Richard Leakey, who until recently was the controversial head of the Kenya Wildlife Service, attempts to negotiate this debate. Co-authored by Harvard biologist Roger Lewin, his book is elegantly written and, for the most part, cogently argued. But it is also deeply frustrating. While Leakey's head is with the sceptics, his heart is with the eco-pessimists.
Leakey's main thesis is a critique of the idea that nature exists in balance or harmony: "Eco-systems are in a constant state of turmoil, both in space and in time, and at any point some populations will be in decline while others will be booming." Imbalance is not only normal, argues Leakey, it is necessary for evolutionary development. Ecological communities that are immune to external disturbances become stagnant. Changes in the habitat, or even the destruction of part of the habitat, open up new ecological niches and greater biodiversity. Leakey shows how the biological richness of the Amazonian rainforest is the result of it having been subject to "tremendous perturbations" in the past. He even suggests that the ancient Mayan civilisation may have aided diversity by levelling much of the forest in certain areas.
Not only are external disturbances and population fluctuations an essential part of nature's development, but extinction, too, "is part of life's flow". The average life span of a species is four million years - a blink of an eye in the context of Earth's history - and 99.9 per cent of all species have disappeared. As one statistical wag has put it: "At a first approximation, all species are extinct."
On five occasions in the Earth's history there have been episodes of mass extinctions, when between 65 and 95 per cent of all species were wiped out. The reasons remain unclear but, in the words of geologist David Raup, in every case "global biology had an extremely close brush with total destruction".
All this would seem to give credence to the sceptics' argument. Disturbance and destruction of habitats, fluctuations in populations, even extinction, are all an essential part of nature. Even if it is true that human activity is leading to a sixth episode of mass extinction, humanity is not doing to nature anything that nature has not done to herself - often much more savagely.
But Leakey demurs from such a conclusion. The "sixth extinction", he claims, will be a catastrophe, and a terrible testament to the destructive tendencies of humanity. In the final few chapters Leakey transforms himself from a hard-headed scientist into a back-to-nature romantic. "Western culture, with its high-tech civilisation," he believes, "has come to ignore the essential connection between the human psyche and the world of nature."
Leakey the romantic has little time for the arguments of Leakey the scientist. He dismisses sceptics such as Julian Simons as "wilfully ignorant", "deliberately obscurantist" and "Panglossian". Where Leakey the scientist considered instability to be the engine of biodiversity, Leakey the romantic considers any "erosion" of nature as destructive. Where Leakey the scientist acknowledged that we have no idea how much biodiversity is necessary for the normal functioning of the biosphere, Leakey the romantic believes that any loss of species "reduces us in some ineffable way".
Leakey's real subject here is not biodiversity, but humanity. He agrees with E O Wilson that humans are "an environmental abnormality" whose very intelligence makes them "fatal for the biosphere", and believes that humanity will become extinct. His pessimism about the human condition leads him to conclusions about biodiversity unwarranted by his earlier, more measured arguments.
At the start of the book Leakey observes that much of the current fascination with extinction stems from our sense of vulnerability and uncertainty and our anxieties about humanity's future. "Many of the attitudes that govern discussion on extinction," he writes, "reflect emotional as well as scientific viewpoints." It is unfortunate that his own conclusions should bear out the truth of this. Biodiversity is too important an issue for the debate to be dominated by gut emotion rather than scientific reason.
Kenan Malik's next book, 'The Meaning of Race', will be published in April.