19 March 2023
Biologists who study evolution have always been interested in animals that developed on islands.
In some situations, animals on islands changed over time and came to look very different from the same species that lived on the mainland.
The experts point to animals such as the dwarf elephant that once lived on the Mediterranean island of Cyprus. The animal is now extinct. When it lived, however, it was only the size of a small horse. Elephants that live today in Africa and Asia are much larger than that.
A mounted skeleton of an extinct Sicilian dwarf elephant s seen at Museo Geologico "G. G. Gemmellaro" in Palermo, Italy. Roberto Rozzi via Reuters.
In the West Indies, researchers found a giant rodent that looked like a rat. However, it was about the same size as an American black bear. Rats are, of course, many times smaller than a bear.
Evolutionary experts came to call this phenomenon "the island effect." They used this term to describe the fact that animals who normally have small bodies "upsize" on an island, while the opposite is true for animals who usually have large bodies.
The "island effect" produces odd-sized animals because large animals require more food than small animals. On an island, there is a limited amount of food. As a result, larger animals become smaller over generations in order to survive with lower food intake.
For small animals, there is not as much risk from predators on an island, so they often grow larger.
Recently, researchers released their findings about 1,231 existing animals and 350 extinct ones that represent 23 million years of life. They found that animals on islands were more at risk of extinction compared to their relatives on the mainland. The arrival of human settlers increased the extinction risk for these odd animals.
Roberto Rozzi is a paleoecologist at Martin Luther University Halle-Wittenberg in Germany. He was the lead author of the study, published in the journal Science.
Rozzi said he and the other researchers concluded that the "extinction curve ... has become even steeper in recent decades."
The researchers call islands "biodiversity hot spots." Even though they only make up 7 percent of the Earth's land mass, they account for 20 percent of the land species.
Two island countries, the Philippines and Indonesia, in southeast Asia have a large number of unique animals.
The Philippine island of Mindoro has a buffalo that is only 21 percent of the size of its mainland relatives. The spotted deer on the islands of Panay and Negros are just 26 percent the size of those on the mainland.
Indonesia's island of Flores is also a laboratory for the "island effect," which is sometimes called Foster's rule. J. Bristol Foster was an animal researcher of the 1960s.
Flores was once home to small elephants, giant rats and a kind of giant stork. There was even a very small human species once living on the island called homo floresiensis that was about 106 centimeters tall. That human species was later called "The Hobbit," and it died out about 50,000 years ago.
Katie Lyons co-authored the study. She is a paleoecologist at the University of Nebraska in Lincoln.
She called the island animals "weird and wonderful." However, she noted that many of those animals are already extinct, and of the ones that are still alive, about 50 percent are at risk of dying out.
She and the other researchers said the speed of island extinctions started increasing 100,000 years ago.
They said humans played a large part in extinctions. The report noted humans hurt the ecosystem that supported the unique animals, hunted them, destroyed their living spaces, and brought disease and unwanted invasive species. Even a species that came before humans – homo erectus – hurt the island animals.
Jonathan Chase of the German Centre for Integrative Biodiversity Research also worked on the report. He said the researchers cannot be 100 percent certain all of the extinctions came because of human involvement, because there were other things happening at the same time on the islands.
However, he said, "extinction rates increased dramatically after the arrival of modern humans." He pointed to the elephants on Cyprus as an example and said they were likely overhunted. He said before humans arrived, there may have only been "a few hundred ... and it didn't take long for them to disappear."
I'm Dan Friedell. And I'm Jill Robbins.
Dan Friedell adapted this story for VOA Learning English based on a report by Reuters.
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Words in This Story
evolution –n. the process by which changes in plants and animals happen over time
unique –adj. used to say that something or someone is unlike anything or anyone else
species –n. a group of animals or plants that are similar and can produce young animals or plants : a group of related animals or plants that is smaller than a genus
extinct –adj. no longer existing
odd –adj. happening in a way that is not planned or regular, different from what is expected
phenomenon –n. something unusual that is difficult to explain
curve –n. a curved line on a graph that shows how something changes or is affected by one or more conditions
steep –adj. rising or falling sharply, a line or a road that goes almost straight up
invasive –adj. tending to spread
dramatically –adv. sudden or extreme