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What an Active Volcano Reveals About Missing Trees

Science
A view of the Piton de la Fournaise crater. Malavika Vyawahare

By Malavika Vyawahare

Giant tortoises and flying foxes once roamed La Réunion, a volcanic island off the eastern coast of Africa. Then humans arrived and decided to stay. Within 150 years of their appearance, large fruit-eating animals like the giant tortoises (Cylindraspis indica) and flying foxes (Pteropus niger), a type of bat, were wiped off the face of La Réunion.


This speck of land in the western Indian Ocean, an overseas department of France, was one of the last corners of the planet to be colonized by humans. It is also home to the Piton de la Fournaise, one of the most active volcanoes in the world. It was recently in the throes of another eruption, having erupted almost 240 times since 1650.

The scars from these eruptions are now helping scientists uncover the lasting effects of permanent human settlement on life on the island, including the loss of large-fruited trees like Sideroxylon borbonicum, known locally as bois de fer de Bourbon, and Labourdonnaisia calophylloides, or bois de natte à petites feuilles.

The fates of flora and fauna are linked. Animals and birds are important to plants; they help to disperse seeds, transporting them near and far. Which seeds and how far they get scattered depend on the kind of creatures that get their paws or beaks on the seeds. The largest fruit eater inhabiting the island today is the Réunion bulbul (Hypsipetes borbonicus), a bird a thousand times smaller than the now extinct giant tortoise. The bulbul can't feed on fruits that the tortoise could eat.

Not many would consider living in the shadow of an active volcano good fortune. For Sébastien Albert and his colleagues at the University of La Réunion, however, the frequency of eruptions at Piton de la Fournaise has proven providential. They recently published a paper in the Journal of Ecology describing the loss of large-fruited trees by creating a timeline over several centuries. Because the time of each eruption is known, it is possible to see how the plant community changed before and after human colonization.

Albert described the island as a "paradise" before humans established themselves there around the mid-17th century. The forests were dotted with towering trees that bore large, fleshy fruits enclosing chunky seeds. Seeds come in all sizes; the biggest in the world is the genitalia-shaped seed of the coco de mer (Lodoicea maldivica) found on Seychelles, an archipelago north of La Réunion. It can grow 30 centimeters (1 foot) across and weigh more than 15 kilograms (33 pounds). At the other end of the spectrum, some orchids produce seeds that are a fraction of a millimeter across, finer than a grain of sand.

The Réunion bulbul can only consume seeds smaller than 13 mm, or about half an inch. A larger species of bird native to La Réunion, the Mascarene parrot (Mascarinus mascarin), went extinct before the turn of the 19th century. So did the Réunion fruit pigeon (Nesoenas duboisi).

Throughout the Mascarenes — the Indian Ocean islands of Mauritius, Rodrigues (which is today part of Mauritius), and La Réunion — this is a familiar tale. The extinction of the dodo (Raphus cucullatus) in Mauritius is emblematic of creatures lost to human exploitation.

Overhunting, habitat loss and invasive predators contributed to the animals' disappearance. Giant tortoises were hunted by European sailors for their meat even before human habitation began. Their eggs fell prey to invasive species like pigs, cats and rats. Henri du Quesne, a French naval officer, wrote of the tortoises: "Their Flesh is very delicate; the Fat better than Butter or the best Oil, for all sorts of Sawces." Another account from 1671 noted that it was not possible to go six steps without coming upon one of these friendly giants. By 1800, they were all but gone.

The difficult part is linking their disappearance to impacts on tree diversity. "There is evidence for declines in large-seeded species that rely on large vertebrates for the dispersal of their seeds. Examples include Africa, Peru, and Brazil," said Elizabeth Wandrag, a plant ecologist at the University of New England, Australia, who was not involved in the study.

"It is not always possible to explicitly link the loss of large frugivores, and wholesale losses of species — many tree species that rely on large frugivores for dispersal are still present in forests that have lost those dispersers but have instead become more aggregated and/or show lower survival," Wandrag told Mongabay in an email.

Which is where the lava flows come in. Lava flow from a volcanic eruption is a major disturbance for any ecosystem; it tears through forests, burning everything in its path. It presents a kind of clean slate (literally) to see how plants recolonize. For existing forests, entangling the effects and even pinpointing the collapse of a species may not be as simple.

By studying how plant communities recovered after each eruption, the researchers realized that large-seeded species were not fully recovering because the fauna around them had changed since the previous eruption. "We analyzed 151 vegetation surveys on lava flows dated between 1401 AD and 1956 AD," the authors write in the paper. Tracking the changing vegetation over five centuries "has helped to pinpoint the important role that these lost animals played for these forests." The scientists say this is because when the animals that were best suited to disperse their seeds disappeared, the trees also started to fade away from the landscape.

"It was nice that the findings of this study mirrored some of those we have seen in our research on Guam without frugivores, some tree species simply aren't colonizing newly disturbed locations," Wandrag said. "In this case, the much longer history of defaunation, long-term chronosequence of plant communities, and botanical records allowed the authors to link that pattern to the resulting adult tree community."

Reposted with permission from Mongabay.

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