Islands are often thought of as small oases of unusual species that challenge the limits of our imagination. Evolving in greater or lesser isolation from continental ecosystems, island animals appear to defy natural laws, from giant species that could dwarf their mainland counterparts to much smaller ones that appear to have shrunk.
Despite the adaptations they have undergone over thousands of years, animals on islands, “giants” or “dwarfs”, are more vulnerable to the risk of extinction than those that inhabit the continental masses.
In an article published in Science, an international team of scientists reveals that around 75% of the extinctions recorded in the last 500 years took place on pieces of land surrounded by water and warn that almost half of the animal species currently occurring on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species living on islands. And island mammals are, of all taxa, the most vulnerable.
Researchers estimate that island animals that are either 10 times larger or 10 times smaller than their mainland counterparts face at least a 75% greater chance of extinction.
Due to the isolation in which they live, island animals, especially mammals, have not developed a “healthy fear” of predators like humans, making them easy prey. And history teaches us, if we are willing to listen, that the less fearful an animal is, the more likely it is to be hunted to the brink of extinction and even pushed beyond it.
The work resorted to the analysis of data from more than a thousand species of mammals living today and 350 extinct species, from 182 islands that are or were.
The researchers say that the arrival of humans on these islands multiplied by 16 the probability of extinction of the species that lived there in complete isolation.
Kathleen Lyons, from the University of Nebraska, in the United States of America, and one of the authors, comments that while most predators do not cause their prey to disappear – because, according to the laws of ecological balance, when there are more predators, prey decline, in turn leading to predator decline and prey recovery, in a dynamic cycle—humans have a long history of doing just that.
“We change prey constantly. We eat something until it’s gone, or until it’s too hard to catch, and then we eat something else until it’s gone too,” says Lyons. Therefore, assessing the vulnerability of species as unique as those that evolved on the islands is necessary to prevent their eventual extinction due to various pressures (such as climate change and direct human action).