His book of life still awaits its completion

Systematists have struggled to preserve the order of species that Linnaeus created, but the rate at which our natural knowledge grows and threats to biodiversity make their task ever more difficult. Peter Crane writes.

This year will see worldwide celebrations of the tercentenary of the birth of the Swedish physician Linnaeus, an inspiring teacher, an accomplished writer and poet, and one of the great figures in the history of science. Linnaeus helped found the Royal Swedish Academy of Sciences, served as its first president and made important contributions to both science and medicine. But his real passion was the diversity of life, and he was the first to try to get to grips with it in a comprehensive and scientific way. In so doing, he cleared the way for Darwin and Wallace, and the methods he established are still in use today.

However, two and a half centuries later we see the world from a very different vantage point. Our Earth bears a more marked human footprint and is changing rapidly before our eyes. So the question arises: how well are we being served by Linnaeus's methods, and are they still appropriate as we seek to understand, conserve and manage biological diversity in the 21st century?

Linnaeus is best known for developing a pragmatic approach to naming species using a binomial structure that includes a generic name (for instance, Homo), followed by a specific epithet (for instance, Homo sapiens). In fact, Linnaeus was not the first to use Latin binomials, nor was he the first to organise biological diversity hierarchically - into species, genera, orders, classes and kingdoms.

But Linnaeus was unique in his relentless application of the binomial approach, and the hierarchy that went with it, dealing with the whole of biology, including material brought back by his students and others from all over the world. His great contribution was much more than developing an elegant new way of labelling species: it was the discipline and scholarship of applying a consistent approach to a near-comprehensive systematisation of biological diversity as it was known at the time.

Linnaeus overcame a key problem that had bedevilled earlier studies and that is still a source of confusion: the curse of synonomy. Linnaeus brought order out of the chaos. Through his industry, and through the audacity of his global ambition, he dominated the field and drew a line under what had gone before. He established a new approach and, through it, a new foundation for the study of biological diversity. Both succeeded not because of some dazzling new theoretical insight, but because they were useful.

Linnaeus's way of working was both comparative and empirical. He based his ideas on data, in the form of reference specimens, previous literature and illustrations, all of which he examined critically in reaching his conclusions. And, while much of his labour focused on describing individual species, he did not shrink from considering the broader implications of what he saw. He was, for example, the first to identify bats and whales as mammals, rather than as peculiar birds or fish. And, perhaps most revealingly in terms of his capacity for then unconventional thought, he was uncompromising in his recognition of humans as primates.

Linnaeus set out 38 principles outlining how work in natural history should be done. These make it clear that he was interested not just in description and naming but also that he saw the names and their associated descriptions as the place to synthesise all that was known about a particular species, from its form and structure, to its ecology and use. And while his approach to recognising species, and delimiting their boundaries, was mainly intuitive, his species have largely stood the test of time. The principles set standards of discipline and documentation that remain essential parts of the scientific study of biological diversity today. By the standards of his time, Linnaeus was methodical and rigorous.

Since the death of Linnaeus, scientists specialising in the diversity of life - systematists, as they have come to be known - have struggled to preserve the order he was able to create, as their task has become steadily more difficult. In the tenth edition of his great work Systema Naturae, Linnaeus enumerated a little over 12,000 plant and animal species. Today, we estimate the likely number of living species of all kinds, excluding bacteria and their allies, as in the vicinity of 5 million to 10 million, of which perhaps only 1.5 million are described so far. New species still continue to be described at the prodigious rate of about 10,000 per year.

The scale of the task is now too vast to ever realistically contemplate undertaking the kind of comprehensive global synthesis that Linnaeus once attempted.

Such a synthesis has been made more complicated too by the distributed, and fundamentally entrepreneurial, enterprise of modern science. In the absence of central control, the old demon of synonymy has re-emerged. And although both botanists and zoologists have developed rules to help decide which of several competing names should be used for a given species, some think that, in their current form, they are as much part of the problem as part of the solution.

