Saproxylic invertebrates are those species which are dependent on dead or decaying wood or associated fungi and microorganisms for at least part of their lifecycle.
These invertebrates may not be dependent on the wood for their entire life cycle but at least some stage is dependent on wood. A good example of this is the larvae of some beetles that feed on decaying wood. The adults may feed on other things (such as nectar).
They include some of the most endangered species due to a reduction in available suitable habitat which leads to fragmentation and increasing isolation of populations. This may occur through intensive land-use such as agriculture and forestry, firewood collection and management practices that reduce decaying wood for safety reasons or aesthetic tidiness. There has, however, been a recent increase in interest in this threatened group and there is now an urgent need for conservation effort.
Saproxylic beetles play important ecological roles in forest habitats. Together with fungi, they contribute to the break-down of decaying wood and are involved in decomposition processes and the recycling of nutrients in natural ecosystems. They interact with other organisms such as mites, nematodes, bacteria and fungi, assisting in their dispersal across the landscape. They also provide an important food source for birds and mammals, and some species are involved in pollination.
In Europe, there are 58 families of beetles (order Coleoptera) with nearly 29,000 species. The exact number of saproxylic species is unknown, but a database of French saproxylic beetles includes 3,041 species. According to expert opinion, there may be closer to 4,000 saproxylic beetle species in Europe. Dead and decaying wood offer a large variety of microhabitats, and different saproxylic species have evolved to exploit these niches, with certain species having very specific ecological requirements. Some saproxylic beetles require live old trees with cavities for their larval development, while others are dependent on trees that have recently died.
Saproxylic beetle richness depends on the quantity and quality of available dead and decaying wood in any environment with trees and woody shrubs, as well as on tree age structure, total number of trees, varying tree density, and habitat continuity. The assemblage of saproxylic beetles can be influenced by the degree of sun-exposure, frequency of habitat disturbance (i.e., forest fires or clear-cutting), hedgerow management, clearance of fallen decaying wood, age of tree stands and presence of certain types of wood-decaying fungi, among others.
The long-term survival of these beetles depends on new generations of trees developing and becoming suitable for colonisation as the host trees decline and disintegrate. This means that certain beetles can be at risk even while the overall population is strong, as new host trees are not becoming available. Old and hollow trees have become increasingly scarce around the world, including in the UK, due to land management practices.
Much is left to learn about the saproxylic beetles of Europe. In comparison with other species groups, and despite all the efforts of generations of entomologists, the biology of many species is still poorly known. Any research on saproxylic beetles enhances our knowledge of the functioning of ecosystems in wooded landscapes.