IB Biology SLProtecting BiodiversityPaper 1 & 2Core idea~11 min read
Loss of Biodiversity
Saying “biodiversity is falling” is easy. Proving it is hard. You need surveys across many habitats, repeated over years, counting both how many species there are and how many of each — and then someone has to pull it all together. This page is about that evidence, and about what is causing the loss.
📚 What you need to know
Trustworthy evidence needs many surveys, across a wide range of habitats, repeated over time, measuring both richness and evenness, and analysed by experts.
Citizen scientists are members of the public who help collect data. They increase quantity but can reduce quality.
IPBES gathers information, recommends policies and communicates findings on global biodiversity.
IUCN is a network of organisations and experts that gathers knowledge and proposes environmental policy.
The IUCN red list ranks species by extinction risk, from least concern to extinct. More than 150,000 species have been assessed and more than 42,000 are listed as threatened.
The main underlying cause of the crisis is the growth of the human population, now over 8 billion.
NOS: to be verifiable, evidence should come from a peer-reviewed published source so the method can be checked.
What makes biodiversity evidence trustworthy
You cannot count every organism on Earth, so conclusions come from sampling. That means the quality of the sampling decides whether the conclusion is worth anything.
🧩 The five things good evidence needs
Enough surveys. Many reliable biodiversity surveys, not one or two.
A wide range of habitats. Otherwise you only learn about the habitats that were easy to reach.
Repeat data over time. A single snapshot cannot show a trend. You need the same sites surveyed again and again.
Both richness and evenness. Species counts alone hide a community collapsing towards one dominant species.
Expert analysis. Raw data means nothing until someone qualified interprets it.
If a question ever asks you to criticise a biodiversity study, run down that list. Almost every weak study fails at least two of the five, and each failure is usually worth a mark.
Citizen scientists: the trade-off
Gathering enough data at national or international scale is beyond what professional scientists can manage alone. So organisations recruit citizen scientists — ordinary members of the public who record what they see.
✓ WHAT THEY ADD
Far more data points than professionals could collect
Coverage of many more locations
Surveys repeated more often
Cheap, so long-term monitoring is realistic
✗ WHAT THEY COST
Usually untrained, so identification may be wrong
Data may not be valid or consistent
Effort is uneven — more records near towns
A trade-off between quality and quantity
How the problem is managed: good experimental design (clear instructions, simple recording forms, photo verification) and high-quality data analysis that can spot and filter unreliable records.
The organisations you need to name
Body
Full name
What it does
IPBES
Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services
Gathers information, recommends policies to governments and communicates findings. Its 2019 report summarised research and set out policy recommendations.
IUCN
International Union for the Conservation of Nature
A network of organisations and individual experts. Gathers knowledge of the natural world, proposes environmental policy, and compiles the red list.
The IUCN red list
The red list is a list of the world’s most threatened species, compiled by scientists and scientific organisations worldwide and used by conservation groups to decide where to act. Species are placed in categories according to how close they are to extinction.
Extinct in the wild is the one worth pausing on: individuals survive only in captivity, so the species exists but no longer plays any part in an ecosystem.
Two extra labels: a species is DD (data deficient) when there is not enough information to place it in a category, and NE (not evaluated) when nobody has assessed it yet. Neither means the species is safe — it means we do not know.
NOS: why published, peer-reviewed sources matter
Evidence is only verifiable if someone else can check how it was produced. Publishing in a peer-reviewed journal means other scientists have examined the methodology before it appeared, and that the method is available for anyone to scrutinise or repeat.
This matters especially for biodiversity work, because so much of the raw data comes from many different sources, including untrained volunteers. Peer review is the step that decides whether the conclusions drawn from that mixed pile of data are sound.
The causes of biodiversity loss
There is wide agreement among scientists that a biodiversity crisis is underway, because of the rate at which extinctions are happening. Almost every named cause traces back to one root.
Population growth is not itself a cause of extinction. It is what scales up every other cause, which is why it sits in the middle rather than on a branch.
Human population growth
There are now more than 8 billion people, and the total is still rising.
Birth rates are falling in many countries, but people are also living longer, so births still outnumber deaths overall.
