Evidence for and causes of the biodiversity crisisSpec A4.2.5, A4.2.6
In short
Evidence for a biodiversity crisis comes from reliable surveys of biodiversity repeated over time in many habitats, summarised in IPBES reports. Repeated surveys show change in species richness and evenness. The overarching cause is human population growth, together with over-exploitation, urbanization, deforestation and land clearance, pollution, and the spread of pests, diseases and invasive alien species by global transport.
Evidence for a crisis
The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) assesses published evidence on the state of biodiversity. Its 2019 global assessment concluded that around one million animal and plant species are threatened with extinction, and that the current global rate of extinction is already at least tens to hundreds of times higher than the average over the past ten million years.
Such conclusions rest on reliable surveys of biodiversity in a wide range of habitats around the world. One survey is only a snapshot: surveys must be repeated to show change in two measures:
- Species richness
- The number of different species present in an area.
- Species evenness
- How similar the population sizes of the different species are. A community dominated by one species has low evenness.
Repeated surveys are only comparable if the method is standardised: same sites, same season and time of day, same sampling effort and same identification methods.
Percentage change from a repeated survey
A butterfly survey on the same transect recorded 240 individuals in 2010 and 150 in 2020. Calculate the percentage change.
- Change = 150 − 240 = −90
- Percentage change = change ÷ original value × 100
- = −90 ÷ 240 × 100 = −37.5
Answer: −37.5 % (a decrease of 37.5 %)
Surveys involve both expert scientists and citizen scientists. To be verifiable, evidence usually has to come from a published, peer-reviewed source whose methodology can be checked.
| Detail | |
|---|---|
| Benefits | Many observers cover far more sites and years than scientists alone could; it builds public interest in conservation |
| Methodological concerns | Species may be misidentified; effort and methods vary between observers; records cluster where people live |
Causes of the current crisis
Human population growth is the overarching cause. More people need more land, food, water, energy and materials. It acts through these specific causes:
| Cause | How it reduces biodiversity | Example |
|---|---|---|
| Hunting and other over-exploitation | Removes individuals faster than populations can reproduce | Overfishing of cod; hunting of the moa |
| Urbanization | Replaces natural habitat with buildings and roads, and fragments what is left | Coastal development replacing mangroves |
| Deforestation and clearance of land for agriculture | Loss of natural habitat | Dipterocarp forest cleared for oil palm |
| Pollution | Kills organisms or alters habitats | Fertiliser run-off causing eutrophication; plastics in the oceans |
| Spread of pests, diseases and invasive alien species by global transport | New species outcompete, prey on or infect native species with no defences | Chytrid fungus in amphibians; brown tree snakes eliminating forest birds on Guam |
Linking question: how does variation contribute to the stability of ecological communities? Communities with more species and more genetic diversity are more likely to keep functioning when conditions change.
Written and checked against the IB Biology SL specification · Updated October 2026