New study reveals flowering plants’ evolutionary history at risk

Animals such as the purple frog and the pygmy three-toed sloth already have conservation rankings that assess their evolutionary distinctiveness.

Those rankings take account of how isolated each animal’s branch is on the tree of life and how few close relatives it has.

Scientists have produced such rankings for endangered animals for almost 20 years. Until now, however, flowering plants – all 335,000 known species – had never been included. A new study has changed that, with results that caught even its researchers by surprise.

Counting flowering plant evolutionary history

Researchers at the Royal Botanic Gardens, Kew (RBG Kew) have completed a project that took several years. Dr Félix Forest, the senior research leader in charge, brought together data on every known flowering plant: all 335,497 of them.

Forest and his colleagues investigated a question seldom addressed on such a scale: how much of the flowering plant tree of life is at risk today? Their newly published answer is more than a fifth.

Specifically, 21.2% of flowering plant evolutionary history is vulnerable. That is nearly twice the figure for jawed vertebrates, which stands at around 13%.

A new metric for endangered plants

Species do not all carry the same weight on the tree of life. Some occupy long, isolated branches with few or no close relatives, having diverged from other groups millions of years ago.

Amborella trichopoda, a small shrub native to New Caledonia, separated from every other flowering plant roughly 130 million years ago. If it disappears, its entire branch will be lost.

This is the principle behind EDGE, a metric whose name stands for Evolutionarily Distinct and Globally Endangered. First introduced for mammals in a 2007 paper, it combines a species’ uniqueness on the tree of life with its risk of extinction.

Priorities on the conservation list

The researchers placed 9,945 species at the top of the conservation priority list, representing about 3% of all known flowering plants.

First on the list is Hondurodendron urceolatum, a tree surviving in just one mountain range in Honduras.

The titan arum, also called the corpse flower because of its rotting-meat smell, is among the highest-priority species. The same is true of the Seychelles’ jellyfish tree and several Magnolia species.

“Compiling this crucial information for flowering plants was no small task,” said Forest.

Flowering plants outnumber ray-finned fish, the largest animal group previously scored using this approach, by more than ten to one.

The single largest species ranking

A practical difficulty confronted the researchers from the outset: at the time, only about 20% of flowering plants had been formally assessed for the IUCN Red List.

The team used computer modelling to address the gap. An earlier study led by Kew researchers had developed a system that uses range size, climate and human pressure to predict whether plants that have not been assessed are likely to be threatened.

By feeding those predictions into the EDGE framework, the researchers produced a ranking for every known flowering plant. It is the single largest species ranking ever attempted in conservation.

Plants on the EDGE list

Around 10% of the world’s flowering plants have at least one documented use as food, medicine, fibre or timber. The EDGE list includes several of them.

Vanilla planifolia, the orchid that supplies almost all the world’s vanilla flavouring, is among the threatened lineages. Gomortega keule, the only species in its family, provides edible fruit and timber along a single stretch of Chile’s coast.

“Losing a deep branch of the tree of life means the potential loss of the next breakthrough cancer drug or antibiotic, with no second chance,” said Dr Matilda Brown, co-lead author of the study.

Already extinct in the wild

Of the EDGE species checked against the Flora of the World database, four have the most severe designation given by the IUCN: Extinct in the Wild.

Three are angel’s trumpets, plants cultivated for centuries by Andean communities that no longer exist anywhere in unmanaged natural populations.

The fourth is Franklinia alatamaha, a flowering tree from Georgia last recorded in the wild in 1803. It survives only because early American botanists continued to grow it from seed.

Rethinking plant conservation

Forest and his team found that protecting just 5.9% of all flowering plant species, selected according to their EDGE scores, would cover half of all threatened flowering plant evolutionary history.

That figure changes how conservation can be planned. When budgets are limited and habitats are shrinking, identifying the 5.9% to prioritise makes an otherwise impossible problem manageable.

The results feed directly into the tree of life indicators of the Kunming-Montreal Global Biodiversity Framework, an international agreement adopted in 2022. Until now, those indicators existed only for animals and a small number of non-flowering plants.

Before the study, nobody had measured how much of the flowering plant tree of life was at risk. The figure is greater than for any animal group measured to date.

The team has already identified its next targets: ferns and lycopods. The flowering plant assessment was the first of its kind, and it has changed how plants will be ranked in future.