In Peru, Kolbert meets with a scientist named Miles Silman. Silman, a professor at Wake Forest University, is a forest ecologist, and studies the tropical ecosystems of South America. While global warming is often interpreted as being most dangerous for animals in cold climates, global warming poses an equally severe threat to tropical life forms.
Having studied the impact of global warming on the oceans, Kolbert turns to another rich, vibrant ecosystem: the rainforests of South America. Her conclusions about rainforests will be similar to the conclusions she’s drawn in previous chapters (increasing temperatures are threatening biodiversity), which provides more evidence for the idea that we are experiencing the Sixth Extinction.
Imagine that you’re standing at the North Pole. You might decide to venture south, toward Greenland, followed by Quebec. As you walk farther south, you’ll see more and more trees. By the time you get about two thousand miles south, you’ll encounter a huge forest, stretching almost a billion acres. Continue walking south and you’ll encounter the forests of the United States, which have significantly more tree diversity than their Canadian counterparts. Continue south to the equator, and the diversity of the forests further increases—indeed, there are more than a thousand different tree species in Peru, where Silman conducts his research. There are a staggering number of species of frogs, birds, fungi, etc.
The purpose of Kolbert’s tour of the Americas is to convey the relationship between biodiversity and climate. Up to a certain point, the combination of moisture and increasing temperature tends to encourage biodiversity; tropical rainforests, with their moist, warm climates, contain millions of different species, while the arctic is sparser. This adds complexity to Kolbert’s previous observations that increasing temperatures are reducing biodiversity in the oceans.
Why are tropical climates so conducive to biodiversity? One theory suggests that, in tropical areas, “the evolutionary clock ticks faster”—animals reproduce faster, leading to more genetic mutations, which leads to more varieties of species. Another theory argues that there’s more biodiversity in tropical climates because tropical climates have relatively low temperature fluctuation, and, therefore, different zones (the tops of trees, the bases of trees, etc.) can only harbor life with very specific thermal tolerances. As a result, the theory posits, different animals self-stratify based on the temperatures they can withstand, and new species gradually emerge over time. Another theory argues that tropical ecosystems are diverse because they’re so much older than other ecosystems and they have been able to accumulate great diversity over time.
There are many different theories about why rainforests are so conducive to biodiversity. Some of these theories suggest that there is an inherently positive relationship between stable climate and biodiversity. Other theories imply that no such relationship exists. Whatever the precise reason, the biodiversity of rainforests suggests that an ecosystem is a complex structure in which the slightest changes (in temperature, moisture, etc.) would interfere with the overall structure of life.
In the thick forests of Peru, Silman shows Kolbert some of the different species of trees that he has discovered in the last few years. He takes Kolbert to some of the different “levels” of the forest, each with its own unique temperature, humidity, and, therefore, life forms. For more than a decade, Silman has been recording the diversity of life at seventeen different “tree plots.” At each one, Silman and his assistants record the average diameter of a tree, the different life forms they find, and other pieces of information. Silman, working with one of his students, Kenneth Feeley, has found that, on average, the increasing temperatures of the Peruvian forests have driven plants and animals to higher altitudes at a rate of eight feet per year. For example, there is a Peruvian tree of the genus Schefflera, which has an especially short lifespan. Every year, Schefflera trees die off, and every year, their replacements show up at a higher altitude, reflecting the overall changing temperatures of the forests. Other trees, such as those in the genus Ilex, are so resilient that they’ve remained largely “inert” in the last decade.
Silman’s research into rainforests suggests that, if given the option, life forms will try to “hold onto” the ecosystem with which they’re most familiar, even if doing so requires them to migrate somewhere else. Thus, plants and animals have slowly “migrated” northwards through the rainforests in the hopes of finding a new climate with the temperature and humidity to which they’re most accustomed. Furthermore, different species adapt to the changing climate with different degrees of success: some migrate successfully, while others don’t. The further implication of this passage, however, is that eventually, plants and animals will have nowhere to go: they will be unable to find the climates they need.
Every species on the planet has evolved in some capacity to cope with changes in temperature. The world’s average temperature changes over time—for example, 35 million years ago, global temperatures declined precipitously, forming the glaciers of Antarctica. The theory of “ice ages” was first proposed by a student of Cuvier named Louis Agassiz; however, it took another hundred years before scientists knew why the world’s average temperature dropped at different points in time. Most scientists now believe that the gravitational pulls of Jupiter and Saturn alter the “distribution of sunlight” across different latitudes of the planet. When other planets’ gravitational pulls are strong, less sunlight hits the northern latitudes, and snow builds up. The buildup of snow further triggers global carbon dioxide levels to fall, leading to a significant drop in temperature.
While scientists have known about ice ages for more than a century, the explanation for ice ages is very recent. The fact that interplanetary forces influence the temperature of the Earth might suggest that there is a limit to how much humans could change their environment. However, Kolbert will show that, in many ways, humans play an even greater role in determining the average temperature of the Earth than Jupiter and Saturn do. This shows just how powerful a force human activity is on the climate.
