The cloud forests of the Andes mountains, bound between the Amazonian lowlands to the west and the peaks of the Andean uplift to the east, harbor worlds upon worlds. Within the mountains' mosaic of high plateaus, deep-cut valleys and steeply climbing slopes, unique ecosystems have flourished side by side for centuries, their equilibrium protected by the rugged terrain and 12,000 years of relatively stable climate. Home to nearly one-sixth of the world's plant species, as well as hundreds of kinds of mammals, birds and amphibians, the Andean cloud forests are one of the most biologically diverse regions on Earth.
They are also among the most vulnerable. Isolation has given rise to a high number of endemic -- or regionally unique -- species with little history of migration, leaving them ill-equipped to respond to the human influences that have crept through the forests in recent decades. Mineral extraction and agricultural deforestation have taken a toll, and incursions into the region show every sign of expanding.
Most alarmingly, warming temperatures have accelerated a process of upslope migration, begun at the close of the last ice age, that threatens to push the region's biodiversity into increased competition, nutrient-poor soil conditions and, finally, thin air, according to forest ecologists working in the region.
Although many South American countries have taken steps to protect Andean ecosystems, their efforts might be insufficient. A new study led by researchers at Duke University has identified and mapped hundreds of endemic plant and animal species across 17,000 miles of east-facing Andean uplift, a section ranging through Bolivia and Peru. It found that only 20 percent of areas with the highest levels of biodiversity are protected by government regulations.
"What our study showed was incredibly scary," said Bruce Young, director of species science at the organization NatureServe and a co-author of the study.
He said that both Bolivia and Peru have taken important steps to protect the forests, but the study "just goes to show that in spite of how much we've done, there's still a lot more to do."
The researchers are concerned that without strong protections that accurately reflect the locations of endemic species, commercial ventures in the area, including oil and gold mining and resource extraction, could disturb and perhaps even wipe out many irreplaceable species. Considering that half of all U.S. prescription drugs include ingredients derived from tropical forest plants, researchers warn that destruction of these biomes poses a significant potential loss for modern medicine.
Dangling by a shoestring
Endemic species -- animals like the reclusive titi monkey or the amber-eyed long-whiskered owlet -- do not distribute themselves evenly across the face of the Andes. The models created by the Duke project depict what ecologists have long known: that mountain species tend to disperse along a horizontal, rather than vertical, axis. Because climate varies with elevation, mountainsides can harbor a gradient of climatic zones. In each of those zones, different species flourish.
"Plants, especially, partition themselves out in temperature gradients, with species literally stacking up the mountainside," said Lee Hannah, a senior fellow with Conservation International who has written extensively about species loss due to climate change. Vertebrates tend to be a little hardier when it comes to resisting temperature variation, though ultimately they follow similar, if somewhat wider, distribution patterns, he said.
This adherence to certain elevations creates horizontal bands of species distribution, also known as "shoestring" distributions, that can extend for thousands of miles horizontally along the mountain slopes but can be as narrow as a few hundred feet from top to bottom.
Not all climatic regions are equally favored by endemic species, however. The models created by the Duke study show that at a certain elevation, species clusters intensify, forming an almost solid band across the mountain. This is because certain climatic conditions suit life better than others, said Young.
"We found that the vast majority of our endemic bird and mammal species cluster at the 2,500- to 3,000-meter elevation range," he said. "Amphibians tend to prefer warmer conditions and stick to the 1,000- to 2,500-meter elevations."
The study notes that the geographic distribution of plants varies widely with taxonomy.
The skirt's hem rises
Scientists estimate that, during the last ice age, many of the species now forming bands along the mountains' slopes were native to the Amazon Basin. As the climate warmed over the past 12,000 years, they migrated upward in search of cooler conditions, lifting like the hem of a skirt raised in curtsy. The rate of change was gradual, and the plants and animals were able to keep relative pace with the increasing temperatures.
Today, that pace has quickened. One team of researchers compared census data over four years and found that, on average, trees were moving upslope at a rate of 2.5 to 3.5 vertical meters a year.
That may seem like a quick trot for species rooted in the ground, but it actually lags behind temperature rise up the mountainsides, said Miles Silman, a professor of biology at Wake Forest University and one of the authors of the study.
"The trees need to move 5 to 7 meters a year to keep up with climate change," he said. If atmospheric carbon levels double, trees will need to shift an average of 800 meters to keep pace with the increase in temperature, he said.
"Nobody knows what will happen with vegetation turnover" -- how many species survive into the future -- "if climate is moving faster than trees can track," he said.
Another Duke University study of bird species found a similar lag in their ability to track climate, with birds moving upward at only about a third the speed that temperatures rose (ClimateWire, Dec. 14, 2011).
If species cannot keep up with temperature rise, researchers worry, they could become stranded in heat levels they have not evolved to tolerate.
At the top of the world
Species that do manage to migrate upslope face an entirely different, but equally concerning, set of challenges.
The area of a mountain diminishes as you move uphill. As species migrate upward, they will likely be crowded into increasing competition, said Conservation International's Lee Hannah. Because of erosion, soil chemistry tends to be poorer the farther up the mountainside you go, he said.
A more immediate concern is that human activity, principally clearing of forests for agriculture, has created a kind of artificial barrier above the tree line.
"We call it the Green Gap -- the place where people exist between the lowland forests and the upland vegetation," he said. "Once the trees reach that point, unless something's changed to allow the area to be reforested, then that's it for your endemic species."
Duke's Silman said not all trees would move in such a uniform manner. "Trees can do other things -- they can shift horizontally, they can form enclaves. It's not purely up-down motion." Many ecologists hold out hope that some species will migrate into sheltered niches where they might withstand climatic effects.
But, he added, "it's hard to imagine that with the incredible diversity you see [in the Andes], there won't be a whole lot of species lost along the way."
Correction: An earlier version of this story incorrectly identified study co-author Young with Duke University. He has no affiliation with the university.
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