The beetles (Coleoptera), with more than 350,000 known species and with new species frequently discovered, rank as the largest order in the animal kingdom.
In Europe, beetles comprise several ten-thousand of species, exhibiting a rich variety of form as well as varied life-cycle strategies. The total number of saproxylic beetle species is not currently known but it is undoubtedly very large, consisting of thousands of different species. In Britain, for example, 7% of all native animals are saproxylic and almost a half of these are beetles (700 species).
Like all insects, beetles' bodies are divided into three sections: the head, the thorax from which three pairs of legs arise, and the abdomen. Beetles are generally characterised by a particularly hard exoskeleton and hard forewings (elytra). The elytra are not used for flight, but tend to cover the hind part of the body and protect the second pair of wings. In some beetles, the ability to fly has been lost.
Beetles undergo complete metamorphosis; beetle larvae pupate, and from this pupa emerges a fully formed, sexually mature adult beetle, or imago. A single female may lay from several dozen to several thousand eggs during her lifetime, depending on the species. Like adult beetles, the larvae are varied in appearance, particularly between beetle families. Adults have an extremely variable lifespan, from weeks to years, depending on the species.
Saproxylic beetles are species which are involved in or dependent on wood decay and therefore play an important role in decomposition processes and thus for recycling nutrients in natural ecosystems. They are associated with both living and dead trees. Wood use has led to morphological, anatomical and metabolic adaptations for the exploitation of a recalcitrant and nutrient-poor resource.
Dead and decaying wood offers a broad range of potential microhabitats and the different saproxylic insects segregate spatially according to tree species, kind of tissue and position in the tree. Aside of this spatial segregation, a temporal segregation occurs in relation to the degradative succession during wood decay. Many stages can be recognized in this decay, each of them having a specific saproxylic fauna. Saproxylic insect richness depends on quantity and quality of the dead wood available in the forest, and on forest size, fragmentation and management. Key factors relate to the host trees themselves: i) the total number of trees needed to maintain population viability; ii) the preferred tree density, as many beetle species require open-grown trees, while others favour shadier conditions; iii) age structure of the tree population; and iv) management history – the four dimensions.
Saproxylic beetles furthermore interact with other groups of living organisms that are very important for the well being of ecosystems and economy, such as mites, nematodes, bacteria and fungi. The beetles may carry these organisms from tree to tree and from shrub to shrub, helping to disseminate them in the habitat. Many are also involved in pollination.
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.