POPULATION AND BIODIVERSITY
umans have been causing the extinction of species for thousands of years. Wherever people have settled they have hunted the local fauna or so altered natural habitats that species within them have suffered. Whether for subsistence or for trade, whether by design or by default, the human impact on the Earth may be bringing about more extinctions than any other single factor since the extinction of the dinosaurs.
The most direct evidence of human-induced extinction comes from North and South America and Australia, where humans are thought to have first arrived around 10 000 to 25 000 years ago. Within a few hundred years, some 70 species of "megafauna" in North America disappeared from the fossil record. These included mammoths and mastodons, giant beavers, sabre-tooth cats, giant ground sloths, horses, camels and more. Climate change may have been partly to blame. But there is abundant evidence that hunting was key .
More conjecturally, human alterations to the land have probably caused extinctions dating back as much as 50 000 years -- when we have the first evidence of the deliberate use of widespread burning of bush and grasslands to round up wild animals during hunting. By 5 000 years ago, Europeans were deforesting large areas for pasture . [Add]
Species extinctions occur as a result of human causes ranging from hunting to climate change. The proportions vary with time and place, but one study of animal extinctions since 1600 found that 39 percent arose mainly from the introduction of alien species, 36 percent from habitat destruction, and 23 percent from hunting or deliberate extermination .
There have been some spectacular examples of our ability to hunt to extinction even the most populous species. One was the passenger pigeon. In the early 1800s, billions of the birds darkened the North American skies. In September 1914, the last died in a Cincinnati zoo. More recently, the tiger has been under severe threat from the Asian medicinal trade in tiger bones, while primates and other mammals in Central Africa face a booming trade in bushmeat. [Add]
But an even bigger cause may be the introduction of non-native species. As people have travelled around the globe, they have taken many other species with them. We have introduced plants that have taken over whole ecosystems and exterminated local species; and predatory animals against which indigenous species had no defence. Sometimes the introductions were deliberate. Plantation crops (such as the rubber plants taken from Brazil to Malaya by British colonial botanists) often thrive best when taken from their natural homes, where there are pests that control them, to new lands where they may be freed from such constraints. Sometimes the introductions have been accidental, and have created widespread ecological dislocation and species extinctions. Many endemic flightless birds have disappeared from the islands of Polynesia and elsewhere following the arrival of humans who, besides hunting, brought rats, cats and other alien species with a liking for birds' eggs.
River and marine ecosystems can be as vulnerable as those on land. One dramatic example was the deliberate introduction of the large and carnivorous Nile perch into the rich ecosystem of the world's second largest freshwater lake, Lake Victoria in East Africa, in the 1960s. The result was the extinction of an estimated 50 species of the indigenous cichlid fish within a couple of decades . [Add]
A recent study by IUCN (the World Conservation Union) found that primates, with the exception of human beings, constitute the most endangered order of mammals; some 46 percent are known to be at risk. Overall, a quarter of mammals are endangered, not just "charismatic" animals such as pandas and tigers but many lesser known species of bats, rodents and marsupials. Some 11 percent of birds were found to be threatened. Much less is known about the status of other groups (see table on page 165). While it was estimated that 3 percent of the amphibians and reptiles that have been identified are threatened, a closer study of representative samples of these lesser known groups found a quarter of the amphibians to be at risk, alongside a fifth of the reptiles . The most threatened group was fish, with up to a third endangered. Marine life at risk includes many species of shark and sturgeon, along with other marine animals from the blue whale to sea horses, of which 20 million are harvested annually. [Add]
Humans do not always have an entirely negative impact on local biodiversity, however. Our activities, while destructive of some species, create new ecological niches for many forms of wildlife, whether birds nesting in the eaves of houses, rats feeding off waste landfills or mosquitoes breeding in the ruts left by tracks in the mud. With the transportation of species around the world, there are more species of flowering plant in England than ever before, thanks to the country's propensity for planting exotic species in gardens.
Humans have also deliberately added to the genetic diversity within species through thousands of years of plant and animal breeding. As recently as 50 years ago, China grew 10 000 wheat varieties. Andean farmers bred 3 000 varieties of potato . [Add]
But our plant and animal breeding has in modern times been directed towards the creation of "superbreeds" distributed so widely and universally that others are relegated to gene banks or, worse, lost altogether. China grows only a tenth of the wheat varieties it grew half a century ago, Mexico a fifth of its former corn varieties . More than 6 000 of the 7 000 apple varieties grown in the United States in the 19th century are now extinct, and just two make up half the national crop .
Crop genetic resources are disappearing from fields at an estimated 1 or 2 percent a year , and domesticated livestock breeds at 5 percent a year . Many traditional plant varieties now live on only in gene banks, where their ability to be transplanted back into fields is sometimes uncertain. This is a biological tragedy. The ability of certain varieties to withstand drought, grow on poor soil, resist insects and disease, prosper in local environments or under global warming, or simply taste better, are contained in these genes. If the genes are lost, so is the future ability to innovate for peasant farmers and global corporations alike. [Add]