Island species evolved to extreme body sizes have higher risk of extinction, study finds

Islands serve as “evolutionary laboratories” and are home to animal species with distinct traits, such as dwarfs that evolved to be much smaller than their mainland counterparts, and giants that evolved to be much larger.

Recently, a team of researchers from the German Center of Integrative Biodiversity Research and Martin Luther University Halle-Wittenberg discovered that species which developed more extreme body sizes compared to their mainland relatives are at a higher risk of extinction than those with less extreme sizes. Their findings, published in Science, also reveal that after the arrival of modern humans, the extinction rates of mammals on islands across the globe have significantly increased.

Despite covering less than 7% of the Earth’s landmass, islands are crucial centers of biodiversity, harboring up to 20% of all terrestrial species on the planet. However, islands also experience a high rate of species extinction, with 50% of today’s IUCN threatened species being indigenous to islands.

Many organisms in island environments undergo remarkable evolutionary changes, including significant modifications of body size, known as gigantism or dwarfism. Typically, large continental species’ relatives tend to become smaller on islands, while small species tend to become larger.

Some remarkable evolutionary wonders, like the dwarf mammoths and hippos that shrank to less than a tenth of their mainland ancestors, and rodents and gymnures that grew over 100 times their original size, have already gone extinct. Others, like the tamaraw of Mindoro and the giant Jamaican hutia, still exist but are threatened with extinction. The tamaraw is a dwarf buffalo standing at around 100cm in shoulder height, while the Jamaican hutia is a rat-like mammal roughly the size of a rabbit.

A recent study led by iDiv and MLU has revealed that these unique features often lead to a higher susceptibility to extinction. “Phyletic giants may offer a larger reward for hunting,” says Dr. Roberto Rozzi, formerly a postdoctoral researcher at iDiv’s synthesis center sDiv and at the Berlin Museum of Natural History, and now Curator of Paleontology at the ZNS of Martin Luther University Halle-Wittenberg. “However, dwarfed species seem to have less protection against hunting or predation by introduced predators.”

Higher extinction risk of extreme dwarfs and giants

The researchers employed data on over 1,200 extant and 350 extinct species of island mammals, both living and fossil, found on 182 islands and paleo-islands worldwide, to quantify how evolution towards dwarfism and gigantism might impact the risk and rate of extinction before and after human arrival.

Their study revealed a previously unknown outcome: species that underwent more extreme shifts in body size, either becoming larger or smaller, were more prone to endangerment or extinction on islands. Comparing the two directions of body size change, the researchers discovered that insular giants were slightly more likely to face extinction than insular dwarfs, though this distinction was only notable when considering extinct species.

Since the period of European expansion, the extinction of dwarfed and giant island mammals has been similar. According to Dr. Roberto Rozzi, this could be attributed to more intense and diverse human pressures such as overexploitation and habitat loss, as well as the introduction of new diseases and invasive predators.

Overlap of human colonization and increased extinction rates of insular mammals

In addition to their examination of living and fossil island mammals, the researchers analyzed the fossil record of global island mammals over the past 23 million years, during the late Cenozoic era. They observed a clear connection between island extinctions on a global scale and the advent of modern humans.

According to Professor Jonathan Chase, the senior author from iDiv and MLU, the overlap in time between insular mammals and Homo sapiens increased the extinction rates of these mammals by over ten times. However, the researchers note that environmental factors like climate change could also have contributed to local extinctions of island mammals.

The study emphasizes the significance of protecting the most extreme insular giants and dwarfs, many of which are already threatened with extinction. While further paleontological data can improve our understanding of extinction chronologies, conservation efforts should prioritize safeguarding these unique and vulnerable species.

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