During the Pleistocene period of Earth’s history, the planet became significantly cooler. Charles Darwin speculated that, during such periods of global cooling, animals tended to migrate toward the equator in search of the temperatures to which their bodies were calibrated—a theory that contemporary scientists have confirmed. Scientists predict that, during the next century, the average world temperature will increase considerably, and at a faster rate than at any other time in the history of the Earth. The speed at which the temperature will increase poses a significant threat to the world’s life, and it’s not clear which species will be able to migrate north quickly enough to survive.
In principle, there is nothing unusual about climate change—the world’s average temperature has changed many times in the past. However, the speed at which the average temperature is rising is unprecedented in planetary history. As a result, it’s likely that many species won’t have enough time to adapt to their new environments, reproduce, and survive climate change—most species will probably be unable to find a stable climate, and, as a result, will die out.
Silman takes Kolbert on several expeditions to observe the tree plots and gather samples. She describes several significant moments with Silman that reveal his character and values. During one expedition, for example, Silman talked about the different trees of Peru, praising them for being “hilarious” or “clever.” On another occasion, Kolbert woke up to learn that, while she was sleeping the previous night, a group of men selling coca leaves (used mostly for medical purposes, especially as a cure for aches) had walked through Silman’s camp and tripped over his tent. Silman was so angry that he yelled at the coca salesmen—not, he later admitted, a wise decision.
In this passage, Kolbert paints a vivid portrait of her guide, Silman. Silman is an eccentric, who seems to excel at “interacting” with trees—he even gives them compliments—and yet sometimes fails to get along with other human beings (such as the coca leaf salesmen). Like many of the individual human beings Kolbert discusses in the book, Silman genuinely cares about preserving the environment; indeed, he has devoted most of his adult life to preserving the rainforests from destruction.
In ecology, there’s a familiar rule that the greater the size of an area, the more species are likely to live there. There appears to be a strong positive correlation between the two variables, such that the number of species in a region is directly proportional to the square root of the region’s total area. This law is helpful to understanding the impact of humanity on biodiversity. Humans decrease the amount of land available for species to inhabit, and therefore decrease the total number of species, too. Taking this principle into account, scientists have estimated that between 30 and 50 percent of the world’s species will disappear by the year 2050. A more optimistic scenario would be that the world’s species could prove to be resilient by migrating north to find temperatures at which they could survive. However, many species won’t be able to find comparable temperatures. Several years ago, National Geographic published a story repeating the conclusion that half of the world’s species could die out by 2050. Since the story was published, other scientists have challenged the original study’s findings—some for overestimating the decrease in diversity, others for underestimating it.
The gist of the law Kolbert describes here is that when the available area for wild species shrinks at a given rate, the total diversity found in the wild shrinks at an even faster rate (for example, when the available land decreases by twenty-five percent, diversity could decrease by as much as fifty percent). Kolbert acknowledges that it’s very difficult to measure the exact rate at which species will go extinct (the study of global warming is so recent that scientists have yet to get a precise figure). However, having a metric by which to measure the relationship between human land use and extinction makes it clear that humans need to do a better job of preserving wild land if they hope to preserve the world’s biodiversity.
Back in Peru, Silman shows Kolbert a tree species called Alzatea verticillata. The tree has bright green leaves and tiny pale flowers. The species is one of the few in the Peruvian forests that hasn’t “migrated” up the mountain in search of familiar temperatures. Silman’s findings suggest an important point about the temperature changes facing mankind in the near future: some species will be able to adapt and survive, while others won't.
Alzatea verticillata is one of the many species that hasn’t been able to migrate away from the rising temperatures; if the decay of the rainforests continues into the 2050s, the tree will, in all likelihood, go extinct. Emphasizing the effect of climate change on an individual species helps to dramatize the tragedy.
One afternoon, Kolbert, Silman, and Silman’s assistants travel through the forest to a cluster of tourist lodges. There, Kolbert is surprised to find a group of different birds, including tanagers and cock-of-the-rocks. These species, Kolbert notes, are “cold-adapted”—for precisely this reason, they survived the last Ice Age. While it’s true that many species will be able to adapt to the new rising temperatures, many other species will die out—a particularly large number, considering how rapid the upcoming temperature changes will be. Silman goes still farther, arguing that the changes facing the world’s life forms in the next fifty years will be “apocalyptic.”
It’s a mark of the unprecedented nature of the Sixth Extinction that some of the same species that survived the last Ice Age will be unable to survive the next wave of mass-extinction. As Darwin argued, the environment is constantly changing—therefore, traits that were evolutionarily advantageous at one point in time won’t necessarily remain advantageous in the future. The birds that Kolbert observes in this passage will have to make extreme adaptations or face extinction.