Mass Extinction Rates
Extinction rates have varied considerably over the history of life on earth. Paleontologists distinguish five episodes of “mass extinctions”–relatively short (1 million to 10 million year) periods during which a significant fraction of diversity in a wide range of taxa went extinct. The most significant mass extinction, as the end of the Permian (250 million years ago), may have eliminated 77 to 96 percent of species.
Even apart from these mass extinctions, background rates of extinction are not constant. For example, for the past 250 million years, relatively high rates of extinction have occurred nine times–at intervals of approximately 26 to 28 million years. Two of these nine episodes were mass extinctions, one in the late Triassic, 220 million years ago, and one in the late Cretaceous, 65 million years ago.
Global biological diversity is now close to its all time high. Floral diversity, for example, reached its highest level ever several tens of thousands of years ago. Similarly, the diversity of marine fauna has risen to a peak in the last few million years.
Humanity’s Contribution to Species Extinctions
Humanity’s first significant contribution to the rate of global extinction may have occurred 15,000 to 25,000 years ago, when hunting of large mammals apparently caused or contributed to significant extinctions in North and South America and Australia. These three continents lost 74 to 86 percent of the genera of “megafauna”–mammals greater than 44 kg–at a time.
While the cause of these extinctions remains a matter of controversy, even if humanity is not wholly responsible, there is no doubt that for millennia, people have significantly altered the landscape with untold effects on native flora and fauna. For at least 50,000 years, intentional burning has occurred in the savannas of Africa. At least 5,000 years ago, in Europe, deforestation and the conversion of wildlands to pasture began and there is evidence in North America that for as long as 4,000 years indigenous peoples influenced the structure of forest communities, provided opportunities for weedy species and such herbivores as bison to expand their ranges, and caused at least local species extinctions. In Central America, forest had already been removed from large areas before the Spanish arrived.
The prehistoric colonization of islands by human beings and their commensals substantially affected the diversity of island species. Fossil evidence suggests that 98 species of endemic birds were present in the Hawaiian Islands in A.D. 400 when the islands were first colonized by Polynesians. About 50 of these species became extinct before the first European contact in 1778.
Most experts believe that these extinctions resulted from a combination of the clearing of extensive tracts of lowland forest for agriculture, predation and disturbance by introduced species (the Polynesian rat, domestic pig, and domestic dog), and hunting. Similarly, following human colonization of New Zealand in A.D. 1000, the introduction of the domestic dog and the Polynesian rat, combined with the deforestation of large areas by fire and intensive hunting of larger birds, led to the extinction of 13 species of moas (large flightless birds) and 16 other endemic birds before the arrival of Europeans.
Humanity is thought to have caused other extinctions following the colonization of Madagascar in A.D. 500 and the Chatham Islands in A.D. 1000. Early human colonization of oceanic islands may have led to the extinction of as many as one-quarter of the bird species that existed several millennia ago.
Endemic vs. Introduced Species
In the 15th and 16th centuries, the global spread of European cultures and their attendant livestock, crops, weeds, and diseases increased the loss of island flora and fauna and added to the threats to continental species. In later centuries, the growth of trans-oceanic human travel and commerce led to the spread of a tremendous variety of species to new regions of the world and to the human colonization of many uninhabited islands.
Between 1840 and 1880, more than 60 species of vertebrates were released in Australia. Between 1800 and 1980, the number of introduced insect species in the United States grew from about 36 to more than 1200.
Many of the European introductions and colonizations, like those of earlier colonizers, significantly influenced native flora and fauna. In Hawaii, the arrival of european explorers added cats, two new species of rats, the barn owl, the small Indian mongoose, and several avian diseases. In the next two centuries, habitat degradation, disease, and predation caused the loss of 17 endemic bird species, reducing the endemic avifauna to 31 percent of the diversity found in A.D. 400; several more species now verge on extinction.
Europeans first visited the uninhabited Mascarene islands (Mauritius, Reunion, and Rodrigues) in the early 1500s and released pigs and monkeys on the islands. In the mid-1600s, the Dutch settled the islands, and in the next 300 years twenty species of birds–including the Dodo–and 8 species of reptiles were lost. The extreme vulnerability of island endemics is exemplified by the fate of the flightless Stephen Island wren, driven to extinction by a single cat owned by a lighthouse keeper on an islet off New Zealand.
