The Sixth Extinction: Chapter 8 Summary & Analysis

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.

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.

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.

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.

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.

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.

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 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.

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.

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.”