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The United Nations has made 2010 its . While there could be as many as 30聽million species on this teeming planet, so far fewer than 2聽million have been identified. That includes a staggering 114,000 catalogued in the past three years alone. Our exploration of life is just beginning. No wonder the UN is keen that this year should be one of celebration.

It is also time to take stock, though. Human activities are causing a mass extinction, but the right action now could pull life back from the brink. At last we are beginning to understand what generates biodiversity (this article, below) and what makes a good conservation programme (How to save an island). We can also predict how our activities today will shape biodiversity in the future (The shape of life to come). It is a sobering vision 鈥� but one that is still in our power to change.

DEEP in the western Amazon lies the Yasuni National Park. Packed within an average hectare of this dense, steamy Ecuadorian rainforest are more species of tree than are native to the US and Canada combined, as well as 150 types of amphibian and an estimated 100,000 insect species. 鈥淚t鈥檚 hard to get very far, because every few minutes you see or hear something new,鈥� says Matt Finer of Save America鈥檚 Forests, based in Washington DC. According to work by Finer and others, published in January, there are more different life forms in Yasuni than anywhere else in South America. It may well be the most biodiverse place in the world.

If Yasuni does indeed hold this title, its tropical location will come as no surprise to biologists. The tropics boast more than 10 times as many species of animal and plant as the Arctic, with diversity decreasing steadily as you approach the poles. This gradient holds true for both land and the abyssal ocean depths. The big question, though, is why? What is it about the tropics that so fosters biodiversity? It is a mystery that has puzzled biologists for decades, and Yasuni might help us find an answer.

According to one classic theory, the reason is simply that there is more habitable space around the equator than at the poles. On the face of it, this seems to make sense. The tropics encompass an area nearly five times the size of the Earth鈥檚 polar regions, and there is some evidence that habitable space is correlated with the number of species on land. Research in the ocean tells a different story, however.

David Jablonski at the University of Chicago and his colleagues are involved in a long-term study of living and fossil marine bivalves, a group that includes oysters, mussels, cockles, clams and scallops. Looking at present-day biodiversity patterns in 4000 bivalve species, they have found no relationship between habitable area 鈥� in this case, continental shelf 鈥� and the number of bivalve species. 鈥淗abitable area just doesn鈥檛 explain marine diversity gradients,鈥� Jablonski says.

Perhaps biodiversity at sea and on land are governed by different rules. The traditional explanation for patterns of marine biodiversity is known as Rapoport鈥檚 rule. The idea here is that ocean-dwelling species in the tropics are very sensitive to temperature, so are restricted to small ranges where the water is just right, whereas species in cooler waters can tolerate a broader range of temperatures, and so spread out. This could explain why large numbers of species are packed together in the tropics, but it doesn鈥檛 seem to hold consistently. 鈥淭here are more exceptions to this rule than strong examples,鈥� says Jablonski, 鈥渨hich means it鈥檚 not much of a rule.鈥�

So what are the alternatives? Some researchers have argued that speciation rates, both terrestrial and marine, could be much higher in the tropics, making them a 鈥渃radle鈥� of biodiversity. Others have suggested that extinction rates are the decisive factor, with species less likely to become extinct near the equator than at higher latitudes, making the tropics a 鈥渕useum鈥� of biodiversity.

鈥淪peciation rates could be particularly high at the tropics, making them a cradle of biodiversity鈥�

To tease out these alternatives, Jablonski and his colleagues focused on three key factors: the rate at which species have evolved in any given location, the local extinction rate, and the immigration rate of new species. In painstaking work published in 2006, they found that three-quarters of the genera of marine bivalves that exist today evolved in the tropics and then spread out towards the poles, while also remaining in their original habitat. So the tropics are a cradle of biodiversity. But that鈥檚 not all. There are also a number of old genera in the tropics, indicating that tropical extinction rates are lower than in temperate regions. So the tropics are also a museum of diversity. The researchers conclude that their findings support an 鈥渙ut of the tropics鈥� theory to explain decreasing diversity towards the poles ().

