Just as Earth is home to 1 trillion microbial species, our planet also nurtures more than 390,000 known vascular plant species -- about 21 percent of which are at high risk of extinction, a new report revealed.

Considered as the first assessment of the planet's flora, the study conducted by scientists from Royal Botanic Gardens Kew suggests that plants are threatened by habitat loss, climate change, disease and invasive species.

There is good news, however: Science Director Kathy Willis says thousands of new plant species are being discovered annually. In 2015, there were 2,034 new species on record.

Still, despite that, Willis says they have seen huge changes in land cover, mostly driven by cultural activity. She says the goal is to better comprehend the factors that push the negative changes and to protect more plants from extinction by altering these factors.

Willis says it's important to know how many plant species are there in our planet, their location and their relationship between different species because they are fundamental to our own well-being. Plants provide us with fuel, food and medicine and can even control climate.

Discovering New Species

Willis and her colleagues drew up information from existing plant databases, but they noticed overlap within these records. Some plant species were given different names by different botanists.

Including mosses, algae, liverworts and hornworts, there are approximately 390,900 plant species of which 369,400 are flowering. Willis says this number is just scratching the surface.

"There are thousands out there that we don't know about," says Willis.

In 2015, scientists discovered a tree called Gilbertiodendron maximum in the forests of Gabon in West Africa. The tree can grow up to 45 meters (148 feet) high.

There were also five new species of onion, new species of an insect-eating plant named Drosera magnifica in Brazil.

Invasive Species

The new "State of the World's Plants" report [PDF] also revealed that at least 30,000 plant species are used by humans, but 4,979 species have invaded other countries. These plants cause billions of dollars of damage every year, the report says.

Head Conservationist Colin Clubbe says invasive species are one of the biggest challenges for native biodiversity because they are a driver of species loss.

One example of highly invasive species is the Japanese knotweed, which was introduced in Britain as an ornamental plant during the mid-19th century. This plant species (Reynoutria japonica) costs the government £165 million ($238 million) to control.

Willis says the Japanese knotweed can survive underground and sprout out any moment. But Clubbe remains optimistic.

"Now that we've got this list and this number, it's certainly a bit like know your enemy," says Clubbe.

Being able to track down invasive species will help scientists know what they are dealing with, see similar patterns, find out what makes a "good" invasive species and see how these information could be used to improve management practices, adds Clubbe.

Photo: Andrew Malone | Flickr

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