: the second pressure
Rising temperatures cause coral bleaching, which the reef would normally have some ability to recover from.
Point 3: the combination
Because resilience is already reduced, the reef cannot absorb the bleaching, so it is far more likely to cross a tipping point into a new, degraded state.
Cumulative effects: the pressures amplify each otheruse “resilience” and “tipping point” — this question is really 1.2 wearing a reef costume
💡 Exam tips
Learn one named example of every threat. The examiner asks for specifics.
Sort threats into direct and indirect before you start writing.
For invasive species, always mention the missing natural population checks.
Use cumulative effects and link it to reduced resilience.
Habitat loss is the largest driver globally — say so if asked which matters most.
Know that transect surveys are the field method for investigating human disturbance.
⚠ Common mistakes
Calling habitat loss a direct threat. It is indirect, however serious it is.
Saying all non-native species are invasive. Only those that spread and cause harm count.
Treating threats one at a time. Real declines almost always involve several together.
Confusing poaching with hunting. Poaching is specifically illegal.
Forgetting fragmentation. Splitting habitat harms species beyond the area actually lost.
Vague transect answers. Say the line is laid perpendicular to the disturbance.
Up next: Judging Conservation Status — once you know what is pushing species down, the next job is working out which ones are closest to the edge.
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