Biodiversity hotspots are regions around the world significant for their biodiversity but threatened with destruction. They’re packed with species because they have been ecologically stable for a long period of time, which has allowed evolution to continue undisturbed, according to a new study.

To be considered a hotspot, a region needs to have at least 1,500 vascular plants as endemics, and retain only 30% or less of its natural vegetation. In other words, they need to have a lot of plants that are found only in that area, and be threatened by human activity. There are 36 areas around the world that meet the strict criteria, representing 2.4% of the Earth’s land surface but supporting more than half of the world’s plant species as endemics.

Researchers have long been trying to understand why some areas of the planet are so biologically diverse. These hotspots provide crucial ecosystem services for human life, such as the provision of clean water, pollination, and climate regulation, so their conservation its highly important.

An international group of researchers mapped the distribution of the 9,400 plant species found in the Cape Floristic Region in South Africa. This is a non-tropical heartland of biodiversity that has ten times as many native plant species as the United Kingdom, concentrated in an area smaller than Belgium.

“Our research focused on the incredible diversity of the Cape Floristic Region because the competing theories of stability versus productivity can’t be untangled in the tropics, where both are true. Exceptions often clarify the rule and our research shows that the environmental history of a place is important to its levels of biodiversity,” said Colin Beale, co-author, in a press release.

The team found that the richness in biodiversity in the region can be explained by the lack of major changes in its climate over the past 140,000 years. Other studies had argued that productivity, defined as the energy flow through an ecosystem, explained these hotspots — but this only played a minor role in the Cape Floristic Region.

The authors also highlighted the threat posed by climate change to the biodiversity hotspots and the importance of giving nature the protection it needs to bloom. The impact of a warmer world will be greater on areas that have been stable for a long period of time, they argued.

“Our study shows that the environmental stability of south-western South Africa, in conjunction with the region’s rugged topography, explains diversity gradients in the region. The same hypotheses can explain tropical diversity; there is no need to invoke productivity,” said Richard Cowling, co-author, in a press release.

Biodiversity, usually defined as the variety of all living things in the planet, has been declining at an alarming rate in recent years, mainly due to human activities such as land-use changes, pollution, and climate change. A UN report in 2019 said one million species are threatened with extinction.