Chapter 1: The Sixth Extinction The ancestors of
frogs crawled out of water around 400 million years ago. 250 million years ago, frogs were the earliest representation of what would become the modern amphibian orders.
Amphibians have been on Earth for longer than
mammals or
birds; they were even here before
dinosaurs. Recently, it has been reported that the extinction rate of frogs is increasing. Based on observed extinction rates far beyond expected
background extinction rates, we can predict that an event of a catastrophic nature is headed our way. A decade ago,
Panamanian golden frogs were plentiful in numbers and easy to find around Panama. However, within the past couple of years, the frogs started to disappear. Kolbert states that studies by the
National Zoological Park in
Washington, DC, and a
mycologist at the
University of Maine, have identified the reason for the increased mortality of Panamanian frogs as a type of
Chytrid fungus. However, Chytrid fungi are not naturally found in Panama. This left a puzzling question: how did the
fungus get to Panama? Evidence indicates that humans were instrumental in how the fungus traveled. Kolbert uses the frog-fungus relationship as a symbol of how humans are introducing
invasive species to various environments, where native species would normally have the proper distribution of
alleles for their environment.
Chapter 2: The Mastodon's Molars Kolbert explains how fossils of the American
mastodon (
Mammut americanum) shaped
Georges Cuvier's views on
catastrophism. According to Cuvier, there was no reason the mastodon should have died out. The mastodon was large enough to avoid
predation, had large enough teeth to consume an abrasive diet, and had other
phenotypes that should have increased its chances of survival. Cuvier concluded there must have been sudden and violent catastrophes that caused mass extinctions of viable species. Kolbert uses the mastodon as a symbol for the idea that
catastrophe is an important mechanism of extinction.
Chapter 3: The Original Penguin The
great auk was a large flightless bird that lived in the Northern Hemisphere. It had a large, intricately grooved beak. When the first settlers arrived in
Iceland, the auk population was probably in the millions. However, the settlers found the auks to be “very good and nourishing meat.” They also used their oily bodies for fuel and fish bait, and their feathers for stuffing mattresses. Despite attempts at protecting the species, by 1844, the last auks were killed. Kolbert uses the great auk as a symbol of how human
overexploitation of resources is another important mechanism of extinction.
Chapter 4: The Luck of the Ammonites Kolbert explains that the main cause of the
Cretaceous–Paleogene extinction event was not the impact of the
asteroid itself. It was the dust created by the impact. The debris from the impact incinerated anything in its path. She states that it is impossible to estimate the full extent of the various species that died out due to this catastrophe. However, one class of animals we know did die out because of the effects of the asteroid's impact, are the
ammonites. Kolbert explains that, even though ammonites were 'fit' for their current environment, a single moment can completely change which traits are advantageous and which are lethal.
Chapter 5: Welcome to the Anthropocene Kolbert uses the extinction of
graptolites and other
clades to explain
glaciation as a mechanism for extinction. She then explains how, when
carbon dioxide levels in the air are high, there typically is an accompanying increase in temperatures and
sea level. Right around the time graptolites became extinct, carbon dioxide levels dropped. Temperatures fell and sea levels plummeted. This caused a change in the chemistry of the ocean, which had a devastating impact on life forms. Kolbert states that human activity has transformed between a third and a half of the land surface on the planet. We have dammed most of the major rivers of the world, increased levels of
nitrogen higher than can be fixed naturally by terrestrial ecosystems, used more than half of the world's readily accessible freshwater
run-off, removed more than one-third of the
primary producers of the oceans' coastal waters and changed the composition of the atmosphere by
deforestation and
fossil fuel combustion.
Chapter 6: The Sea Around Us Since the beginning of the
Industrial Revolution, we have seen increasing levels of carbon dioxide in the atmosphere at an alarming rate. Studies show we have added approximately 365 billion tons of it by burning fossil fuels, and an additional 180 billion tons as a result of deforestation. We add another 9 billion tons or so a year, an amount that has been increasing by 6 percent annually. Essentially, we have increased the concentration of carbon dioxide in the air to higher than it has been in the last several million years. Some of this carbon dioxide is being absorbed by our oceans to create
carbonic acid. This lowers the
pH of our ocean and kills much of our marine life. Kolbert uses the drastic decline in life forms around the
Castello Aragonese as a warning sign of what is to come if we continue to increase carbon dioxide levels in the atmosphere.
