Island-inhabiting giants, dwarves more vulnerable to extinction
Human arrival on islands catalyzed disappearance of iconic animal species
Peer-Reviewed PublicationIMAGE: ISLAND-DWELLING MAMMAL SPECIES OFTEN EXPAND OR CONTRACT IN SIZE, BECOMING GIANT OR DWARF VERSIONS OF THEIR MAINLAND COUNTERPARTS. A NEW SCIENCE STUDY FROM A GLOBAL TEAM SHOWS THAT THOSE GIANTS AND DWARVES HAVE FACED EXTREME RISK OF EXTINCTION — AN EXISTENTIAL THREAT EXACERBATED BY THE ARRIVAL OF HUMANS. view more
CREDIT: SCOTT SCHRAGE | UNIVERSITY OF NEBRASKA–LINCOLN
Forget the sci-fi trappings of ray guns, Pym Particles and gamma radiation: For animals both supersized and miniaturized, look no further than islands, where rodents can swell to 100 times their mainland mass and mammoths once shrank from 20,000 pounds to 2,000.
Those same island-dwelling giants and dwarves contend with far greater risks of disappearing from the planet than do other species, says a new study in the journal Science. Yet it’s not so much the size that counts, the researchers concluded, as how much that size varies between mainland and island.
Island inhabitants, even those of standard size, face more than their share of existential peril. Roughly 75% of the documented extinctions over the past 500 years took place on water-encompassed patches of land. About half of the animal species now listed as threatened by the International Union for Conservation of Nature live on islands, too.
But ecologists from the German Centre for Integrative Biodiversity Research (iDiv), Martin Luther University Halle-Wittenberg, the University of Nebraska–Lincoln and elsewhere found that island-dwelling mammal species larger or smaller than their continental counterparts are even more likely to be endangered — or have already gone extinct.
Extinction risks generally rose in tandem with the size disparities between mainland and island species, meaning that the most extreme giants and dwarves were dealt the longest survival odds, the team discovered. Island-inhabiting mammals whose evolution multiplied or divided their mass by at least four were 75%-plus likely to be classified as threatened. Those that evolved to be 10 times larger or smaller than their mainland peers, meanwhile, faced at least a 75% chance of going extinct.
“We think it has to do with the associated ecological changes that go along with the morphological changes on islands,” said Kate Lyons, associate professor of biological sciences at Nebraska. “Islands are generators of evolutionary novelty. You get all sorts of weird things on islands that you don’t get on the mainland.”
Gigantism and dwarfism are notable symptoms of what ecologists call “island syndrome,” which frequently affects animal species — from the enlarged but endangered Komodo dragon to the extinct pygmy mammoth — that either immigrate to islands or originate there. Smaller mammals, like mice, generally encounter fewer predators and, having less reason to hide or flee, may evolve into giant versions of their mainland species or sister species. Larger mammals, including buffalo and hippopotamuses, tend to confront more constraints — less territory on which to forage for vegetation or prey, and smaller quantities of both — that limit their growth and ultimate size.
Species emigrating from a mainland often exhibit another trait: Being unfamiliar with the meat-eaters on their newfound home, they may lack appropriate fear of the neighbors most motivated and best equipped to kill them. The fact that some of the mammal species most prone to expanding or contracting in size also make for unsuspecting prey could help explain why island-confined giants and dwarves are so vulnerable, the researchers said.
“They’re going to be really naïve to predators, especially any large primate predator, like us, that shows up,” Lyons said. “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,” she said, “they would just easily catch and eat animals with no issues.”
Data from 1,231 surviving mammal species, and fossils from 350 extinct ones, allowed Martin Luther’s Roberto Rozzi, iDiv’s Jonathan Chase and the global team to take stock of those very human footprints across 182 current and former islands. For as much danger as giants and dwarves already faced on islands, the arrival of modern humans, or Homo sapiens, multiplied the probability of extinction by 16. That far outweighed even the impacts of earlier, less advanced Homo species, whose appearance coincided with a doubling in extinctions.
Those rises in human-linked extinctions manifested as pulses in the fossil record that together represent a “protracted extinction event” stretching back roughly 100,000 years, when the first pulse occurred. Another emerged about 16,000 years ago, near the end of the last ice age, with a third arising just 2,000 years ago. That latest pulse yielded an extinction rate about 88 times higher than that of the first.
“The reason they’re pulsed like that is because Homo sapiens got to different islands at different times,” said Lyons, whose prior research has linked the extinction of large mammals with human encroachment. “It’s similar to how we got to different continents at different times — except that for islands, it took us much longer to get to some of them, especially the really remote ones.”
The pulses also help illustrate differences in how humans and other predators alter the food webs of ecosystems — differences that can lead not just to the thinning but the snipping of threads that make up those webs. Most predators, Lyons said, will not drive their prey to extinction. When the population of prey plummets due to hunting, predators have less to eat and eventually see their own numbers drop. That allows the prey population to rebound, with predators following suit, and so on.
“Humans (historically) don’t do that,” she said. “We switch prey constantly. We eat something until it’s gone, or until it’s hard to catch, and then we eat something else until it’s gone. But we don’t stop eating the thing that we were first eating. If we come across it, we’re going to continue eating it, so the pressure on that population is still there.”
Efforts to prevent the further disappearance of species might benefit from incorporating the study’s findings, Lyons said. Current conservation policies do prioritize so-called endemic species that, by inhabiting only one small part of the world — often an island — are more vulnerable to extinction. Many conservationists also triage species according to genetic diversity, so that those featuring more distinct blueprints receive more attention and resources.
