The questions from the village council on one of the forested mountains of Luzon went like this: What are you going to do with the animals you catch? How many people from the village will you hire and how much will you pay them? How deep will you dig your latrines?

Lawrence Heaney and his colleagues answered the questions with as much detail as possible. They explained that they wouldn’t be selling the mice and rats they caught, but preserving them for future study. They wanted to know more about the unique mammals of the tropical island nation—the cloud rats and earthworm mice that had intrigued Western biologists for more than a century. (In 1898, the mammalogist Oldfield Thomas described new species discovered on the island as “a proportion of novelty that has perhaps never been equaled in the history of mammal collecting.”) Finally, the council gave their permission for the American and Filipino scientists to trek through the mountains in search of new life forms.

This was 2000, when Luzon—the largest island of the Philippines and home to the nation’s capital, Manila—was known to be the home of 28 native non-flying mammal species. But no one had ever done surveys of the island’s higher mountain elevations. On field surveys over the next 12 years, Heaney and his team doubled the native-species number, discovering 28 new creatures on Luzon’s peaks. Their discoveries have earned an island the size of Indiana the title “most diverse place on earth.”

“This little mountain range right there has more endemic species of mammals than any country in continental Europe,” Heaney, the head of Division of Mammals at Chicago’s Field Museum, said from his office this summer. He was referring to a map in a recently published study that details his work on the mammals of Luzon. He had just returned from a conference in Portugal’s Azores Islands and was preparing for another trip to the Philippines to distribute a textbook on the country’s mammals he’d written with his Filipino research partner, Danny Balete.

Heaney’s discovery of so many new species in the mountains is part of a revolution that’s overturned a previous foundation of biodiversity. For decades, scientists believed lowland forests held the greatest concentration of species living in one area. As elevation increased, they said, biodiversity decreased—a hypothesis based more on lack of research on mountains and several famous studies of the Andes from the 1970s and ’80s. Heaney’s research turns this tenet on its head. Mountains are often more diverse than the lowlands—but until recently, no one had thought to look.

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Around the time Heaney and his colleagues were hiking through the cloud forests of Luzon, Science magazine was celebrating its 125th anniversary. For the occasion they compiled a list of the top 25 unsolved questions in science. Among them: what determines species diversity?

More species of cloud rat
(D. Balete / Field Museum)

Scientists have grappled with this since Charles Darwin published his theory of natural selection. Darwin himself hypothesized about “centers of creation” (places where species originated) and whether wider varieties of life develop more readily in certain conditions, but came to no ready answers. The problem is the complexity of the question. Understanding biodiversity demands knowing everything about a particular habitat: its climate, its geologic history, the interactions between all the species of plants and animals, the differences between taxonomic groups, the evolutionary history of its inhabitants—the list goes on.

“Biodiversity has to come from somewhere,” says the mammalogist Christy McCain, an assistant professor at University of Colorado, Boulder. “But there are hundreds of hypotheses of what drives diversity.”

According to McCain, the main four mechanisms thought to do so are evolutionary history, climate, area, and biotic interaction—or, more simply put, how things have changed over time; the impact of temperature, precipitation, weather; how much space animals have; and their place in the food web. Think of each element as an ingredient in the biodiversity cake. They all play some role in the end product, but no one knows which is the most important, because scientists can’t pull out the discrete elements of ecosystems and compare them in a more static setting, where one element changes while everything else stays the same. Is climate the flour holding the cake together? Or is it area? As McCain points out, “We only have one planet. You can’t replicate!”

That’s where mountains come in. There are mountains all over the world, in different climates with different evolutionary histories and different assemblages of species. But they share one thing in common: height. Scientists can compare the number of species at 1,000 meters to the number of species at 2,000 meters, at 2,500 meters, and so forth, on mountains around the world to test whether a particular elevational range is more conducive to speciation. McCain’s research has found that regardless of geographical location or evolutionary history, 45 percent of the world’s vertebrates are clustered around mid-elevational peaks of diversity, making them more common at elevations in the middle of mountains than anywhere else.

McCain thinks the percentage’s stability might be related to levels of precipitation on the mountains, which means climate, more so than the other three mechanisms, would be a major driver of diversity. But there’s still a lot of work to be done to understand just how big a role climate plays.

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Like Heaney, when McCain began studying mammalian diversity, in the 1990s, everyone believed the most diverse hubs were concentrated in the lowlands. Research on the Andes showed greater numbers of bird and bat species around the lowlands—yet fieldwork was rarely conducted in mountains. “People got a famous study stuck in their heads and forgot everything else, even their own data,” she says, referring to the Andes research. She’s taken the mistake to heart, resisting any overly simplistic rules about where diversity is found, even when it comes to mid-elevational peaks.

“I fight against the idea of people now trying to make the mid-elevational peak the only thing that’s there,” McCain says. She wants researchers to focus more on variability—the anomalies that can’t be explained in the standard model.

One example of such an anomaly came in a 2013 study, which found that harsh environments, like those in northern latitudes, contain more mammal and bird subspecies than other regions—that is, a greater variety within the species that live in harsh environments, even though the number of different species itself was lower. “It is surprising given that most indirect evidence suggests that the tropics, not the temperate regions of the world, are hotbeds for biological diversity,” the authors wrote. The researchers (who included McCain) reviewed 2,365 species of mammals for the study in hopes of understanding whether the tropical regions had greater diversity because the conditions helped produce more species (a phenomenon known as the “cradle of diversity” hypothesis), or simply because fewer species went extinct (the “museum of diversity” hypothesis).

“Any kind of change is a driver of evolution,” says study co-author Carlos Botero, assistant professor of biology at Washington University in St. Louis. “If the environment is really harsh and selection is really strong, that could be an engine for the generation of diversity. But even though the potential for the generation of species appears to be greater in temperate regions, when you subtract the cost from the benefits you see fewer species.”

In other words, harsh climates might promote faster adaptation and evolution—but they might also cause a greater number of extinctions. So in addition to understanding everything else about a given habitat, scientists also need to know how many new species are being created and how many are going extinct if they truly want to understand the source of biodiversity.

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It may seem like an impossible problem, one with too many variables to solve. McCain predicts it will take at least 50 more years to find a definitive answer to the puzzle of biodiversity, and she acknowledges that some skeptics doubt an answer can ever be found. But a better understanding could mean major advances in conservation.

“Knowing about the process [that drives biodiversity] could help in focusing efforts on the most pressing needs,” Botero says. “Which groups are the rarest and most potentially threatened? Those are things we can identify by understanding general principles.”

In the meantime, Heaney will continue his research on the Philippines to advance our knowledge of biodiversity. “If we can go to Luzon and double the number of species of mammals, what does that say about how much we know about the diversity of mammals globally?” he asks. “How many are there out there?” About 6,000 mammals have been formally described; Heaney estimates there are probably 8,000 to 10,000 in total, meaning there are as many as 4,000 science has yet to discover.

To the question of where to look next, Heaney turned his attention to Mindoro, an island in the Philippines one-tenth the size of Luzon. Until now, researchers have believed Luzon is the smallest island that can support an endemic population of non-flying mammals. But Heaney and his colleagues have already found endemic species on Mindoro.

“What’s the lower limit on how big an island has to be to support speciation by mammals?” Heaney says. “I think I’ve got a pretty good idea of what the answer is, and it’s another question: How big is a mountain? You have to have two.”

Two mountains in the middle of an ocean—just enough land to divide one species into two groups, allowing them to follow separate evolutionary trails and provide one more clue for solving biodiversity’s riddle.