Lava flows tell story of biodiversity loss

There’s quite a story to be told on the slopes of one of the world’s most active volcanoes, if you’re game to look.

Researchers from the University of Réunion Island were, and after surveying vegetation on more than 600 years of lava flows on Piton de la Fournaise they have a clearer picture of how human colonisation has changed the forests.

Réunion is part of the Mascarene archipelago of the east coast of Africa, which also includes Rodrigues and Mauritius. They were among the last tropical islands to be inhabited by humans; permanent settlement began in 1665.

Before then, Sébastien Albert and colleagues found, Réunion’s forests were dominated by large fleshy-fruited plant species, usually big trees. 

During and after human settlement, when fruit-eating animals (frugivores) such as giant tortoises and flying foxes became extinct, these plants were found to be far less present. By 1800 they were nearly gone.

That’s significant, because large frugivores swallow large fruits and leave the seeds far away from the parent trees. Seed dispersal is important for ecosystem recovery after a major disturbance like a lava flow, so when fruit eating animals are lost, tropical forests cannot fully recover.

The largest remaining native frugivore is the Réunion bulbul, a bird that’s nine times smaller than the flying-fox and 1000 smaller than the giant tortoise.

“The global decline of large vertebrate populations is likely to have dramatic consequences on the regeneration of tropical forests worldwide,” says Albert. “The conservation of plant-animal mutualistic interactions is imperative.”

The study was possible because the high volcanic activity of Piton de la Fournaise creates frequent lava flows. 

By analysing historical data and updating it with modern information like GPS coordinates, the researchers selected 151 vegetation surveys where they could identify the age of the substrate the vegetation was growing on.

Substrate age was grouped into five categories: 1401 to 1665, when frugivores were abundant and diverse before permanent human settlement; 1665 to 1800, when large frugivore populations were drastically reduced; 1800 to 1900, when large frugivores went extinct but small frugivores were still abundant; and 1900 to 1956, when even small frugivore populations had declined.

Although the decline in large fleshy-fruited plant species is mostly shaped by the loss of large frugivores, the researchers warn that there are other potential factors at play, such as seed predation from invasive species or environmental selection against this form of seed dispersal. 

They are now looking to focus more on the processes behind plant dispersal on the island. 

“We are currently monitoring seed dispersal, conducting sowing experiments and regeneration surveys to monitor survival and growth of candidate plant species in which early stages are naturally lacking at large scale in the island forests,” Albert says.

The findings are published in the British Ecological Society’s Journal of Ecology.