Professor Iain Stewart – from salt flats to savannah:
It’s one of the most truly extraordinary landscapes on Earth. The Salar De Uyni. The biggest salt flat on the planet, stretching out across the Bolivian altiplano plateau, 3700 metres up in the Andes mountains of South America.
In winter, especially during the snowmelt of late March, water ponds on the Salar’s flat surface, turning the land into a vast natural mirror. But at this altitude, any runoff from the mountain is subject to the cyclical and seasonal evaporation, laying down layer after layer of salt brine.
A stunning white vista made all the more remarkable because this vast expanse of high plateau was originally part of a complex of great lakes at low altitudes, which gradually became uplifted over millions of years as the Andean mountains themselves began to rise.
The upheaval of the Andes has shaped South America’s natural history, not least by creating some of the world’s most rich, unique and biodiverse natural habitats. The continent is a true patchwork of ecological zones: from deserts, flooded plains and savannah, to lofty salt flats, cloud forest and mountain peaks. And at the continent’s heart, a vast rainforest that overflows with life: the Amazon.
That Amazon basin is itself a creation of the towering Andes. Before the Andes, the main rivers on the continent flowed in the opposite direction to today, westward into the Pacific ocean. When the mountain barrier started to rise it diverted rivers to the north, where they flowed out into the Caribbean, creating a huge expanse of wetlands close to the growing mountains.
Later, further uplift blocked the route north and forced the rivers to converge towards the Atlantic. From around 20 million years ago, the resulting wall of high mountains disrupted rain-bearing air masses. With the increased rainfall on its eastern flank, more and more sediment flowed off the Andes in new rivers that drained down into the Amazon basin. And with that flow came nutrients that enriched the Amazon forest and its flora.
With the growing Andes on one hand and the flourishing Amazon on the other, South America saw the development of an astonishing diversity of life.
Many species are still only found on this continent. Species like the xenathrans – armadillos, anteaters, and sloths – found in diverse habitats across South America, not only in the rainforest but also in the savannah-like regions of southern Brazil and Argentina. Or the iconic llama, restricted to the steep Andean peaks and high salty plateau.
Few animals are better suited to the high-altitude mountainous terrain of the Andes than the llama. Unlike other hoofed animals, their feet have two toes. The bottom part of the foot is divided in two and is covered by a tough leathery sole. Because of these flexible pads, llamas have an amazing foothold on rocky and slippery ground. What’s more, they have unique blood that adapts well to the poor oxygen in the high elevations where they live. Llamas have more red blood cells (haemoglobin) per unit volume of blood than any other mammal. So their bodies are super-charged with oxygen.
These clever adaptations make llamas perfectly at home in the Andean high lands. But they didn’t originate in the mountains. In fact, they are not even from South America. Instead, they evolved in the low plains of North America. They are living proof of a dramatic tectonic union of the two continental Americas which fundamentally shaped the destiny of South America and its wildlife.
For it is the llama, perhaps more than any other animal, that highlights the remarkable biological transformation of the South American continent. When their ancestors first appeared in North America, about 40 million years ago, the northern and southern Americas existed as separate continents. Slowly, plate tectonic motions were edging them closer together and, by around 30 million years ago, an intervening archipelago of scattered volcanic islands linked up as stepping stones between the two approaching land masses.
And so it was that, 3 million years ago, that a narrow land bridge emerged and the llamas crossed into South America. Their arrival was part of an epic intermingling of species – the Great American Interchange. In reality, it wasn’t an especially even interchange; roughly half of today’s South American species are derived from North American forms, including cats, rabbits, bears, deer, and foxes. In contrast, only 10% of North American animals are descended from South American – armadillos and opossums being the most successful.
It seems that North American animals were more robust. Perhaps the relative isolation of the South American animals made them more vulnerable when new species encroached on their habitat. Whatever the reason, North American carnivores flooded south to find a wealth of large and lumbering herbivores.
In among the bloody northern invasion, the formation of the Central American isthmus dramatically increased animal and plants both north and south. But it was South America in particular that witnessed a true transformation in its biological wealth. And of course, among the most successful arrivals was that most quintessential of all South American mammals, the llama, now long extinct in the north, and gradually adjusting to the new heady heights of the Andean peaks.