Tactical moves in a floral dance

In 1917, the British botanist Arthur Tansley published a study of two plants of open ground, the heath bedstraw and the slender bedstraw. Put simply, the two could not get along. Grown in the same plot, one bedstraw always dominated, while the other disappeared. Many experiments since then, on animals as well as plants, have shown that if two species are made to compete over a single pot of resources, one will oust the other.

So why are there currently about 400,000 different ways to be a plant? All plants need the same small set of resources: light, water, carbon dioxide and a handful of chemical nutrients such as nitrogen and phosphorus. After what Tansley saw, why have a few super-competitors not taken over the floral world? There are lots of possible answers, but no consensus among researchers. What controls biodiversity is still one of the big unanswered questions in biology, and it forms the centrepiece of Jonathan Silvertown's brisk and readable tour of what is going on in plant ecology.

Tansley himself provided one answer. He found that, by altering the playing field, he could control which species won his experimental contests. In acid soil, heath bedstraw prevailed; in alkaline, slender bedstraw won out - even though on their own each species flourished in both soils. By evolving to take advantage of different conditions, the species had divided their environment and ruled; each one's ability to monopolise some places came at the cost of inferiority in others.

The set of any species' tolerances and preferences - the soil chemistry it prefers, whether it does best in wet or dry, sunny or shady conditions, for example - is called its niche. The process whereby species avoid competition by specialising in different conditions is called niche differentiation. Silvertown describes, with evident pride, his work on the influence of niches on plant diversity; he and his colleagues have shown how species can coexist in meadows by specialising on either wet or dry patches of soil, with many species preferring different points on the spectrum, from soggy to parched.

But niche differentiation alone cannot explain plant diversity. In particular, ecologists are challenged by the diversity of tropical forests, where every tree in a 10m x 100m plot containing about 250 individual trees can belong to a different species. There are not nearly enough different niches to explain such a blizzard of diversity, but there are other forces that can stop plants driving each other to oblivion.

One is natural enemies. Look at almost any leaf on a wild plant and you will see holes made by herbivores. Meanwhile, fungal and bacterial pathogens work away from within. Adult plants can stand up to these assaults but infant mortality is high, and any seedling in the vicinity will have a hard time getting established. In effect, trees foul their nests.

But diseases and herbivores are also specialists, attacking one or a few species of tree. So although a tree's own kind cannot thrive near it, other species can. Thus might tropical diversity be maintained. Herbivores, in the form of grazers, also help maintain grassland diversity, by keeping fast-growing but juicy species in their place, allowing slower, tougher plants to hold their ground.

Silvertown's treatment of these issues is excellent. He also mentions another possible reason for diversity, championed by the US ecologist Steve Hubbell, called neutral ecology, which has it that plants coexist not because they are different, but because they are the same, and that chance, not competition, controls what lives where.

This is one of the most interesting current ideas in ecology; it is also controversial, because it throws away so much past ecological thinking.

Silvertown gives neutral theory slightly shorter shrift than it deserves - he seems to think of it as disproven, but studies are appearing on both sides in this debate, showing systems that do and do not match the predictions of neutrality. It will be at least a decade before there is any consensus.

For the past few centuries, people have unwittingly experimented on plant diversity by moving species around. We do not understand enough to predict which alien species will be troublemakers, such as Japanese knotweed in Britain; most slot into their new homes unobtrusively. But freedom from natural enemies seems to be one important factor that allows invasive species to thrive.

Silvertown visits Florida, one of many places where the native flora have been almost entirely displaced by invaders, to see how aliens such as the Australian paperback pine are filling the forests and water hyacinth is clogging the Everglades. And he goes to Mexico (being an ecologist is a good way to see the world, provided you can hack the endless collecting and measuring), where the native forest is being felled for cattle ranching.

Such examples provide the answer to what now controls plant biodiversity. We do.

John Whitfield is a science writer and journalist whose book Life, Energy, and the Unity of Nature will be published this autumn by Joseph Henry Press.