Why Do We See More Species in Tropical Forests? The Mystery May Finally Be Solved

Surveying 2.4 million trees showed that predators may help keep the trees at sustainable levels

A view from within the Tyson Forest Dynamics Plot in Missouri. (Jonathan Myers)
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When Charles Darwin first sailed into the tropics aboard the HMS Beagle in 1835, he was stunned. The 26-year-old naturalist had expected to find the same level of diversity of plants and animals as he had left behind in the higher latitudes of Plymouth, England. Instead, on the balmy Galapagos Islands, he found a multitude of strange and diverse creatures thriving together.

Rowing ashore to explore, Darwin jotted in his notes that the number of different “vegetable and animal” inhabitants on tiny tropical islands was strikingly higher than at other sites along his voyage. He wondered: How was it possible that the tropics seemed to hold so much more diversity than the more northerly forests of Europe? Shouldn't these tightly packed creatures have battled it out to extinction long ago?

Darwin never found out the answer to that particular mystery (after all, he had a lot on his mind), and so the question persisted for another century. Finally, in the early 1970s, two ecologists independently came up with the same hypothesis to explain the mysterious phenomenon—at least with trees. 

Daniel Janzen and Joseph Connell put forth a seemingly counterintuitive explanation. Perhaps, they posited, the astonishing plant diversity we find in tropical forests is enabled by two factors: the presence of “natural enemies” that target specific species and keep population size in check, and the tendency of youngsters of one species to settle far away from their parents, beyond those predators' reach.

Until recently, researchers have only been able to prove that the Janzen-Connell hypothesis holds true in localized studies. The problem was, they lacked access to the kind of global datasets necessary to explain the broader planetary pattern of decreasing diversity from equator to poles. Now, in a new study published last week in the journal Science, researchers show that this hypothesized mechanism is indeed responsible for global trends in forest biodiversity.

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Myers holds a tropical tree seedling in the Amazon Rain Forest in Peru. (Jonathan Myers)

Last year, forest ecologists Jonathan Myers and Joe LaManna traveled to a workshop in Hainan, China focused on analysis of data generated by the Smithsonian’s Forest Global Earth Observatory (ForestGEO), a network of 60 forests across the planet that are exhaustively monitored. Myers and LaManna, both of Washington University in Saint Louis, Missouri, knew that ForestGEO could provide the global dataset they needed to answer the question that has been vexing them and other ecologists since Darwin’s voyage.

“One of the striking differences between temperate and tropics is that all of those 'extra' species are very rare,” says LaManna, a post-doctoral researcher and first author of the new study. Consider that temperate forests can be packed wall to wall with redwood trees, whereas the tropics are dotted with a bevy of unique trees that often exist in isolation from others in their species. “How can those rare species persist in the face of extinction?” asks Myers, a professor of biology and co-author on the study. 

Answering that question required a massive undertaking. The dataset tallied 2.4 million trees from 3,000 species in an exacting fashion to ensure comparability across each forest. More than 50 co-authors from 41 institutions including the Smithsonian then analyzed the data, which spanned 24 ForestGEO plots around the planet. “It was a lot,” says LaManna. “Every stem down to one centimeter in diameter is mapped, measured, tagged and identified.” 

The herculean effort paid off. After analyzing the data, they found a surprising trend: In areas with higher numbers of adult trees, there were fewer young saplings of the same species. This pattern was strikingly more pronounced in the tropics than in the temperate regions they sampled. 

This means that, unlike in higher latitude ecosystems, near the equator trees are less likely to coexist around neighbors in the same family. It’s as if, at some point, the tree parents and their sapling kids unanimously agreed that was time to move out of the basement. Except in a forest, living farther apart doesn't just allow the parent trees to luxuriate in their empty nest. It’s a matter life and death for the species. 
 
“With trees it’s less a direct effect of the parent tree on the offspring,” Myers says. “It’s an indirect effect where the natural enemies that attack the adults also attack the offspring.” These enemies could be pathogens, seed predators or herbivores that target one species. Just as dense human populations in cities enable the rapid spread of communicable diseases, these enemies can rapidly devastate a dense forest of the same species. 

If your saplings settle down farther away, however, it’s less likely that any one enemy will wipe them all out. “You think of enemies as being bad influences on trees, especially ones of low abundance,” LaManna says. “But they can be a strong stabilizing force—[enemies] can actually buffer them and keep them from going extinct.” You might say: With enemies like this, who needs friends?

“It’s changed the way I think about ecology,” Myers says. “The enemy can actually have a beneficial effect in maintaining the rare species in these communities, especially in the tropics.” 

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Herbiverous predators leave behind holey leaves in Madidi, Bolivia. (Jonathan Myers)

The data provides compelling explanation for why we see the global biodiversity patterns we do, says Gary Mittelbach, a forest ecologist and professor of integrative biology at Michigan State University who was not involved in the study. “The fact that they were able to show it on a worldwide basis with standardized methods helps solidify the idea,” says Mittelbach. 

One weakness of the study is that, while it implies a global trend, there are no samples from north of Central Europe or south of Papua New Guinea. “I kind of wish they had more [forests] in Asia and Europe so not all the high latitude ones are in North America,” says Mittelbach. Even with the dearth of samples from high latitudes, however, “I’m still pretty convinced of the pattern,” he says. 

Though the researchers succesfully showed that the trend put forth by Janzen and Connell holds true, the question of what exactly is causing the tropics to be so diverse still remains. 

Myers speculates that the stability of the tropical climate may contribute to its rich biodiversity, compared to the drastic changes that have taken place over geologic time in the higher latitudes. “There’s been a lot more disturbance in the temperate zone” over the past thousands of years, he says. By “disturbance,” Myers means ice sheets that repeatedly bulldozed across North America in Earth’s past. 

The tropics have not endured such disturbances. Researchers attribute the high reproduction and low extinction rates in tropical species of plants and animals to the relatively comfy climate. That’s worked out well for them until now, but forests around the world are changing as a result of more volatile climate patterns. For instance, as higher latitudes become warmer, temperate trees are migrating slowly north.

“There might be a direct or indirect influence of climate in mediating the strength of the biotic interactions between enemies and trees,” Myers says. “Where it’s warmer or wetter you might expect pathogens to have a stronger influence.” 

The global trend these researchers have uncovered illustrates just how much the diversity of biological life on Earth can hinge on small-scale interactions. “This mechanism is a global scale process, and we’re talking about interactions between adults, young and their specialized enemies at the scale of 10 meters,” LaManna says. “That very local-scale interaction is contributing to a pattern of biodiversity across the entire globe.”

About Kyle Frischkorn

Kyle Frischkorn is a graduate student in oceanography at Columbia University, and a 2017 AAAS Mass Media fellow at Smithsonian Magazine. In between being in the lab, on a boat, or in a lab on a boat, he’s written about science for GQ, Lucky Peach, Eater, Scientific American and Atlas Obscura.

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