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.
