Island dwarfs and giants are more susceptible to extinction than other species, particularly following the arrival of humans to their insular homes, according to a new analysis of island species over millions of years. The findings highlight the vulnerability of some of Earth's most unique species and could be used to inform conservation strategies to preserve them. Although they cover less than 7% of the planet's surface, islands are hotspots of biodiversity. Due to their isolation, islands often contain species that have led unique evolutionary trajectories resulting in peculiar features, including unusually large or small body sizes. For example, islands have hosted dwarf mammoths and giant rodents. However, islands are also known hotspots of extinction – particularly human-mediated extinction – with species that exhibit extreme body size shifts seemingly at greater risk.
To better understand the relationship between body size evolution and susceptibility to extinction, Roberto Rozzi and colleagues evaluated data on extinct and living island dwarf and giant mammal species and their risk and rate of extinction through time, both before and after human arrival. Rozzi et al. combined data on extinction risk, body mass, and body size change for 1231 extant and 350 extinct species of insular mammals from islands and paleo-islands worldwide spanning the last 23 million years. They found that extinctions and extinction risk were highest among island dwarf and giant species. Although the authors show that ongoing biodiversity loss observed on islands is part of an extended island extinction event that began more than 100,000 years ago, the Late Pleistocene/Holocene arrival of humans to distant islands, which began roughly 12,000 years ago, greatly accelerated its pace, increasing extinction rates by more than 10-fold. "Looking forward to the future, we recommend that conservation agendas give special priority to protecting insular giants and dwarfs – the surviving evolutionary marvels of island life," write Rozzi et al.