And regrettably this is not just a matter of process. It is also a matter of scientific judgment because of an unfortunate weakness that is inherent in Linnaean nomenclature itself. Uniform application of the binomial approach requires uniform concepts of both genus and species identity. The binomial system, rather unfortunately, intertwines the delivery of straightforward species labels with the much trickier issue of how similarities and differences translate into hypotheses of evolutionary relatedness at the generic level. When generic concepts change, so too can Latin binomials. One can understand the frustration of gardeners when they are told that the Leyland Cypress, a hybrid conifer that is widely planted all over suburbia, will now be called x Cuprocyparis leylandii , rather than x Cupresssocyparis leylandii .

Against this background, can the systematists of the 21st century deliver what society demands of them: efficient information storage and retrieval, combined with user-friendly means of species identification? For example, in the case of plants, is there any prospect that "a widely accessible list of known plant species, as a step towards a complete world flora", will be delivered by 2010, as envisaged in the Global Strategy for Plant Conservation that was adopted in 2002 by the Parties to the Convention on Biological Diversity?

For plants, the scale of the synonymy problem is considerable. The International Plants Names Index maintained by the Royal Botanic Gardens, Kew, the Harvard University Herbaria and the Australian National Herbarium contains almost a million names, compared with a best estimate of between 350,000 and 400,000 described plant species. For comparison, mammals and birds have about 5,000 and 10,000 species respectively. But despite the challenges, the prospects of completing a global plant checklist are better than one might expect. Based on work at Kew and elsewhere, the job is perhaps already two thirds complete. But a list is just the beginning. It then needs to be kept up to date and, just as Linnaeus's principles would have it, it needs to be used to organise all we know about each species, including their uses and conservation status. To do this, the essential raw materials are scientific expertise and excellent collections of specimens and literature.

In the coming months, the celebrations around the birth of Linnaeus in the UK will be second only to those in Sweden. In many ways, this is appropriate. Through a quirk of history, Linnaeus's collections are now housed in a vault at the Linnean Society of London. The results of a long-term joint project undertaken by the Natural History Museum and supported by the Linnean Society sifting the evidence that Linnaeus considered for the 9,000 plant species he described will be published in a matter of months.

But Linnaeus's collections are only part of the story. For reasons to do with colonial history, most of the key reference specimens for the names of all plant species are housed in the preserved plant collections of North American and European institutions. There are more in the UK than anywhere else. For this and other reasons, Britain is still a global leader in recognising, circumscribing, describing and identifying the variety of life on our planet. The Natural History Museum, the Royal Botanic Garden, Edinburgh, and the Royal Botanic Gardens, Kew collectively represent the most powerful force anywhere in the world for understanding the variety of plant life and how it can be conserved and used in sustainable ways.

These internationally preeminent organisations are in the forefront of developing new ap-proaches for organising and making available the vast amount of information on the world's plant species. However, as part of the broader community of plant systematists, they face a key challenge in deciding how best to deploy their resources in the face of many competing demands. On the one hand, with biological diversity being steadily eroded all over the world, now is not the time to stop collecting and describing the many new species that are still waiting to be discovered. We cannot conserve what we do not know, and working in the tropics with local specialists is also one of the most effective ways to help increase the capacity for plant identification and systematic work where it is needed most. But, on the other hand, these organisations have to find the right balance between continuing to develop systematics as an intellectually exciting and viable discipline in its own right that is capable of attracting high-quality new practitioners, while also providing the kind of reasonably stable, user-friendly classifications the world needs.

Both the challenge and the task are greater than Linnaeus could ever have imagined but, through computer-assisted informatics, the right kind of tools are available. The hard part, as always, will be negotiating the politics of funding and support around a shared sense of purpose. But the opportunity to follow Linnaeus by fashioning a new blend of pragmatism and ambition is exciting. The key will be striking the right balance between the service and scientific functions of biological diversity research, while at the same time increasing the speed with which knowledge of the natural world is gathered, synthesised and disseminated. And with time of the essence, it is vital to move fast.

In the face of accelerating environmental degradation, the present generation of plant systematists may be the last with the opportunity to deliver what Linnaeus was striving for while it is still relevant - a compendium of all the world's plant species and what we know about them.

Sir Peter Crane FRS is the John and Marion Sullivan university professor in the department of the geophysical sciences at the University of Chicago. He was director of the Royal Botanic Garden, Edinburgh, from 1999 to 2006. He is giving a Darwin College lecture at Cambridge University on species identity on March 9.