Researchers have long known that islands are hotspots for species diversity. But, until recently, there weren’t precise figures to quantify the diversity of plant life on islands. A study involving an international team of scientists has filled this gap. The researchers developed a database of vegetation information from more than 3,400 geographical regions worldwide, including about 2,000 islands.
The Conversation Africa chatted with the study’s lead author, Julian Schrader, about the findings on African islands’ plant life, including the fact that Madagascar has the world’s highest percentage of plant species endemic to a single island.
Why did you want to map plant diversity on islands?
Until now we didn’t know much about the global distribution of plants on islands. We didn’t have the answers to questions like: how many plant species are native and endemic to islands worldwide? What is their threat status, and which evolutionary lineages do these species come from? It’s surprising that we didn’t know; many islands are very appealing to researchers and they have always played an important role in ecology and biogeography.
Beyond being inherently interesting questions, these topics are also crucial from scientific and conservation perspectives. Only by understanding what exists can specific and effective conservation strategies be developed.
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Asking these questions and uncovering the answers has been thrilling for me. I still remember the moment when the numbers first appeared on my screen, after we had assembled all the necessary data. I didn’t expect that about 20% of all plants globally were endemic to islands. I was also surprised that most of these – about 44,000 species – were endemic to a single island only, such as Madagascar or New Guinea. These numbers were far higher than expected, given that islands make up only about 6% of terrestrial land.
What makes islands so rich for plant biodiversity?
Some, like New Guinea, Cuba, Borneo and Madagascar, have many endemic species because they are large, tropical and have a lot of different habitats. Others have a high proportion of endemic plants because they are isolated from the mainland or other islands. This causes species to evolve into new species due to limited genetic exchange with other populations, as is the case for Hawaii and La Réunion (an island in the western Indian Ocean about 700km east of Madagascar).
And then there are the fragment islands, of which Madagascar is also an example. These islands have, at some point in the distant past, been connected to the mainland. They then broke off, drifting away and taking their species with them. These species then either evolved in isolation or went extinct on the mainland, making them what scientists call paleo-endemics.
Madagascar ticks all these boxes. That’s what makes it so diverse in sheer species numbers and number of endemic plants.
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You found that 83% of the plants growing on Madagascar are endemic to the island. Is that the highest rate of endemic plants on any island?
Yes, Madagascar tops the list globally in terms of the percentage of plant species endemic to a single island. This is because, as I’ve said, Madagascar is highly species-rich. It is an ancient fragment of a larger continent that drifted away from Africa millions of years ago, carrying with it many species that subsequently went extinct or evolved into new species on the mainland.
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This long isolation has led to very high endemism, which is still evident at higher taxonomic levels, such as genera and families. In fact, of all plant families globally, 17 are entirely endemic to islands, and Madagascar contributes the most.
The islands in the Gulf of Guinea also yielded some interesting results. Which in particular stood out?
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In the Gulf of Guinea, the islands of Bioko, São Tomé and Príncipe host a high diversity of endemic plant species. These islands offer diverse habitats, supporting high niche differentiation for species.
However, I am uncertain about their uniqueness in a global context – São Tomé and Príncipe are classic oceanic islands populated through long-distance dispersal from the mainland. Once species arrived, they could evolve in isolation into new species — a process that generally drives high endemism on oceanic islands.
How does knowing this information help researchers, conservationists and governments?
This work is highly valuable for future research. Having comprehensive data on the global distribution of plants can enhance our understanding of evolutionary drivers and why species occur where they do.
From a conservation perspective, this dataset offers a clearer picture of where threatened species are located, which can help in designing targeted conservation strategies and may even raise international awareness of islands’ global significance for biodiversity preservation. For governments, it’s crucial to understand the importance and natural heritage that they are responsible for protecting.
While it’s hard to predict the direct effect on conservation efforts, I sincerely hope this work will help place islands firmly on international conservation and biodiversity agendas.
