How Many Species Exist on Earth Today?
Despite centuries of scientific exploration, we have formally described fewer than 2 million species. Yet this figure almost certainly represents only a fraction of life on Earth.
Estimates for the true total vary considerably:
- Eukaryotes (organisms with a nucleus): somewhere between 2 and 10 million species
- Prokaryotes (bacteria and archaea): likely in the billions, though their classification is complicated by enormous genetic diversity
Why is there so much uncertainty? Several factors make an accurate count extremely difficult:
- Many species live in inaccessible habitats , the deep ocean floor, the canopy of tropical rainforests, or inside other organisms
- Cryptic species exist , organisms that look physically identical but are genetically distinct, making them easy to overlook
- Microbial diversity is vast and still poorly understood; we have barely scratched the surface of bacterial and archaeal diversity
Cryptic species: Species that are morphologically (physically) very similar or even indistinguishable from one another, but which are genetically distinct and do not interbreed. They can only be reliably identified using molecular tools.
Biodiversity hotspot: A region with exceptionally high species richness and endemism that has also lost a significant proportion of its original habitat. Conservation International's widely used definition requires an area to support at least 1,500 endemic vascular plant species and to have lost more than 70% of its original habitat.
DNA barcoding is a powerful modern technique that uses short, standardised gene sequences (like a barcode) to identify species based on genetic differences , even when individuals look identical. This has been especially useful for identifying cryptic species and has dramatically increased species counts in many groups.
Today, the planet's biodiversity is not evenly distributed. Tropical rainforests cover only about 6% of Earth's land surface but are estimated to contain roughly 50% of all species. Coral reefs cover less than 1% of the ocean floor yet support around 25% of all marine species. These are classic examples of biodiversity hotspots.
Biodiversity Through Deep Time: The Fossil Record
To appreciate current biodiversity, we need to compare it with the past. The fossil record , although incomplete , provides our best window into how species diversity has changed over hundreds of millions of years.
Fossil record: The collective body of all known fossils, providing evidence for the history of life on Earth, including the appearance, evolution, and extinction of species over geological time.
Key events in the history of biodiversity:
1. The Cambrian Explosion (~540 million years ago)
This period saw the rapid appearance of most major animal body plans (phyla) in marine environments over a geologically short period. It represented the first dramatic increase in animal diversity, though overall species numbers were still far lower than today.
2. The Five Major Mass Extinctions
Earth has experienced five recognised mass extinction events , periods when a significant proportion of species were lost in a geologically short time:
- Ordovician-Silurian (~443 mya): ~85% of marine species lost (approximately 60–70% of all species)
- Late Devonian (~375 mya): ~75% of species lost
- Permian-Triassic (~252 mya): The largest known extinction , approximately 90% of marine species and ~70% of terrestrial species were eliminated
- Triassic-Jurassic (~201 mya): ~70–75% of species lost
- Cretaceous-Paleogene (~66 mya): Ended the reign of non-avian dinosaurs, opening ecological niches for mammalian diversification
3. Post-Extinction Recovery
Following each mass extinction, surviving lineages diversified to fill vacant ecological niches. However, these recoveries took millions of years.
4. The Modern Era (~66 mya to present)
Animal and plant diversity have been increasing over the past 66 million years. Key drivers include the radiation of flowering plants (angiosperms), insects, and diverse mammalian groups. Based on available fossil evidence, animal and plant diversity appear to be at or near their highest recorded levels , though this claim is difficult to extend to all life given the incompleteness of the fossil record and our limited knowledge of microbial diversity.
