There is abundant evidence that human beings are beginning to alter some of the planet’s basic physical, chemical and biological systems,endangering other species and disrupting ecosystems in the process and ultimately threatening human health. When Homo sapiens evolved some 120 000 years ago, the number of species on Earth was the largest ever,8 but human activity has resulted in species extinction rates that are currently 100 to 1000 times the pre-human rate.
Although the record demonstrates that humans hunted to extinction scores of large mammals and birds as early as tens of thousands of years ago, it is only in recent times that these extinctions have spread to virtually every part of the planet and to almost every phylum. Species numbers are now being reduced so rapidly that some experts have predicted that 25% or more of all species currently alive may become extinct during the next 50 years if these rates persist.Such losses have prompted biologists to refer to the present period as “the sixth extinction” (the last great extinction event, the fifth, was 65 million years ago, at the end of the Cretaceous period, when dinosaurs became extinct) and to warn that evolutionary processes would not replace these losses with a stock of new species for several million years. From this perspective, the loss of species may be said to be the most destructive and permanent consequence of human-caused degradation of the global environment.
Global climate change,stratospheric ozone depletion, chemical pollution,acid rain, the introduction of alien species and the overhunting of speciesall threaten biodiversity, but it is the degradation, reduction and fragmentation of habitats that is the greatest threat, particularly in species-rich areas such as tropical rain forests and coral reefs.
The relation between human health and the health of other species has been given little attention by scientists and public health experts and has not been a part of medical education. This article reviews some of the ways that plant, animal and microbial species support human health and, by their interactions with each other and with nonliving components of the environment, produce what are called “ecosystem services,” which make all life, including human life, possible on Earth. Understanding these connections will be increasingly important to physicians and other health care professionals in coming decades, as the number of species driven to extinction continues to mount.