“So they do tend to look at various axes of diversity that they want to try to preserve. But they don’t take into account what this study shows,” Lyons said, “which is that the species that get onto islands, and either dwarf or get giant, are at particular risk.”
Rozzi, Chase and Lyons authored the study with Mark Lomolino, from the State University of New York; Alexandra van der Geer of the Naturalis Biodiversity Center in the Netherlands; Daniele Silvestro, from Switzerland’s University of Fribourg; Pere Bover, from Spain’s University of Zaragoza; Josep Alcover, from Spain’s Mediterranean Institute for Advanced Studies; Ana Benítez-López of the Spanish National Research Council; Cheng-Hsiu Tsai of National Taiwan University; Masaki Fujita, from Japan’s National Museum of Nature and Science; Mugino Kubo, from The University of Tokyo; Janine Ochoa, from the University of the Philippines; Matthew Scarborough, from the University of Cape Town; Samuel Turvey, from the Zoological Society of London; and Alexander Zizka, from the Philipps University of Marburg in Germany.
JOURNAL
Science
ARTICLE PUBLICATION DATE
9-Mar-2023
Insular dwarfs and giants more likely to go extinct
Higher extinction rates of insular mammals after arrival of modern human
Peer-Reviewed PublicationIMAGE: ILLUSTRATION OF SARDINIAN DWARF MAMMOTH, SARDINIAN GIANT OTTER, DEER, SARDINIAN DHOLE AND GIANT PICA view more
CREDIT: PETER SCHOUTEN, HTTP://STUDIOSCHOUTEN.COM.AU/
Leipzig/Halle. Islands are “laboratories of evolution” and home to animal species with many unique features, including dwarfs that evolved to very small sizes compared to their mainland relatives, and giants that evolved to large sizes. A team of researchers from the German Centre of Integrative Biodiversity Research (iDiv) and Martin Luther University Halle-Wittenberg (MLU) has now found that species that evolved to more extreme body sizes compared to their mainland relatives have a higher risk of extinction than those that evolved to less extreme sizes. Their study, which was published in Science, also shows that extinction rates of mammals on islands worldwide increased significantly after the arrival of modern humans.
Islands are hotspots for biodiversity – they cover less than 7% of the Earth’s land area, but account for up to 20% of all terrestrial species on the planet. However, islands are also hotspots for species extinction as 50% of today’s IUCN threatened species are native to islands.
In response to the unique characteristics of island environments, many organisms undergo remarkable evolutionary changes, among the most notable of which include extreme modifications of body size. This phenomenon is known as gigantism or dwarfism – in general, relatives of large continental species tend to become smaller on islands and small species tend to become larger. Some of these are already extinct evolutionary marvels such as dwarf mammoths and hippos that shrunk to less than one-tenth the size of their mainland ancestors, and rodents and gymnures of unusual size that increased by over 100-fold. These also include dwarf and giant species currently threatened with extinction, such as the tamaraw of Mindoro (Bubalus mindorensis), a dwarf buffalo with a shoulder height of approximately 100 cm, and the giant Jamaican hutia (Geocapromys brownii), a rat-like mammal about the size of a rabbit.
A team of researchers led by iDiv and MLU now confirmed that evolution towards these features frequently goes hand in hand with increased susceptibility to extinctions. “On the one hand, phyletic giants might provide bigger reward for hunting”, explains Dr Roberto Rozzi, former postdoctoral researcher at iDiv’s synthesis centre sDiv and at the Berlin Museum of Natural History, and now Curator of Palaeontology 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.”
Higher extinction risk of extreme dwarfs and giants
To quantify how evolution towards dwarfism and gigantism may have affected the risk and rate of extinction (before and after human arrival), the researchers used data on fossil and living island mammals including over 1,200 extant and 350 extinct species of insular mammals on 182 islands and paleo-islands (formerly isolated landmasses that are now part of the mainland areas) worldwide.
Their findings indicate a previously unknown result that those species that underwent more extreme body size shifts, either larger or smaller, were more likely to be endangered or to go extinct on islands. Comparison between the two directions of body size change showed that insular giant species have a slightly higher extinction risk than insular dwarfs. However, this difference was only significant when extinct species were included. Since the European expansion around the globe, extinctions have similarly affected dwarfed and giant insular mammals. “This likely reflects the impact of more intense and multifaceted human pressures, such as overexploitation and accelerated habitat loss, but also introductions of novel diseases and invasive predators”, says Dr Roberto Rozzi.
Overlap of human colonization and increased extinction rates of insular mammals
The researchers also analyzed the global fossil record of mammals on islands over the last 23 million years (late Cenozoic) and found a clear correlation between island extinctions at a global level and the arrival of modern humans. “We recorded an abrupt shift in the extinction regime from pre-sapiens to sapiens-dominated island ecosystems. Time overlap of insular mammals with H. 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”, says senior author Prof Jonathan Chase from iDiv and MLU. “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.”
Kalibasib, the world’s last captive tamaraw, died in 2020
CREDIT
Gab Mejia, https://gabmejia.com/
JOURNAL
Science
METHOD OF RESEARCH
Data/statistical analysis
SUBJECT OF RESEARCH
Animals
ARTICLE TITLE
Dwarfism and gigantism drive human-mediated extinctions on islands
ARTICLE PUBLICATION DATE
10-Mar-2023
Island dwarfs and giants are disproportionality prone to human-mediated extinctions
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.
JOURNAL
Science
ARTICLE TITLE
Dwarfism and gigantism drive human-mediated extinctions on islands
ARTICLE PUBLICATION DATE
10-Mar-2023
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