The Sixth Extinction: An Unnatural History (2014)

In eastern Peru, accompanied by professor and forest ecologist Miles Silman, Kolbert hikes through Manu National Park, one of Earth’s “great biodiversity ‘hot spots’” (148). Silman primarily studies how forest communities are affected by climate change. To determine how forests may be impacted in the future, he created a series of tree plots, which Kolbert visits with him. The seventeen parcels sit at different elevations and are thus exposed to different temperatures annually.

Silman argues that global warming will have a more significant impact on tropical environments than on any other, because the tropics are where the most species in the world exist. Unlike other ecosystems, which have relatively few species of trees, the tropics, such as those in Central America, have an enormous variety of species. In Silman’s plots alone there are 1,335 distinct tree species, “roughly fifty times as many as in all of Canada’s boreal forest” (152). The wide variety of life forms in tropical areas is explained by the latitudinal diversity gradient (LDG), a measure that indicates that life forms are most abundant at low latitudes and least abundant at the Earth’s poles.

Although the LDG has been established, the reasons it is true are still being investigated by scientists. One possible theory is that the tropics provide an environment where animals can procreate and produce more than they can in other areas around the world. The larger the number of generations, the better the chance of genetic mutation, which leads to better odds for the development of a new species. Another theory suggests that tropical species are sensitive and fussy, and so a tropical environment is the only place where temperatures are stable and less likely to fluctuate. A third argument is that the tropics are ancient and therefore have been afforded far more time to develop a higher number of new species.

While hiking with Silman and his students, Kolbert is introduced to a new genus of tree and walks through the rainforest, astounded at the number of trees surrounding her. In 2003, Silman planted his plots with the plan of returning annually to examine how the trees responded to climate change. He observed a Birnam Wood scenario wherein the trees would start “moving” to higher elevations as the climate warmed. That is, the trees disperse seeds that grow into new trees which would start to appear upslope of their original location. Eventually, Silman’s work showed that the trees were indeed in motion. He found that global warming was “driving the average genus up the mountain at a rate of eight feet per year” (158), and that different types of tree reacted differently to temperature changes.

Kolbert writes that global temperatures have changed in past epochs and periods, hence the advent of ice ages, but that today’s increase in temperature cannot be compared to those earlier changes because of the speed at which it is happening. Global warming is occurring at rates ten times faster than at the end of the last ice age. To continue to survive, “organisms will have to migrate, or otherwise adapt, at least ten times more quickly” (162). That adaptation or migration is unlikely to occur in time to save multiple species from extinction.

A rule of ecology, the “species-area relationship” (SAR), involves the theory that the larger an area sampled, the more diverse the species that will be found. In 2004, a group of scientists decided to test this rule to discover as estimate on possible species extinction within a given area. The findings were disturbing: “between 22 and 31 percent of the species would be ‘committed to extinction’ by 2050” (165). The group ran a scenario where highly mobile species were able to relocate to new areas and, while this method would work for some species, many other species would be exterminated.

In Peru, Silman shows Kolbert a tree, the Alzatea verticillata. This tree is the only one of its genus and also the only species in its family. The tree towers over everything at its extensive height of 5,900 feet, making this tree one that helps structure the ecosystem around it. It may very well be an ecosystem that will eventually become restructured due to global warming. Silman warns that extinction events—or “biotic attrition” (171)—will be devastating to many species should evolution continue to work the way it does. 

Kolbert travels to Reserve 1202, a section of an archipelago of the Amazonian islands. Reserve 1202 is a square-shaped twenty-five acres of untouched rainforest bordered on all sides by scrub. This plot of land is part of an extensive experiment run by the Biological Dynamics of Forest Fragments Project (BDFFP), a group of conservationists and cattlemen who work together to make appropriate decisions regarding the future of the Amazon rainforests. The cattlemen and scientists determine and agree on what parts of the rainforests can be removed to provide expansion for cattle, while scientists get the chance to study the effects of deforestation in a controlled environment.

The 50 million square miles of ice-free land on the planet are used as the baseline for studying human impact on the environment. The Geological Society of America recently posited that human beings have “ ‘directly transformed’ more than half of this land— roughly twenty-seven million square miles” (175) through conversion of land to pastures and crops, mining and logging, and building cities. The remaining square miles are covered by forest, mountains, tundra, or desert. What makes Reserve 1202 one of the most important ecological studies is that it is a virtual replication of the shape of the world.

Kolbert speaks with Mario Cohn-Haft, an American ornithologist who works at the National Institute of Amazonian Research in Manaus. She accompanies him into Reserve 1202 to listen for and view some of the bird species there. According to Cohn-Haft, “[b]y the end of the day you could have heard a hundred and fifty species of birds and only seen ten” (178).

When he started his job, Cohn-Haft was responsible for catching and banding birds, eventually tagging roughly twenty-five thousand birds. In this process, Cohn-Haft realized that in areas where deforestation was occurring, the bird capture rate was raised in the first year. Birds who had been displaced due to deforestation had relocated to the untouched fragments, and “as time went on, both the number and the variety of birds in the fragments started to drop. And then it kept on dropping” (179). This degradation began to emerge in other Amazon species as well.

Kolbert addresses the habitat natural to islands, which are typically less diverse in species due to the isolation factor. However, on land-bridge islands, created by rising sea levels epochs earlier, species continue to die out as opposed to stabilizing at lower levels, as happens on other types of islands. The name for this process is “relaxation,” and ecologists believe it occurs because “life is random” (179). Relaxation relates to the studies done by the BDFFP, whose researchers discovered that some birds would not recolonize after the population had been lost, resulting in local extinctions that become regional, and eventually, worldwide.

While heading back to their camp, Kolbert and Cohn-Haft find a group of army ants, Eciton burchellii. Known for their ferocious appetites, army ants support a variety of other species, including birds who are ant-followers. The ants set in motion a chain of life: the birds eat the ants, the bird droppings are food for butterflies, mites use the ants for transport. In all, three hundred species live in connection with these ants.

Kolbert notes that the estimated number of insects in the tropical forests is incredibly high, with research suggesting at least two million species and maybe as many as seven million total. The number of insects means that any disaster that befalls the tropics can lead to the loss of thousands of species each year, or “roughly fourteen species a day, or one every hundred minutes” (185). The only issue with these numbers is that, after a quarter-century, the numbers established in the original studies didn’t match up with what was truly occurring. Kolbert believes this discrepancy is due to the time it takes for a species to become extinct, hence her reference to “extinction debt.” Extinction debt refers to the difference between the number of species destined for extinction due to climate change and those species that have already disappeared. Another possibility is that habitat lost through deforestation can, over long periods of time, regrow.

Back at the BDDFFP camp, Kolbert meets Tom Lovejoy, the scientist credited with coining the term “biological diversity” and known for his extensive conservation efforts, which have resulted in roughly half the Amazon rainforest receiving legal protection. Lovejoy shares the results of recent studies that show that changes in land use also impact atmospheric conditions and circulation. If deforestation occurs on a large enough scale, the rainforest will disappear, but so would the rain. Species loss has already been recorded in the Amazon, with certain monkeys, birds, and frogs found in fewer numbers in larger land fragments, or having entirely vanished.