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

But gardeners would need to a microscope to see the incredible diversity which exists almost entirely among micro-organisms in the soil.

Dr Tom Curtis of the department of civil engineering at Newcastle University, ;use3d a new methods of analysis to estimate that a tonne of soil coiuld contain some four million separate species of bacteria.

This is compared to only 1.75m plant and animals species which have so far been identified globally.

The new findings may cause scientists to revise the lower estimates for biodiversity which had previously been put at between three and 100 million species.

Dr Curtis said studying the unseen biodiversity of bacteria is important because it helps experts fully understand the way ecosystems work.