Antarctica is the coldest, driest and windiest continent on Earth. About 98 percent of it is covered by ice up to 2.9 miles thick, and temperatures have reached lows of minus 135 degrees. Yet as inhospitable as it is now, it was even more so during past ice ages, when glaciers blanketed virtually the entire landmass along with its offshore waters. Scientists reported this week that in order to survive such difficult times, many species likely depended on patches of ice-free land near volcanoes or on ice caves formed by geothermal steam. From those safe havens, they then apparently re-colonized other parts of Antarctica once the glaciers began to recede.

Despite the obvious challenges of living on Antarctica, certain plants and animals have managed to make a go of it. Millions of birds, for example, breed each year near the coastline, including emperor penguins, Adélie penguins, snow petrels and south polar skuas. There are no terrestrial mammals, reptiles or amphibians. But a number of tiny invertebrates reside there, such as nematode worms, mites, springtails and an eight-legged segmented creature called a tardigrade. Meanwhile, although most of the flora consists of lichens, mosses, terrestrial algae and liverworts, there are also two native flowering plants: a grass and a cushion-forming pearlwort.

Many of these species, including about 60 percent of invertebrates, are found nowhere else on Earth, leading scientists to believe that they have been isolated there for millions of years. Genetic evidence backs this up. Yet scientists remained puzzled about how Antarctic organisms that require ice-free habitat could have survived through long glacial periods, the last of which reached its peak about 20,000 years ago. Now we have an answer. According to a paper published Monday in the Proceedings of the National Academy of Sciences, birds and seals simply flew or swam north, whereas plants, lichens and bugs rode out the weather near volcanoes. “Volcanic steam can melt large ice caves under the glaciers, and it can be tens of degrees warmer in there than outside,” Ceridwen Fraser, a biogeographer at Australian National University and a lead author of the paper, said in a statement. “Caves and warm steam fields would have been great places for species to hang out during ice ages.”

In order to test their theory, the scientists studied nearly 39,000 records of Antarctic species. They selected 33 non-geothermal sites and 10 geothermal sites, all with a radius of about 60 miles, and determined that the areas near volcanoes supported the greatest diversity of life. This was particularly true for mosses and other plants and to a lesser extent for lichens and invertebrates. “The closer you get to volcanoes, the more species you find,” Aleks Terauds, a terrestrial biologist at the Australian Antarctic Division and the other lead author of the paper, said in a statement. “This pattern supports our hypothesis that species have been expanding their ranges and gradually moving out from volcanic areas since the last ice age.”

The scientists asserted that an Antarctic peninsula with rocks heated through radiogenic decay may have served the same function for life as volcanoes, of which there are at least 16 active ones on the continent. They also took note of the existence of small ice-free rocky patches called nunataks. But the organisms there are largely different from those elsewhere, the scientists said, and therefore can’t be the main source of later repopulation movements. A row of valleys near Antarctica’s Ross Sea was also partly ice-free during the last ice age, but very low precipitation appears to have sharply limited the number of species there.

In their paper, the scientists stated their belief that geothermal heat has provided ice age refuges in other areas of the world as well, including for freshwater crabs in southern South America. Meanwhile, they expressed concern about the effects of climate change on Antarctica, and suggested that conservation efforts focus on biodiversity hotspots near volcanoes. “We can learn a lot from looking at the impacts of past climate change,” Fraser said.