Projections generally show the population continuing to grow towards the year 2100 before levelling off.
The six branches, with examples
Overexploitation. Taking more than can be replaced — overfishing reducing fish populations, hunting for ivory, whale oil or fur.
Urbanisation and habitat loss. Building towns removes habitat directly, roads fragment what remains, and farming is pushed outwards into new land.
Deforestation and agriculture. Land is cleared for crops, timber, mining and quarrying. Farmland often becomes monoculture with very low biodiversity, and soil micro-organisms are lost.
Pollution. Fertilisers and organic waste wash off farmland into rivers and lakes; microplastics now turn up even in remote environments.
Invasive species. Humans move species around the world, deliberately or by accident. Examples include grey squirrels and signal crayfish in the UK, cane toads in Australia and rats in the Galapagos Islands.
Disease and pests. Pathogens spread further as people and goods travel and as climate change widens their range — the chytrid fungus is thought to have wiped out around 90 amphibian species. Meanwhile monocultures with fewer natural predators let pest species boom.
🤔 Why biodiversity loss makes pest problems worse
It sounds circular, and it is — that is the point. In a diverse ecosystem, a pest species has plenty of predators and parasites keeping its numbers down, and its food plants are scattered among many other species. Strip the diversity out and you remove the predators and hand the pest a field of nothing but its favourite food. Numbers explode, more pesticide gets used, and biodiversity falls further.
Worked examples
WORKED EXAMPLE
A study surveyed one woodland once and concluded that national biodiversity is stable. Suggest three reasons this conclusion is unreliable. [3]
Reason 1: one siteA single habitat cannot represent a whole country. Surveys are needed across a wide range of habitats.Reason 2: one point in timeA single survey gives a snapshot. Repeat data over a period of time is needed to show any trend.Reason 3: what was measuredSpecies richness alone can stay constant while evenness collapses, so both must be assessedInsufficient sample size or lack of expert analysis would also be accepted.
WORKED EXAMPLE
Discuss the use of citizen scientists in gathering biodiversity data. [4]
Advantage 1: scaleMembers of the public can collect far more data, over more sites and more often, than professional scientists alone.Advantage 2: coverageThis makes national and international pictures of change possible, which is otherwise impractical.Disadvantage: reliabilityCitizen scientists are often untrained, so data may not be valid — there is a trade-off between quality and quantity.How it is managedGood experimental design and high-quality data analysis reduce the effect of unreliable records
WORKED EXAMPLE
A species is moved from “vulnerable” to “critically endangered” on the IUCN red list. State what this means and suggest two human activities that could have caused it. [3]
Step 1: what the change meansIts risk of extinction has increased; it is now in the highest threatened category before extinct in the wildStep 2: causes — pick any twoHabitat loss through deforestation or urbanisation; overexploitation through hunting or overfishing; pollution; introduction of an invasive species; spread of disease.Always give an example alongside the cause. “Overexploitation, e.g. overfishing” scores better than “overexploitation”.
💡 Exam tip
Learn the red list codes in order: LC, NT, VU, EN, CR, EW, EX, plus DD and NE.
Know what IPBES and IUCN stand for and one sentence each on what they do.
For “is this evidence reliable?” questions, use the five-point list: enough surveys, wide range of habitats, repeated over time, richness and evenness, expert analysis.
On citizen scientists, always give the trade-off — more data, lower reliability — and how it is managed.
When listing causes of biodiversity loss, pair each one with a named example.
For NOS questions, mention peer review and the fact that it lets the methodology be checked.
⚠ Common mix-up
Thinking “threatened” is one category. It covers vulnerable, endangered and critically endangered.
Confusing extinct in the wild with extinct. EW species still survive in captivity.
Reading “data deficient” as “safe”. It means nobody knows, which is a different problem.
Swapping IPBES and IUCN. IUCN compiles the red list; IPBES produces assessment reports and policy advice.
Dismissing citizen science entirely. The exam wants the balanced view, not a one-sided answer.
Blaming population size alone. Population growth is the root, but the mark is for naming specific mechanisms like habitat loss or overexploitation.
Up next: Preserving Biodiversity — what actually works, from national parks and rewilding to seed banks and frozen tissue.
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