IB ESS SL & HL3.2 Human Impact on BiodiversityPaper 1 & 2~12 min read
What Is Driving Species Loss
The threats split neatly into two kinds: things we do to a species directly, and things we do to the world around it. Knowing which is which matters, because the exam asks for it — and because real ecosystems almost never face just one.
📘 What you need to know
Biodiversity is affected by both direct and indirect human influences.
Direct threats: overharvesting, poaching, the illegal pet trade.
Invasive species often have no natural competitors, predators or pathogens to limit their growth.
Named case study: grey squirrels outcompeting red squirrels in the UK and carrying squirrelpox.
Most ecosystems face several threats at once, producing cumulative effects that amplify the damage.
Human impact on biodiversity can be investigated using transect surveys.
Direct and indirect threats
Biodiversity is crucial for ecosystem stability, resilience and functioning — and it is being negatively affected by human influences of two distinct kinds.
The test is simple: does the activity kill or remove the organism itself, or does it alter the environment the organism needs?
Direct threats
Threat
What it means
Example
Overharvesting
Harvesting species faster than their natural reproduction, causing population decline. Continued overexploitation can drive a species to extinction
Overfishing of Atlantic cod in the North Sea, leading to population collapse. Tropical rainforests are also overexploited, with trees cut far faster than reforestation replaces them
Poaching
Illegal hunting or capturing of wildlife, often for trade or consumption. If too many individuals are killed, the population becomes too small to survive
Poaching of African elephants for their tusks
Illegal pet trade
Trafficking of live animals for the exotic pet market
Capture of wild animals for private collections
Indirect threats
Threat
What it means
Example
Habitat loss
Destruction or fragmentation of natural habitats. Without their habitat, species lose the support systems and resources they need to survive
Clearing of Amazon rainforest for cattle ranching. Aquatic causes include destructive fishing, dredging of wetlands, ship damage, tourism and pollution; terrestrial causes include inland dams, deforestation, desertification, agriculture and pollution
Climate change
Changing global climate patterns from greenhouse gas emissions, disrupting habitats, shifting species distributions and increasing extreme weather
Melting polar ice caps threatening polar bears
Pollution
Introduction of harmful substances into air, water and soil
Plastic pollution in oceans endangering marine species
Invasive alien species
Non-native species that disrupt native species and ecosystems
Japanese knotweed in the UK, outcompeting native plants and damaging buildings
Why invasive species are so damaging
When humans travel between countries and continents they exchange animal and plant species, whether intentionally or not. These non-native species are often highly problematic because they have no natural competitors, predators or pathogens to limit their population growth. Without those natural checks, their numbers can increase enormously, harming native species through competition and disease.
Case study: the grey squirrel in the UK. Grey squirrels (Sciurus carolinensis) were introduced from North America in the 19th century as ornamental additions to estates. They outcompete native red squirrels (Sciurus vulgaris) for food and habitat, and carry the squirrelpox virus, which is fatal to reds but harmless to greys. Management includes culling programmes, forest management such as selective tree planting and corridors that favour the more arboreal red squirrel, and ongoing research and monitoring.
Cumulative effects
Most ecosystems face multiple human impacts simultaneously, and this produces cumulative effects: the negative effects are amplified when different threats act together, reducing ecosystem resilience.
Overfishing weakens the reef’s resilience to bleaching. Neither threat on its own would necessarily cause collapse.
Cumulative effects are where the higher marks live. A weak answer lists threats separately; a strong one explains how one threat makes a species or ecosystem less able to withstand the next. That is exactly the resilience concept from 1.2 — diversity and storage size determine how much disturbance a system absorbs, and each threat chips away at both.
Measuring the impact
You can investigate the impact of human activity on biodiversity using transect surveys:
Study the change in species diversity along a transect laid perpendicular to a site of human interference, to see how diversity changes as you move away from the disturbance.
Or randomly sample within transects before and after the human activity takes place.
Worked examples
WE 1
Direct and indirect
Distinguish between direct and indirect threats to biodiversity, giving one example of each. (4 marks)
Direct threats
Human activities that remove or kill organisms themselves, such as overharvesting, poaching and the illegal pet trade.
Direct example
Overfishing of Atlantic cod in the North Sea, where fish were harvested faster than they could reproduce, causing population collapse.
Indirect threats
Human activities that alter the environment organisms depend on, such as habitat loss, climate change, pollution and invasive alien species.
Indirect example
Clearing of Amazon rainforest for cattle ranching, which destroys the habitat species need to survive.
Direct removes the organism; indirect removes what it needsa named example for each is what separates 2 marks from 4
WE 2
Invasive species
Using a named example, explain why invasive alien species threaten native biodiversity. (4 marks)
Step 1: why they spread
Non-native species are introduced when humans travel between countries, either intentionally or accidentally.
Step 2: why they thrive
In the new ecosystem they often have no natural competitors, predators or pathogens to limit population growth, so their numbers can increase enormously.
Step 3: the example
Grey squirrels were introduced to the UK from North America in the 19th century as ornamental additions to estates.
Step 4: the impact
They outcompete native red squirrels for food and habitat, and carry the squirrelpox virus, which is fatal to red squirrels but harmless to greys.
No natural checks, so competition and disease overwhelm native speciesthe squirrelpox detail is the strongest single fact in this case study
WE 3
Cumulative effects
Explain what is meant by cumulative effects, using an ecological example. (3 marks)
Step 1: the definition
Most ecosystems face several human impacts at once, and cumulative effects occur when negative effects are amplified because different threats act together, reducing ecosystem resilience.
Step 2: the example
In a coral reef, overfishing removes species that help maintain balance, weakening the reef’s resilience.
Step 3: the combined outcome
That weakened reef is then far less able to recover from coral bleaching caused by climate change, making ecosystem collapse more likely than either threat alone would.
Each threat reduces the system’s ability to withstand the nextlink explicitly to resilience — it ties this to systems thinking in 1.2
💡 Exam tips
Be able to sort any threat into direct or indirect, and justify which.
Learn at least one named example per threat. The syllabus asks for it explicitly.
For invasive species, say they lack natural competitors, predators or pathogens.
Know the grey squirrel case study including squirrelpox and the management strategies.
Use the term cumulative effects and explain the amplification.
Mention transect surveys if asked how impact could be investigated.
⚠ Common mistakes
Classifying habitat loss as a direct threat. It changes the environment, so it is indirect.
Confusing poaching with overharvesting. Poaching is specifically illegal hunting or capture.
Saying invasive species are simply non-native. The harm comes from having no natural checks.
Listing threats without examples. Named examples carry the marks here.
Treating threats as independent. Cumulative effects amplify each other.
Forgetting greys are unaffected by squirrelpox. That asymmetry is why it matters.
Up next: Judging Conservation Status. You know what threatens species. The next question is how anyone decides which ones are actually in trouble, and how urgently.
Want this explained one-to-one?
Book a free session with an experienced IB ESS tutor and get your trickiest topics made simple.
