By Alister Doyle, Environment Correspondent
Animals such as lions, crocodiles or snakes thrive elsewhere on the planet, but Antarctica's most fearsome land predator is a reddish bug.
The continent is best known for penguins, seals and whales, but all rely on the sea for food, unlike its Lilliputian land-based creatures and plants -- so far almost unaffected by humans.
Scientists are stepping up their study of these tiny creatures in Antarctica for possible early warnings about how climate change may disrupt life around the planet in coming decades.
"Antarctica is strikingly different to other continents in terms of what you find on land," Pete Convey, a biologist at the British Antarctic Survey, said while peering at an apparently barren pile of rocks on the Antarctic Peninsula.
"There are no land mammals, there are no grazing animals like gazelles, no land birds," he told Reuters near the British Rothera Base.
One of the first rocks he picked up had a tiny, reddish mite racing around the surface.
"It's the lion of the ecosystem -- it's the top predator," he said of the Rhagidia mite, about 1 mm (0.04 inch) across. The mites have eight legs and are related to spiders.
And the biggest land animal on the entire continent, which covers more land than the United States, is a flightless midge about 0.5 cm (0.2 inch) long.
Such tiny animals have found ways to live year-round on land and shut down their bodies to survive the deep winter freeze.
The simplicity of the ecosystem means the impact of new threats such as climate change can be more easily assessed.
"There are only two (land) predators within 500 miles of here," Convey said. "It makes it a lot easier to understand the way the ecosystem functions."
"Everywhere people go they take roads, they take pollution, they take farming, they move species around," said David Vaughan, a glaciologist at BAS.
"It's very hard to see how climate change affects a natural ecological system, except somewhere like this," he said of the Rothera area, ringed by mountains and with icebergs crowding the bay.
"The Antarctic Peninsula, because the climate is warming so rapidly, is the one place on the world's surface where you can come to see the effects on the ecology in a pure form," he said.
The peninsula, sticking up toward the southern tip of South America, is the part of the southern hemisphere that has warmed fastest in the past 50 years, apparently because of an increase in temperature stoked by human use of fossil fuels.
Temperatures on the Antarctic Peninsula have risen by 3 Celsius (5.4 Fahrenheit) in the past half century, almost the difference in mean annual temperatures between France's southern city of Nice and Paris.
And the Antarctic ecosystem may already be changing -- with both benefits and possible disruptions. Global warming "is going to make life easier for (tiny creatures on the peninsula), almost certainly," Convey said.
Warmer temperatures would help plants grow, turn parts of the peninsula greener and so benefit the animals that feed on them. But rising temperatures might also dry out the climate, threatening life.
And higher temperatures could make the Antarctic Peninsula more open to invasive species -- such as seeds, insects or spores unwittingly brought by tourists or scientists on their clothing, blown by the wind or stuck to birds.
"More than 50,000 people a year come to Antarctica," Convey said of tourists, scientists and other visitors. "That carries a far greater risk of bringing an alien biological organism into the Antarctic than natural colonization," he said.
Many invasive species will die because of the cold -- the winters are still too cold for rats or mice.
Midget creatures have evolved in Antarctica wherever ground is exposed and there is fresh water in summer -- temperatures around Rothera reach a maximum of about 7 Celsius (44.60F) in summer. It even rained briefly at the weekend.
Rhagidia hunts for springtails, a primitive type of insect that Convey likened to the elephants of Antarctica -- or maybe gazelles since springtails can jump.
The springtails live off vegetation. Sparse patches of green, black or orange lichen dot some rocks. Antarctica also boasts two flowering plants, some tiny worms and countless microbes.
Many of Antarctica's animals have blood proteins that act as a natural anti-freeze. "I can come here in winter and collect them," Convey said.
"They are absolutely stationary ... they are perfectly well capable of surviving months and months and months of minus 10 to minus 20 Celsius" (14.0 to minus 4.00 Fahrenheit)," he said.
-- For Reuters latest environment blogs click on: blogs.reuters.com
(Editing by Charles Dick)