Islands often contain distinctive ecological conditions that can lead to unusual evolutionary trajectories such as dwarf mammoths and giant rats. In new research, scientists from the German Centre for Integrative Biodiversity Research and colleagues looked across living and extinct species from islands to determine whether these evolutionary oddities were more threatened and found that both dwarf and giant species were more at risk for extinction.
Sardinian dwarf mammoth, Sardinian giant otter, deer, Sardinian dhole and giant pica. Image credit: Peter Schouten, http://studioschouten.com.au.
Although they cover less than 7% of the planet’s surface, islands are hotspots of biodiversity.
Due to their isolation, they 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.
“On the one hand, phyletic giants might provide bigger reward for hunting,” said lead author Dr. Roberto Rozzi, a former postdoctoral researcher at the German Centre for Integrative Biodiversity Research and now curator of paleontology at the ZNS of Martin Luther University Halle-Wittenberg.
“On the other hand, dwarfed species seem to have less deterrence power, facilitating hunting or predation by introduced predators.”
To better understand the relationship between body size evolution and susceptibility to extinction, Dr. Rozzi and co-authors 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.
They combined data on extinction risk, body mass, and body size change for 1,231 living and 350 extinct species of insular mammals from islands and paleoislands worldwide spanning the last 23 million years.
They found that extinctions and extinction risk were highest among island dwarf and giant species.
“Island dwarf and giant species are going to be really naïve to predators, especially any large primate predator, like us, that shows up,” said co-author Dr. Kate Lyons, a researcher at the University of Nebraska.
“So they’re going to be much easier to catch and kill and eat. And because islands are isolated, and there’s no source population for them, it’s also going to be easier for a new predator to drive them to extinction.”
“If you think about what we know from the recorded history of what happened to a lot of these islands when sailors arrived, they would just easily catch and eat animals with no issues.”
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.
“We recorded an abrupt shift in the extinction regime from pre-sapiens to sapiens-dominated island ecosystems,” said senior author Professor Jonathan Chase, a researcher at the German Centre for Integrative Biodiversity Research and the Martin Luther University Halle-Wittenberg.
“Time overlap of insular mammals with Homo sapiens increased their extinction rates more than 10-fold.”
“However, our results at the global level do not rule out the concomitant contribution of environmental drivers such as climate change on local extinctions of island mammals.”
“While it is important to acquire more paleontological field data to further refine extinction chronologies, conservation agendas should, at the same time, give special priority to protecting the most extreme insular giants and dwarfs, many of which are already threatened with extinction.”
The study was published in the journal Science.
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Roberto Rozzi et al. 2023. Dwarfism and gigantism drive human-mediated extinctions on islands. Science 379 (6636): 1054-1059; doi: 10.1126/science.add8606
Source : Breaking Science News