Biodiversity and the number of speciesSpec A4.2.1, A4.2.2
In short
Biodiversity is the variety of life in all its forms, levels and combinations, including ecosystem diversity, species diversity and genetic diversity. Millions of species have been discovered, named and described, but many more remain to be discovered. Evidence from fossils suggests that more species are alive on Earth today than at any time in the past.
Biodiversity is the variety of life in all its forms, levels and combinations. It is usually considered at three levels.
| Level | Meaning | Example |
|---|---|---|
| Ecosystem diversity | The variety of habitats and ecosystems in an area | A coastline with coral reefs, seagrass beds, mangroves and desert |
| Species diversity | The number of different species (richness) and how evenly individuals are spread between them (evenness) | A tropical rainforest has more tree species than a pine plantation |
| Genetic diversity | The variety of alleles within a species or population | The many varieties of rice; the low genetic diversity of cheetahs |
Genetic diversity matters because a population with more alleles is more likely to include individuals that can survive a new disease or a change in conditions.
How many species are there?
About two million species have been discovered, named and described, but there are many more species to be discovered. Most estimates of the total run to several million eukaryote species, and most bacteria and archaea have not been described. Many undescribed species are insects, fungi and organisms of tropical forests and the deep sea.
Evidence from fossils suggests that there are more species alive on Earth today than at any time in the past. Over long periods speciation has outpaced extinction, despite five earlier mass extinctions. The fossil record is incomplete, so past numbers are estimates.
Nature of science: classification is pattern recognition, and the same observations can be classified differently. Splitters recognise more species than lumpers in a taxonomic group, so species counts depend partly on who does the counting.
Written and checked against the IB Biology HL specification · Updated October 2026