Books and Arts

Nature 438, 27 (3 November 2005) | doi:10.1038/438027a; Published online 2 November 2005

Green in tooth and claw

Peter D. Moore1

BOOK REVIEWEDDemons in Eden: The Paradox of Plant Diversity

by Jonathan Silvertown

University of Chicago Press: 2005. 192 pp. $25, £17.50

Apart from a few parasites and decomposers, all plants face the same problem: how to capture light energy and fix atmospheric carbon dioxide more effectively than their neighbours. Actually, a darwinian would argue that their ultimate problem is how to leave more progeny than their neighbours, and that photosynthetic competition is just one aspect of this struggle. Growth may help, but pollination, seed maturation, fruit dispersal and seedling establishment all contribute to the final outcome in the next generation.

The theory of natural selection is based on the idea that heritable variation, acted on by the pressures of the environment, ultimately favours excellence. Only the fit survive. This being the case, one might predict that one supremely fit plant species, capable of photosynthesis, vegetative growth, pollination and seed dispersal in a way that outperformed all other plants, might ultimately prove the victor in the evolutionary struggle and would dominate the world's primary production. But this has not happened.

On the contrary, Earth displays an extraordinary diversity of plant species (around 400,000), almost all of which contribute to the planet's energy-trapping potential. Why? How can biodiversity be reconciled with natural selection? This is the question Jonathan Silvertown asks in Demons in Eden as he explores the remarkable variations of form and the elaborate systems of coevolution with animals that are found throughout the plant kingdom.

Among plants there are some darwinian demons that seem unstoppable in their spread, such as the kudzu vine, which seems intent on burying Florida, or the rhododendron, which is running amok in British woodlands. But both of these were introduced by humans, taken to parts of the world where there seem to be no constraints upon their spread. And herein seems to lie the answer. Constraints on population expansion decelerate the tendency for any plant species to spread indefinitely and become the ultimate demon. Such constraints include herbivores, seed predators, parasites, diseases and new evolutionary developments in competitor plants.

When a plant escapes from these constraints, it may for a while become a darwinian demon. For example, genetic evidence seems to show that the olive tree invaded the Canary Islands just once. Freed from predators and competitors it may well have spread unrestrained. There is still nothing on the Canaries that eats olive seeds, but the arrival of other tree species and the introduction of goats by people supplied the constraints that put the demon olive back in chains.

Another type of constraint on indefinite population expansion in plants is the supply of resources. Soil elements, such as nitrogen, phosphorus, potassium and calcium, are not always abundant, and the one that falls farthest behind the plant's demands may become the limiting factor for its growth. Silvertown describes the long-running Park Grass experiment at Rothamsted in England, and shows how the addition of an element, such as nitrogen, that may be limiting to some plant species, causes changes in the composition of grassland, releasing some species from the constraints of element limitation and causing others to be suppressed by the growth of the unchained demons.

The great diversity among plants does not therefore contradict darwinian principles, but upholds them. Vegetation illustrates the complex balance of interacting forces and trade-offs that evolution generates among competing species. In this highly readable and pleasantly anecdotal account of the dynamics of the plant world, Silvertown suggests that tasting the fruit of evolutionary knowledge may provide us with a ticket for readmission to the Garden of Eden, where we can exercise the privilege by ensuring that biodiversity is conserved. Let us hope that he is right.

  1. Peter D. Moore is in the Division of Life Sciences, King's College London, Franklin–Wilkins Building, 150 Stamford Street, London SE1 9NH, UK.


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