Habitat destruction is, without a doubt, the greatest threat to biodiversity around the world. The human population, which surpassed 7 billion in 2012, increasingly dominates Earth. As our population grows and we alter the planet to meet our own needs, we put untold thousands of species at risk. In Chapter 4, we will consider how human impacts on the environment challenge the diversity of whole communities of organisms. Consider the extensive agriculture and urban sprawl in southern California coastal sage scrub. The region, home to 300 plant species found nowhere else, once covered approximately 1 million hectares (2.5 million acres). Today, as little as 10% of that remains. Native mammal and bird species, such as the California gnatcatcher, have been largely restricted to remnants of their habitat. Many of these small populations are unable to sustain themselves and will likely die out within a short period of time (Figure 3.20).

We destroy terrestrial habitats when we plow prairie and plant wheat, cut down a tropical rain forest, or drain and fill in marshes. With each pass of a bulldozer, we reduce the habitat available for other species, thereby reducing their total population size. In these situations, smaller, isolated populations, such as butterflies or mountain gorillas, have a lower chance of survival than larger populations. According to the IUCN, habitat loss and degradation are affecting 86% of all threatened birds, 86% of threatened mammals, and 88% of threatened amphibians.

Environmental scientists estimate that we are losing approximately 90,000 km² of tropical rain forest annually, which is approximately the area of the state of Indiana. In the countries of Southeast Asia, including Malaysia, Indonesia, and Papua New Guinea, approximately 11 million hectares of tropical forest have been cleared for oil palm plantations. Palm oil is used in foods, beauty products, and even as a biofuel. One casualty of this transformation has been the orangutan, a great ape that spends most of its life high in the tree canopy on the islands of Sumatra and Borneo. The orangutan population has declined by 50% since the 1950s. If current trends continue, conservationists believe that the Sumatran population could vanish in the next 10 years and that the Bornean population is unlikely to survive into the next century.

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Temperate habitats are also under threat. More than 50% of historic grasslands and more than 30% of desert habitat have been lost. The Kermode (or spirit) bear, a white subspecies of the black bear, is one temperate species that is seriously threatened by habitat destruction. The bear lives in the temperate rain forests of northwest Canada (Figure 3.21). The bear’s habitat is shrinking due to the logging of old growth forests in the region and will shrink even further as a result of a recently approved project to build an oil pipeline on Princess Royal Island in British Columbia.

Marine and freshwater species are also increasingly impacted by human activity. According to the IUCN, freshwater species are going extinct faster than either terrestrial or marine species. In 2014 the IUCN estimated that human pressures on freshwater ecosystems threatened one-third of freshwater species with extinction. The impact of humans on freshwater wetlands and other aquatic ecosystems is considered in Chapter 6, which focuses on managing freshwater resources and associated environmental issues. Pollution and warming of the oceans have placed 33% of reef-building corals in danger of extinction. In addition, overfishing has severely depleted marine fish populations around the world. Ways to reverse these impacts on fish stocks are discussed in Chapter 8.