The word 'saproxylic' is from the Greek sapros meaning 'rotten' and xylon meaning 'wood' and the saproxylic fauna and flora is a major element in woodland ecology.
A large log of wood, half a tree trunk, in an advanced state of decay like the one shown above will contain hundreds, maybe thousands of different species. These include various moulds and fungi, woodlice and centipedes, snails and pseudoscorpions, the larvae of flies beetles and other invertebrates feeding directly on the decaying wood or plant life associated with it. Many of the invertebrates have parasites and predators and the wood itself will attract different suites of species as it goes through the processes of decay.
It is salutary to think that before the arrival of Homo sapiens trees and branches just lay where they fell unless there was a forest fire. Today wood is harvested and removed, the brash is often burnt and material that has fallen naturally is cleared away. This has caused an enormous reduction in the dead wood habitat with profound consequences higher up the food chain to birds and bats and the things that prey on them as there simply is only a fraction of the invertebrate food that would naturally have been available.
The Woodland Trust (and other conservation-minded organisations) are, of course, fully aware of this and will try to ensure the balance, insofar as it is possible, is redressed. While some of our rarer saproxylic species may have been lost for good, an increase of this resource should quite rapidly produce results as the quantity and quality of dead wood biodiversity increases.