Since then, a few other teams have found more evidence that the tropics are both a cradle and a museum of biodiversity. Not all research backs the idea, though. Last October, Martin Buzas of the Smithsonian Institution in Washington DC and Stephen Culver of East Carolina University in Greenville, North Carolina, published a study of the 259 species of foraminifera 鈥� single-celled, seabed-dwelling animals that secrete a durable shell 鈥� living along the Atlantic coast of North America. They found not only that equal numbers of these originated in tropical and temperate regions but that three-quarters of the species that had evolved at higher latitudes were now also found in the tropics (). These results are very interesting, says Jablonski, although he points out that the Caribbean underwent a major extinction event between 2 and 3 million years ago, which might at least partly account for Buzas鈥檚 findings.

Some like it hot

If the tropics are indeed the 鈥渆ngine鈥� of biodiversity, with more species evolving here than anywhere else, why could this be? Shane Wright at the University of Auckland, New Zealand, has a possible explanation. He compared the genes of 45 common tropical plants with plants from cooler regions, and found that the tropical species had more than twice the rate of molecular evolution (). Warmer temperatures could increase metabolic rates and rates of DNA replication, Wright suggests. This would raise the mutation rate, which, via natural selection, could lead to a proliferation of new species.

The idea has its detractors, however. 鈥淚 don鈥檛 think DNA replication rates are driving speciation,鈥� says Stuart Pimm at Duke University in Durham, North Carolina. 鈥淚f it were just temperature, then deserts would have more species.鈥� He also points out that marine diversity varies considerably within equatorial regions.

鈥淐onserving the tropics is essential. Disrupt them and you cut off the source of diversity at all latitudes鈥�

Nevertheless, warmer temperatures may at least be important for sustaining biodiversity by providing plenty of energy to fuel crowded ecosystems. This seems to be the case in Yasuni National Park. It is consistently warm, with average monthly temperatures between 24 掳C and 27 掳C, and temperatures by night never dropping below 10 掳C, which can damage tender plants. The steady climate means fruit and flowers are always available, providing plenty of food for animals. As well as sunshine, Yasuni also has plenty of that other stuff of life 鈥� water 鈥� with higher rainfall than the Amazonian average (). 鈥淎s best as we can tell, one of the driving forces for Yasuni鈥檚 extraordinary biodiversity is that it is ever warm and ever wet,鈥� says Finer.

Jablonski agrees that such conditions may help explain high tropical biodiversity. 鈥淓nergy input 鈥� perhaps not its mean annual value but some combination of annual average and seasonality 鈥� is very likely to be an important factor,鈥� he says. In the oceans also, Jablonski鈥檚 team has found a strong relationship between the amount of energy from the sun entering a region of water and the number of species present. In addition, stable levels of sunlight can mean stable levels of nutrients in the ocean, though more work is needed to understand the impact of this on marine diversity, he adds.

Finding that biodiversity is correlated with temperature and nutrient availability is just the start. The mechanisms by which factors such as these might generate the global biodiversity gradient are still open to debate. Unravelling this puzzle is a huge challenge that will require the analysis of massive environmental and diversity data sets. Improvements in computing power are making this increasingly possible, says Jablonski. But he cautions that there may not be one simple solution to explain global biodiversity. 鈥淚t might be that we鈥檒l look back and think that our biggest step forward was the realisation that different groups of organisms built their latitudinal gradients by different mechanisms and over different timescales,鈥� he says.

Meanwhile, the knowledge we already have can be put to good use to guide our conservation efforts. 鈥淭he tropics aren鈥檛 just the biggest reservoir of biodiversity, they are also the engine of biodiversity 鈥� the crucible where major adaptations and new lineages are formed,鈥� says Jablonski. 鈥淪o the conservation of the tropics is essential. Disrupt that engine beyond recovery, and you have not only stalled tropical diversification, you鈥檝e cut off the source of diversity at all latitudes. It鈥檚 a global issue.鈥�

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