Chapter 7: Dropping Acid Coral reefs support thousands of species by providing food and protection. Subsequently, many species have co-evolved with corals. Due to
ocean acidification, corals may become extinct by the end of the century. Before the Industrial Revolution, underwater reefs had an
aragonite saturation state between 4 and 5. However, if current
emission intensities remain as they are today, by 2060, there will no longer be a region above 3.5. This will lead to an increase in the energy needed for
calcification. This extra energy that will eventually be expended on calcification is currently vital to corals, as they use it to recover from being eaten away by marine species and battered by waves. Thus, ocean acidification is a mechanism of extinction.
Chapter 8: The Forest and the Trees Global warming is most commonly seen as a threat to cold-loving species. As temperatures rise, the ice at the
North Pole and
South Pole will melt. Any living thing that depends on the ice will be faced with extreme challenges that could ultimately drive them to extinction. Kolbert points out that the poles are not the only places affected by global warming, and that other areas have much higher
latitudinal diversity gradients. She discusses the work of scientists who have used measures of
species-area relationships to model the possible effects of global warming. The extent to which species are mobile and can relocate to new areas, in response to shifting climate conditions, is predicted to be a significant factor in possible species extinction. This has particular importance for
trees and other
plant species. Even more difficult to estimate is the extent to which ecological communities of species will be able to tolerate disruptive changes.
Chapter 9: Islands on Dry Land Kolbert points out how everything in life is interconnected, and discusses the importance of
patch dynamics. Over time, the fragmentation of environmental areas leads to a
decrease in the number of species in an area. This occurs, in part, because the size of such "islands" is too small to support a stable number of species members. Also, smaller populations are more vulnerable to these changing events. In addition, the disconnection of islands makes it more difficult for species to reach and recolonize them. One researcher describes this as "an obstacle course for the dispersal of biodiversity." Kolbert also notes that the habits of many species can be highly specialized to their environment. She explains that one minor change can cause a
domino effect in various ecological systems.
Chapter 10: The New Pangea Kolbert points out that there is an
evolutionary arms race, in which each species must be equipped to defend against their potential predators and need to be more fit than their competition. A species has no defense if it encounters a new
fungus,
virus, or
bacterium. This can be extremely deadly, as it was in the case of the American
bats killed by the
psychrophilic fungus
Geomyces destructans. Another example of this occurred in the 1800s. The
American chestnut was the dominant
deciduous tree in eastern American forests. Then, a fungus (
Cryphonectria parasitica) started to cause
chestnut blight. It was nearly 100 percent lethal. This fungus was unintentionally imported to the US by humans. Kolbert then explains that global trade and travel are creating a virtual "
Pangaea", in which species of all kinds are being redistributed beyond historical geographic barriers. This furthers the first chapter's idea that invasive species are a mechanism of extinction.
Chapter 11: The Rhino Gets an Ultrasound The
Sumatran Rhino was once so abundant in numbers it was considered an
agricultural pest. However, as
Southeast Asia's forests were cut down, the rhino's habitat became fragmented. In the 1900s, the rhino population had shrunk to just a few hundred. A captive breeding program was widely regarded as a failure and resulted in the deaths of several rhinos, and it was decades before a single baby was born. Today, there are only one hundred living Sumatran rhinos. Kolbert uses this rhino species to illustrate
habitat fragmentation as another mechanism of extinction.
Chapter 12: The Madness Gene Europe was home to the
Neanderthals for at least 100,000 years. Then, about 30,000 years ago, the Neanderthals vanished. Fossil records show that modern humans arrived in Europe 40,000 years ago. Within 10,000 years, Neanderthals were bred out. Through
molecular sequencing, scientists have found that there is one to four percent Neanderthal DNA in all non-African humans. This indicates that humans and Neanderthals interbred, and the resulting
hybrids reproduced. The pattern continued until Neanderthals were literally bred out. Kolbert states there is every reason to believe that Neanderthals would still exist if it weren't for
Homo sapiens.
Chapter 13: The Things with Feathers Kolbert concludes with hope in humanity, pointing to various efforts to conserve or preserve species. Whether meaning to or not, we are deciding which evolutionary pathways will be shut off forever, and which can be left open to flourish. ==Sources==