A study by University of Queensland and international researchers has found that most of Madagascar's unique present-day fauna may originally have arrived across long distances by air or sea.
Long considered as a biodiversity hotspot, the island of Madagascar is the sole home of many species, most of which are now thought to have arrived after the island was fully isolated from other continents during the last 65 million years (the Cenozoic Era).
To reach the island vertebrates would have rafted, swum, or flown to the island, with the study suggesting that animals relying on swimming and rafting were most successful when winds and ocean currents worked in their favour and that, after the ocean currents reversed, the number of arrivals decreased markedly.
“We believe that flightless animals arrived virtually exclusively from Africa during the early part of the Cenozoic era by rafting and, after the shift in ocean currents, flying species became the dominant immigrants,” according to UQ researcher Karen Samonds.
The study is the first to directly test the hypotheses of rafting and future studies are planned to scour the fossil record for “ghost lineages”: animals that came to Madagascar but then went extinct and were replaced by later arrivals.
“The findings of this study help us understand how islands accumulate biodiversity over time and can be used in modelling the time frame required to readjust following climate change or natural disasters,” Dr Samonds said.
“Rafting as a general phenomenon applies to any landmass isolated by oceanic waters and the same kinds of modeling could be used to investigate other islands.
“Some types of arrivals were not across the ocean (such as marsupials which arose while Australia was still part of Gondwana), but other arrivals to places like New Zealand and Fiji could be investigated by modeling ocean currents and winds in the same way.
“These findings are a step toward explaining the uniqueness of the fauna and the current species diversity of the world's fourth largest island,” she said.
To determine how and when Madagascar's vertebrates arrived, Dr Samonds and colleagues from the USA, Germany, China, and Madagascar combined biogeographic, geophysical, and oceanographic information, to analyse how arrival rates changed over time and relative to area of origin, prevailing ocean currents, and the island's distance from other land masses.
The findings of the study are being published in the Proceedings of the National Academy of Sciences, USA.