Islands make small animals giant, and giant animals shrink, but these unique species are threatened with extinction
A dwarf elephant the size of a pony once roamed the Mediterranean island of Cyprus. In the Antilles, a giant rat-like rodent weighed more than 180 kg and rivaled the size of the black bear.
These are just two examples of the “island effect,” a rule of evolutionary biology that describes how large species tend to shrink on islands, while small species grow larger. Island dwarfs and giants include miniature hippos, buffaloes and wolves, which have long faced a high risk of extinction. According to a new study published in the journal Science, this risk is intensifying, putting some of Earth’s most unique creatures at risk.
Focusing on island mammals, researchers at the Martin Luther University Museum of Natural History in Halle-Wittenberg (Germany) examined 1,231 extant species and 350 extinct in the last 23 million years. The greatest risk of extinction was observed among species that underwent more extreme changes in body size compared to their continental relatives. The arrival of humans on the islands increased extinction rates more than tenfold.
Some island species threatened today are: the Tamaraw dwarf buffalo from the Philippine island of Mindoro, 21% larger than its closest mainland relative; the spotted deer from the Philippine islands of Panay and Negros, 26% larger than its closest mainland relative; and the Jamaican hutia, a rodent four and a half times larger than its closest mainland relative.
The island of Flores in Indonesia is a remarkable laboratory for the island effect, also called “Foster’s rule,” based on observations by mammalogist J. Bristol Foster in the 1960s. It was home to a dwarf relative of the elephant, giant rats, and a giant stork, as well as a dwarf human species: Homo floresiensis, nicknamed the “Hobbit”, just 106 cm tall. The Hobbit disappeared about 50,000 years ago, shortly after our species, Homo sapiens, arrived on Flores.
The islands are biodiversity hotspots. Although they cover less than 7% of the earth’s surface, they are home to up to 20% of terrestrial species, and close to 50% of them are in danger of extinction. The researchers documented an accelerated uptick in island extinctions, beginning more than 100,000 years ago. Humans, and even our ancestors, are behind this catastrophe.
Our species has played a leading role through hunting, habitat destruction, and the introduction of disease and invasive predators, destabilizing pristine island ecosystems. Even the earlier arrival on the islands of extinct human species such as Homo erectus coincided with a doubling of extinctions.
Dwarfism and gigantism drive human-mediated extinctions on islands
Photo: By Ninjatacoshell – Own work, CC BY-SA 3.0,