From Tasmania to Madagascar to New Guinea, islands cover just over five percent of Earth’s land but are home to 31 percent of the planet’s plant species. A recent study indicates that over half of the world’s threatened plants are unique to these islands, facing dangers from habitat destruction, climate change, and invasive species.
A recent study shows that roughly one in three of the global plant species is found on islands, even though they account for just over five percent of the Earth’s land area.
Dr. Julian Schrader from Macquarie University’s School of Natural Sciences led a team of twelve researchers from various countries including Australia, Germany, Spain, the USA, Greece, and Japan to analyze data on more than 304,103 plant species — nearly all known plant species globally — revealing a wealth of island biodiversity.
The researchers discovered that 94,052 species are native to islands, out of which 63,280 are endemic, meaning they can only be found in those specific locations — this constitutes 21 percent of the world’s plant diversity.
This research, published in Nature, marks the first thorough assessment of vascular plants that are native to and endemic on islands worldwide.
Native plants exist naturally on an island, while endemic plants are restricted to that particular island or a group of islands and are not found anywhere else.
Vascular plants, which include trees, shrubs, herbs, ferns, and grasses, represent the majority of plants on Earth and possess a circulatory system, unlike non-vascular plants such as mosses and liverworts.
“This is the first time we have achieved such a comprehensive understanding of plant species distribution globally,” remarks Dr. Schrader. “We can now evaluate the conservation status of some of our most vulnerable plants and develop targeted strategies to safeguard them, like identifying botanical gardens suitable for establishing rescue populations.”
The study highlights that only six percent of islands that host endemic species meet a United Nations objective of protecting 30 percent of land and sea regions by 2030.
Hotbeds of island diversity
The research pinpointed several regions of high plant endemism — areas boasting large numbers of species found nowhere else. Most of these regions are substantial tropical islands characterized by diverse landscapes and prolonged isolation.
Leading the list is Madagascar, which harbors an impressive 9,318 endemic plant species. Following Madagascar are New Guinea (8,793 endemic species), Borneo (5,765), Cuba (2,679), and New Caledonia (2,493).
“Geographical separations and distinct climates and environments, differing from other island groups or mainland areas, foster a significant occurrence of new species development or ‘speciation’,” explains Dr. Schrader.
This isolation has resulted in extraordinary examples of plant evolution; for instance, in Hawaii, 126 lobeliad species can trace their ancestry back to a single colonization event.
Nevertheless, many plants that have adapted to their isolated environments may struggle to compete with introduced species.
Climate change adds another layer of threat, with rising sea levels and an increase in extreme weather events potentially posing serious risks to low-lying islands and their distinctive plant life.
The research team has established a standardized checklist of all known vascular plants on islands, detailing their geographical and phylogenetic distribution as well as their conservation status.
This dataset is essential for monitoring shifts in island plant communities over time and can serve as a guide for prioritizing conservation efforts.
“In French Polynesia, I was in search of one of the rarest plants on Earth, the flowering shrub tiare apetahi (Sclerotheca raiateensis), which has only a few individuals remaining in the wild,” shares Dr. Schrader.
This plant features large, aromatic flowers and holds cultural significance; however, it has been over-harvested and is vulnerable to predation by rats. “No one has managed to cultivate this species in botanical gardens yet — it is at risk of extinction in the near future,” warns Dr. Schrader.
