Back garden biodiversity

The average back garden may contain twice as many species as have so far been identified on the whole planet, according to a study published today by British scientists.

But gardeners would need a microscope to observe the massive biodiversity, which exists almost entirely among micro-organisms in the soil.

Using new methods of analysis, Dr Tom Curtis, of the Department of Civil Engineering, Newcastle University, England, and colleagues, estimated that a tonne of soil could contain some four million separate species of bacteria.

This is surprisingly high, since only 1.75 million plant and animal species have so far been identified globally, although these tend to be the larger, observable species.

The new findings, published in the Proceedings of the National Academy of Sciences (PNAS) of the USA, may cause scientists to revise the lower estimates for global biodiversity, which had previously been put at anywhere between three million and 100 million species. It is generally accepted that it is impossible to identify all species, because micro-organisms are so many and so small.

Dr Curtis argues that studying the unseen biodiversity of bacteria is important because we cannot fully understand the way ecosystems work without this knowledge. For example, the way we manage soil affects the bacterial content, which in turn may affect productivity. This is not only important for farmers but for ecologists, such as those who manage rainforests.

Other applications of this knowledge may lead to better sewage treatment and pollution control, since bacteria are employed to break down human waste products and to destroy toxins as polluted water flows through wetlands.

Dr Curtis and colleagues based their findings on two key measurements of soil samples: the total number of bacteria present and the abundance of the commonest species. They devised a statistical technique, based on a branch of mathematics known as log-normal species abundance curves, to estimate the total number of species.

The method was then applied to other ecosystems, calculating that typical samples of ocean might contain 160 species per millilitre, and sewage systems contain surprisingly fewer, only 70 species per millilitre.
According to the calculations, the entire ocean may contain about two million species, while a tonne of soil could contain double that number.

1. “Estimating prokaryotic diversity and its limits” by Thomas P. Curtis, Jack W. Scannell, and William T. Sloan will be published by PNAS on their website www.pnas.org and can also be obtained from Newcastle University press office