With the most recent studies estimating there are 8.7 million, knowing how many species are on Earth is one of the most important questions for scientists but also one of the most difficult to answer.

A total of 1.3 million species have so far been identified but many more live on the planet.

A British burnet moth. Credit: Philip McErlean (Flickr)

Current estimates range from the whopping one trillion species on the planet to the much lower 5.3 million. A massive difference, which actually starts with the disagreement among biologists on what the term species actually means – with at least 50 definitions to choose from.

A conventional definition says two organisms belong to the same species if they can interbreed and produce fertile offspring. But this has been challenged, as the concept can’t be used to define asexual organisms and also ignores that many living things we consider separate species can interbreed.

At the same time, researchers have so far been unable to count all life forms on the planet. A large number live on places that can’t be accessed by humans, so what scientists do is look for patterns in biodiversity and try to estimate the total number of species.

US entomologist Terry Erwin did one of the first studies, back in 1980. He sprayed pesticides into the canopy of trees, with 1,200 species of beetle falling to the ground. He concluded there are 30 million species on Earth, assuming each tree species had a similar number of beetles that beetles make up about 40% of insects.

But the figure has been repeatedly challenged, with later estimates arriving at figures under 10 million. In 2011, scientists used a technique based on patterns in the number of species at each level of biological classification to arrive at a much lower prediction of about 8.7 million species

“Knowing how many plants and animals there are on the planet is absolutely fundamental,” said Bob May, a zoologist at the University of Oxford, UK. “Without this knowledge, we cannot even begin to answer questions such as how much diversity we can lose while still maintaining the ecosystem services that humanity depends upon.”

Classification and distribution

Despite the disagreement on the number, what’s clear is that there’s a long way to go until completing the catalog of species, with the risk of not discovering all of them before they face extinction.

Scientists have so far named and cataloged 1.3 million species. How many more species there are left to discover is a question that has hovered like a cloud over the heads of experts for centuries.

Researchers constantly find new species, at a rate of roughly 18,000 per year. For example, in Los Angeles experts found 30 new species of scuttle fly living in urban parks, while also in the US more than 1,400 new species of bacteria living in the belly buttons of university students were found.

The main group of species is animals, which represent 76% of all known species, according to the classification system of Margulis and Schwartz. Within animals, arthropods are the group with the most species, with about 1.2 million species – one million of which are insects.

Plants represent 17% of the species studied, with about 292,000 species. These include four large groups: angiosperms (87% of species), gymnosperms (0.3%), ferns (4.3%) and bryophytes (9%).

Drives of change

The fact that many species haven’t been discovered yet is faced with a difficult reality. Human activities are causing a decline in many of them, with several reports now warning over an age of mass extinction of species in the land and the ocean.

The expansion of agriculture is largely to blame, as abusive use of pesticides is causing the death of many inspects that are key for pollination. At the same time, the use of more land is leading to the destruction of large areas of native forest, displacing species from their habitats.

Exotic species also present difficulties for biodiversity. When a species is introduced in an area where it’s not native, it competes for space and resources with native ones. This can harm local species and even displace them, having an invasive behavior. The number of many species has been reduced because of this.

Climate change is also to blame. It alters the habitats and conditions in which species leave, forcing them to move to new locations that are in line with their climate patterns. WWF estimates most species will have to move 1,000 meters per year to keep within their climate zone.

Hunting and exploitation of animals can also play a big role in reducing the number of species, a practice that remains in many countries. The same occurs with the trade of exotic species and collecting or capturing animals with supposedly curative properties, leading to the decline of many species.

Warning signs

The International Union for the Conservation of Nature (IUCN) red list is an annual report on the global conservation status of plant, animal and fungi species and assesses the extinction risk of a species should no conservation action be taken.

According to its most recent update, there are more than 28,000 species threatened around the world. That is a 6% increase from 2018 when 26,840 species said to be threatened.

Species are assigned to one of eight categories of threat based on whether they meet criteria linked to population trend, size and structure, and geographic range. They are listed as critically endangered, endangered or vulnerable.

The new report classified 6,127 species as critically endangered, meaning they are one step away from global extinction. This is up from 5,826 species last year. However, the IUCN says this may be due to greater efforts at assessing species.

In line with these findings, the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) report found in May that a million animal and plant species are threatened with extinction, many within decades.

“This Red List update confirms the findings of the recent IPBES Global Biodiversity Assessment: nature is declining at rates unprecedented in human history,” said Jane Smart, Global Director of the IUCN Biodiversity Conservation Group, in a public statement. “Decisive action is needed at scale to halt this decline.”