Islands that were connected 12,000 years ago when sea levels dropped. Species on each island evolved differently, creating a natural experiment in isolation and adaptation.
Marie-Lan Taÿ Pamart, CC BY 4.0, via Wikimedia Commons
A sunken continent
Beneath the rough waters of the Bass Strait lies a submerged landscape known as the Bassian Plain. During the last glacial period, which peaked around 20,000 years ago, vast amounts of water were locked in continental ice sheets. This caused sea levels to drop by as much as 120 meters, exposing a dry, cold land bridge that connected mainland Australia with Tasmania. For thousands of years, this plain was not a narrow corridor but a sprawling ecosystem of grasslands and sparse woodlands.
Around 14,000 years ago, as the ice age ended and the planet warmed, the seas began their steady rise. The Bassian Plain was gradually flooded. By about 8,000 years ago, the process was complete, and Tasmania was an island. The highest points of the former plain—the peaks of ancient granite mountain ranges—remained above the water, forming the islands of the Bass Strait we see today, including King Island and the Furneaux Group. Animal populations that once roamed freely across the plain were now trapped on these new islands, led to for rapid, isolated evolution.
Natural laboratories of change
The isolation of animal populations on the Bass Strait islands created distinct evolutionary pressures. The King Island emu (Dromaius novaehollandiae minor), for instance, evolved into a dwarf subspecies, reaching only about half the size of its mainland relatives before being hunted to extinction in the early 19th century. This shows insular dwarfism, where species shrink in size in response to the limited resources of an island environment. The last captive King Island emus died in Paris in 1822.
Some tiger snake (Notechis scutatus) populations experienced insular gigantism. On islands like Mount Chappell, the snakes evolved to be much larger than their mainland counterparts. This change was driven by their diet; with frogs absent, the snakes adapted to prey on the large, seasonally available chicks of nesting seabirds. A larger gape size was necessary for baby snakes to consume the only available food source, selecting for overall larger bodies over generations.
The common wombat also shows variation. The Flinders Island wombat (Vombatus ursinus ursinus) is a distinct subspecies, smaller than those found in Tasmania and on the Australian mainland. An adult Flinders Island wombat averages around 75 cm in length, compared to 85 cm for the Tasmanian subspecies and up to 1 meter on the mainland.
💡Fun Facts
The King Island emu was about 87 cm (34 in) tall, compared to the mainland emu which can reach up to 190 cm.
Some island tiger snakes are among the largest of their species, weighing up to three times more than mainland snakes.
Aboriginal Tasmanian (Palawa) oral traditions, recorded in the 1830s, accurately recall a time when Tasmania was connected to the mainland by land.
The Furneaux Group of islands, which includes Flinders Island, consists of over 50 islands that were once peaks of a mountain range on the Bassian Plain.
