Half-Earth: Our Planet’s Fight for Life

Wilson explains in detail how to calculate amount of biodiversity and argues that “the best unit to use by far is the species” (20). The author prefers this unit of analysis over ecosystems and genes, neither of which allows researchers to quantify the number of living organisms.

In the mid-1700s, Carl Linnaeus, a Swedish naturalist and explorer, was the first to create a system for naming, ranking, and classifying organisms according to their characteristics and relationships with one another. Linnaeus introduced a binomial system based on the combination of two Latin names. The first denotes the genus, and the second denotes the species. Genus is a taxonomical rank that comes above species and comprises a group of related species. For example, the binomial species name for wolf is Canis lupus. Scientists all over the world use this taxonomic system, which enables them to group species that are related to one another into genealogical trees. These trees provide the evolutionary lineage of modern organisms from common ancestors.

Even with this classification system, Wilson emphasizes that Earth “remains a little-known planet” because we do not know the actual number of species alive today (21). According to Wilson, this is partly because scientists’ work of describing and analyzing each new species takes a significant amount of time. The most publicly and scientifically well-known group of animals is the vertebrates, or animals with a backbone, which include mammals, birds, reptiles, amphibians, and fishes. Over 50,000 species of vertebrates are currently identified. In contrast, invertebrates, or animals without a backbone, represent the majority of animal species on Earth. These include insects, crustaceans, and earthworms, and they number over one million species. Neither of these two categories includes lower plants (algae, mosses, and fungi), flowering plants, microorganisms, or bacteria, which include millions of species. Wilson suggests that scientists have only discovered around 20% of “Earth’s biodiversity at the species level” (23). It will take until the 23rd century to complete the global census.

This chapter examines the human role in the decline of rhinoceroses. This decline began during the last Ice Age, when hunters killed off the woolly rhinoceros. Human-induced extinction of rhinos did not stop in the Ice Age. Rather, it continues today, and there are now only 27,000 individual rhinos left (down from one million a century ago). There are currently only five rhino species, all of which are endangered. Many of the last individuals of these species live in captivity. It is difficult to breed rhinos in captivity, so births are rare.

Wilson attributes the decline of rhinos in the wild to sport hunting, habitat destruction, and poaching. Powdered rhino horns are a key ingredient in traditional Chinese medicine. According to Wilson, the demand for powdered rhino horn was “the crushing blow” for rhinos (30). Substantial population increases in China and Vietnam have increased the demand for rhino horns, as attested by the fact that the price per gram is equivalent to that of gold. For the author, the rhinos’ extinction is ironic because “their horn has no more medicinal value than a human fingernail” (30).

The story of the rhinos highlights the true extent of what the planet is losing because humans are obliterating species for trivial reasons. Wilson notes that their story signifies “millions of years of evolutionary glory [coming] to an end” (31).

In this chapter, Wilson addresses “the menace of invasive species” and the role humans play in moving them to non-native areas (37). Wilson utilizes three examples to illustrate the “apocalyptic force” of invasive species. First, the “deadly chytrid fungus” (35), which has spread via freshwater aquariums as part of the global pet trade, has decimated frogs and salamanders, two major groups of amphibians. Similarly, imported shrubs native to Central America and Mexico have destroyed two-thirds of Tahiti’s native forests. Thirdly, two-thirds of the Pacific Islands’ land birds have disappeared, representing around 10% of global bird species. These extinctions began as soon as humans colonized these islands 3,500 years ago. The land birds’ evolution on islands made them particularly vulnerable. Most of these birds did not evolve with mammalian predators, and they are tame because of this. Some have also become flightless and are thus easy prey.

Wilson articulates that among the greatest threats to protecting global biodiversity are scholars who believe that invasive species “will form ‘novel ecosystems’ that replace the natural ecosystems wiped out by us and our hitchhiker companion species” (37). The economic costs of invasive species, Wilson reminds us, are astronomical. For example, by 2005, the United States spent $137 billion annually to try to combat invasive species.

The author also discusses the vulnerability of freshwater species to extinction, due to human demand for clean fresh water. Fresh water is a limited resource, and therefore humans are in direct competition with both plant and animal species that also depend on this resource. Dams, while important to local economies, cause great damage to freshwater species through “aquatic habitat destruction” (42). Specifically, dams block traditional upstream and downstream fish migration routes, alter the depth of water upstream, increase stilling, and increase pollution. Wilson identifies several species that are particularly at risk, such as sturgeons, salmon, and mollusks. He discusses 19 species of mollusks that have gone extinct from river basins in the United States, reiterating how unfamiliar the public is with invertebrates.