From the simplest sketches to the most advanced scientific models, illustrations of mountains pretty much all look the same. Their classic pyramid form, wider at the bottom and narrowing all the way up to the top, has been ingrained in the human mind, and scientists have always assumed that land area in mountain ranges decreases the higher you climb. Until now, that is.
New research published Monday in Nature Climate Change reveals a surprising discovery that not only changes the way we think about mountains but could also have big implications for how we understand, monitor and protect the organisms that call them home.
It turns out mountain ranges don’t just come in the familiar pyramid form — in fact, most of them have a different shape entirely.
Researchers Morgan Tingley and Paul Elsen used satellite data on mountain ranges from around the globe to analyze how the amount of land area changed with increasing elevation. They learned that pyramidal mountain ranges account for just 32 percent of the mountain ranges on Earth. Of the remaining mountain ranges, six percent have an inverse, or upside-down, pyramid form, with land area increasing toward the top; 23 percent have an hourglass shape, being wider and at the bottom and top and pinched in the middle; and 39 percent have a diamond form, with less land area at the top and bottom and more available in the middle.
“I did expect that we’d see some patterns that were not this classic pyramid,” says Elsen, lead author and PhD student in Princeton University’s ecology and evolutionary biology department. In fact, Elsen got interested in conducting the study while doing field research in the Himalayas. He noticed that as he hiked to the tops of the mountains, the land area seemed to increase, rather than decrease, at high elevations. Still, he says, “I had no idea that pyramid mountains would be the exception to the rule.”
It’s hard to tell the true form of a mountain range just by looking at any given mountain peak, since most individual mountains still come to a point at the very top. But mountain ranges are so big, and their topography so complex, that it would be impossible to observe their true shapes just by looking at them. That’s why the researchers had to analyze satellite data, looking at the total surface area in relation to elevation across the whole mountain range, to complete their study. The way land area is distributed on a landscape scale — whether the greatest area lies at the top, bottom or in the middle when you take into account all the slopes, ravines and plateaus that make up the mountains — is what determines a mountain range’s designation as a pyramid, inverse pyramid, diamond or hourglass.
The finding doesn’t just flip our view of mountain topography. More importantly, it changes our understanding of how climate change can affect mountain-dwelling species, the authors say.
Organisms that live on mountains are in a particular pickle when it comes to climate change. These species tend to be highly specialized and do best in particular habitats and narrow temperature ranges. As global temperatures rise, the best way to find cooler spots is to move higher up on the mountain. But in pyramidal mountain ranges, which get narrower toward the top, moving higher also means losing land area. Having less available space can cause populations to shrink and can put them at an increased risk of dying out entirely.
But Elsen and Tingley’s research shows that the pyramid model doesn’t hold true for all, or even most, mountain ranges, meaning space shortages might not always fall where scientists think they do. In hourglass mountains, for example, the most constricted space will be in the middle of the mountain, rather than at the top. On the other hand, species on diamond mountains will see the widest spaces in the middle. And species on inverse pyramids will enjoy increasing land area all the way up to the top of the mountain.
“I think this is critical information that will really inform our understanding of mountain species,” says Robert Guralnick, a biodiversity scientist and curator at the University of Florida’s natural history museum, who was not involved with the study. “The models we’ve been using are typically that mountain ranges are narrowing toward the top.” More realistic models and a better understanding of mountain topography can help conservationists make better decisions when monitoring and managing mountain species, the paper’s authors say.
“This is absolutely an important study for informing our conservation policy,” Elsen says. Knowing where land area is likely to be scarce can help conservationists target the right places and the right species.
In some cases, new knowledge could even indicate that climate change doesn’t threaten a species in quite the way scientists thought. The Himalayan monal, for example, is a colorful bird that lives in the Himalayan mountains, which have the hourglass form. Currently, the bird prefers an elevation that’s right in the middle of the hourglass, says Morgan Tingley, senior author and assistant professor of ecology and evolutionary biology at the University of Connecticut. So space may be pinched for it now, but if warming temperatures force the bird into higher elevations, it will likely enjoy more space as it moves upward.
On the other hand, a bright little bird called the beautiful nuthatch — which is already classified as a vulnerable species by the International Union for Conservation of Nature — lives just below the pinched part of the hourglass. If it were to flee to higher ground, it would lose land area. “This current research is showing that there are potentially optimistic futures for some species, and it’s also highlighting these bottleneck zones,” Elsen says.
The research is also relevant for species that move downslope in response to climate change, chasing the increased precipitation that comes with warmer temperatures. Before now, most scientists might have assumed that any species moving downhill would be able to take advantage of greater and greater land area as it moved along. Now we know that in certain mountain ranges, these species may actually encounter a shortage in space as they move toward the base of the mountain, and their populations may shrink as a result.
Tingley cautions that other factors must also be taken into account when predicting how climate change will affect mountain-dwelling species. Just because more space is available on certain parts of a mountain doesn’t mean the habitat there will be suitable for a species moving into the area. For example, Tingley says, “if you’re a hummingbird, you’re going to need a source of nectar that might be coming from flowers of specific sizes and shapes. So you can’t move up [the mountain] unless the flower moves up, as well.”
Understanding these finer points is important for making accurate predictions about how climate change will affect an organism. Guralnick, the University of Florida curator, says combining the large-scale topographical data presented in Elsen and Tingley’s study with fine-scale data on local mountain landscapes — including information on microclimate, habitat and terrain — can help scientists come up with the best conservation plans for individual species.
The study includes a “graphical representation of every single mountain range” the authors analyzed for conservationists to use in their planning, Elsen says. Guralnick believes the new information should cause a fundamental change in the way scientists create models for the impact of climate change on mountain-dwelling species. “We’ll be drawing different mountain shapes when we make our papers now,” he says.
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