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The Evolutionary Future of Biodiversity

Now we are altering the planet more rapidly and profoundly than ever, and much of the diversity produced by half a billion years of evolution could be lost in the next few centuries. We are triggering a mass extinction that could be as severe as the one that ended the reign of the dinosaurs.
Given enough time, biodiversity will recover. Extinctions create new evolutionary opportunities for the survivors: the blossoming of mammals after the dinosaurs died out ultimately led to our evolution, after all. But the aftermath of this Anthropocene ( 地质史上的新时期，称为“人类世”) extinction will not be like any other. Humans have become the main driving force in evolution--and life will never be the same again.
The list of threats we pose to biodiversity is long. We are killing many creatures directly, destroying habitats, introducing exotic predators and diseases, and pumping out pollution. Already, a tenth of birds, a fifth of mammals and a third of amphibians(两栖动物)are regarded as threatened.
Rapid climate change will make matters even worse. Warming threatens a lot of species that might otherwise be able to cope with the changes inflicted by humans, says Chris Thomas at the University of York in the UK. To work out how many species are at risk, Thomas and colleagues looked at the climatic conditions required by 1000 representative species and used them to work out how much habitable area would remain for each if the world warmed by between 1.5 and 2.5℃. Based on these figures, the team estimated that between 15 and 37 per cent of species will be "committed to extinction" by 2050.
"There are very large uncertainties," Thomas admits. "But it is equally likely for things to be worse than we are suggesting." Indeed, without drastic action the world will warm by far more than 2℃. "We will be subjecting our creatures to environmental conditions not seen for more than 10 million years," Thomas says.
Loss of diversity is not just the result of these challenges, it is also part of the problem. Decreasing population levels have already greatly reduced the genetic diversity within many species, decreasing their chances of adapting to changing environments by depriving them of the raw material needed for evolution. As well as this, extinctions can lead to further extinctions, because so many species depend on others. And as ecosystems become less diverse, they generally become less flexible to change. "The worse it gets, the worse it gets," says Jablonski.
The collapse of ecosystems will have huge economic consequences. From flooding to dust storms, the effects of environmental degradation are already hitting us hard. The loss of more coral reefs, for instance, would be a disaster for many fisheries and tourist resorts, and their death and erosion will leave formerly protected coastlines vulnerable to the ocean.
Some believe there is still time to avoid the worst. "The level of extinction can be considerably modified," says David Western of the African Conservation Centre in Nairobi, Kenya. For example, we must restore animals’ freedom to move, as the current strategy of trying to protect areas of high biodiversity will not work if species are trapped in increasingly unsuitable climate zones. Transporting species to areas that have a more suitable climate is also an option, although this would be expensive so could only be used for a few species.
Other researchers are more pessimistic. "We can turn the ship a little," says Jablonski. The main problem he sees is a lack of political will.
No one can predict exactly what the Earth of our descendants will look like. However, there are some clues in what’s happening right now. Our influence is so profound that we are altering the evolutionary pressures that shape life. There have already been very large and quick behavioral changes as landscapes become "humanscapes", says Western. Foxes and coyotes (郊狼) are adapting to life in cities, and elephants have started moving out of parks at night to feed at the fringes of settlements before returning in the morning to avoid us. Human pressures are also producing genetic changes in wildlife. As a result of hunting for ivory, for instance, tuskless elephants are evolving. "There will be a new round of evolution," says Western. "We are already seeing that."
Almost all biologists believe that the age of giant animals is over. Large, slow-to-reproduce animals are the most likely to become extinct and, at least on land, those that do survive will not have the vast expanses of habitat needed for further evolution. The greater pressures on large animals will downsize entire communities, says Western. For example, small antelope (羚羊)are likely to become more common than larger vegetarians on the African grassland, which would lead to lions becoming smaller too. Lions may also become more tame as we kill off the aggressive individuals that intrude upon human settlements. "There will be a transformation of large animals to ones that are compatible with the humanscape," Western says.
The species most likely to thrive will be small ones that are easily spread around by humans and good at colonizing new territory. "It’s not good to be big or rare," says Jablonski. "You want to be a rat, or a weed, or a cockroach(蟑螂)." In theory, as humans fragment habitats, evolution may throw up new species, especially small mammals and insects--but these might not be very flexible. They may limp along and easily go extinct, Jablonski says. So the ecosystems of the future are likely to be far poorer affairs, with fewer species, fewer links between species and a shortage of large animals.
After previous mass extinctions, the recovery of biodiversity took millions of years. For starters, we are preserving samples of endangered species so that they can be revived if necessary. There are over 1400 plant "gene banks" worldwide storing millions of seeds, mostly from food plants but also some wild species. Animals are also being stored as frozen tissue samples.
Even assuming that civilization survives and that gene banks get the funding they need to store many more samples, to look after them for the next few centuries and to revive species as suitable habitat becomes available, only a tiny fraction of the world’s species could be saved this way. Nevertheless, that fraction could include not only many charming large animals, but also keystone species that play a vital role in maintaining ecosystems, such as corals.
The second thing people could do to aid the recovery of biodiversity would be to manage habitats in a way that allows evolution to continue. This is now being tried in the Cape region of South Africa, home to some of the most diverse flora in the world. "Species come and species go," says Richard Cowling of the Nelson Mandela Metropolitan University, Port Elizabeth. "You’ve got to preserve the processes.,"
Finally, we might generate diversity in an even more direct way. We have already created millions of new varieties of plants and animals through selective breeding, and many of these creatures have established wild populations. In the vacuum left by a mass extinction, many more domesticated species may turn wild--and genetically modified domesticates could have a particularly dramatic impact.

According to the author, human pressures have resulted in（）.

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