“Extinction is for ever” used to be a green movement slogan. It is no longer completely true. Thanks to the wonders of genetic engineering, it is becoming possible to recreate lost creatures provided we have their DNA. The quagga, a species of zebra whose last member died in Amsterdam Zoo in 1883, is one animal that might get the recreation treatment soon.

In case you are wondering, it is far trickier to do this with long-extinct animals with no living relatives, for example dinosaurs. One reason is that the plants they used to eat have also gone extinct and those that exist today might be toxic to them. More importantly, you need a near relative to give bith to the revived species.

Although some creatures can be driven to extinction by humans killing them off, far more are put in danger by having their habitat removed around them. So like climate change, extinction is important in its own right but also for what it tells us about how we are changing the Earth.

Just how many species are in danger of being wiped out isn't an easy question to answer. Our knowledge of the world around us is not perfect. The excellent Recently Extinct Animals website run by Peter Maas in the Netherlands lists dozens of species, such as the Gomeran lizard (in the Canary Islands) and the Bavarian pine vole, that have been spotted alive and well some time after reports had been issued of their dying out.

The International Union for Conservation of Nature and Natural Resources runs the Red List of endangered species. It turns out that the bigger and more noticeable a species is, the likelier biologists are to know about it in detail. Thus there are 5416 known species of mammal of which we have detailed knowledge of 4853. Of these, according to IUCN, 1101 were threatened with extinction in 2004. That is a startling number, almost exactly a fifth of the total. For birds the figure is 12 percent, at 1213 out of 9917 species to have been described; for amphibians it is an even more alarming 31 percent, 1770 out of 5743 described species.

However, things get a lot less clear when less adorable species are being considered. For example, there are about 950,000 described species of insect. Of these, IUCN was only able to evaluate the status of 771
properly for its 2004 return. It turned out that 569 of these were endangered. That is 73 percent of those evaluated, but less than 0.1 percent of the total number we know of. Although the species chosen for analysis were probably the most imperilled, it is likely that the real number of threatened species is far higher than the headline figure. The same goes tor the lichens, creatures made up by the co-operative union of fungi and algae. There are said to be 10,000 species of lichens of which two are in danger, but in fact these two are the only ones to have been properly investigated.

The IUCN s tables show that the numbers of species apparently at risk have been increasing consistently since the mid-1990s. Perhaps most alarming is the fact that the list contains over 8000 species of plants. While people might hunt bears or wolves to death, the extinction of a plant usually stems from gross alteration of the environment, and in turn threatens the animals that eat it. The number of endangered plant species counted by IUCN has risen by about half since 1996. The figure is slight compared to the number of plants known – 287,655 species, to be exact – but also adds up to 70 percent of the species studied in detail.

The IUCN's headline figures suggest that only about 1 percent of species are threatened with extinction. However, a far higher percentage of the species that have been studied in detail are in trouble. The figures support an estimate that extinction is now running at anything up to 100 times its normal rate.

What are we doing to cause all this extinction? Some species, particularly large land mammals, have been deliberately hunted to the point of extinction. Now, tigers and elephants are seen as having value in their own right, if only as something for lucrative tourists to come and look at, which may help them survive. Others, such as whales, have been hunted for food, although here, again, there has been an improvement in recent years despite some nations' continued insistence on hunting whales. The southern right whale population plummeted to about 300 in 1935, but has now grown back to about 7000 members with the cessation of most hunting. A similar story can be told about a number of bird species.

Most of the species that are in danger of being wiped out directly by human activity are fish, because demand for fish, and fishing technology, have both developed apace in recent decades. Climate change has been blamed for severe declines in some other species such as frogs and toads in Costa Rica, reduced radically in numbers by unusually long periods of drought.

But as we saw above, the greatest cause of extinction is damage to a species' native habitat. The IUCN points out that up to half the world's freshwater turtles and a third of its amphibians are threatened with extinction, a fact which reflects the huge stress that pollution, agriculture and development are placing on the lakes, rivers and wetlands that they need.

Quite simply, it is the sheer percentage of the Earth's biological activity that we are making use of that is sending species extinct, as we concrete the river banks and plough up the jungles. Although it is unlikely that we'll be responsible for a mass extinction on the scale of the one which saw off the dinosaurs, the prospect is a gloomy one. Some espedally shroud-waving experts claim that up to a fifth of the Earths could vanish within thirty years.