Saproxyl beetles: the “decomposing community”

During the development of a project to study saproxyl entomofauna, the technical team of Ideas MedioAmbiental had the opportunity to enjoy the presence of two of the most emblematic beetles in our entomofauna: the flying deer (Lucanus cervus Linnaeus, 1758) and the Alpine Rosalía (Rosalia alpina Linnaeus, 1758). But... what are saproxyl species?

What are saproxyl insects?

Saproxyl insects are defined as those that, at some stage of their life cycle, depend on dead or decaying wood, associated both with dying or dead trees (standing or fallen), and with living trees colonized by fungi or linked to the presence of other strictly saproxylic species (Speight, 1989; Kirby & Drake, 1992). This definition was specified and expanded by Alexander (2008), who emphasized living trees, which may have cavities with tree humus, dead parts and/or be already affected by lignicolous fungi.

In addition to insects, nematodes, gastropod mollusks, arachnids, crustaceans such as isopods (ball bugs) and myriapods (centipedes and millipedes) are also included as saproxyl species (Méndez Iglesias, 2009). In general, they are considered to constitute a vast functional group of organisms united by their dependence on a fundamental process exclusive to forests and other woody formations: the death and decay of wood (Pagola-Carte, 2016).

The definition of the term goes back a long way in history. According to the book on saproxyl beetles from France (Bouget et al., 2019), as early as 1913 Silvestri used the term Saproxylophile, which literally means “those who love dead wood”. Later, in 1966, Dajoz used the word saproxylyque (in French), until the English term was finally consolidated in international ecological and entomological literature saproxylic.

Due to the complexity and great diversity of existing saproxyl insects, in this article we will focus on saproxyl beetles.

What is its importance?

Saproxyl beetles live in dead or partially decayed wood, under loose bark, in holes, cavities and galleries created by other saproxylics, in tree humus and in saproxyl fungi (Speight, 1989). Together with other saproxyl arthropods and fungi, they participate in numerous ecosystem functions, their fundamental role in the degradation of dead wood and the subsequent reincorporation of nutrients into the soil.

Saproxyl beetle species have very particular requirements. These include the type of decay, the stage of the process, the precise location of the decaying wood, and the location of the host tree (Alexander, 2008).

In addition, they genuinely represent the problems associated with the survival of faunas adapted to threatened microhabitats. Therefore, from the point of view of conservation, some taxons can be used as “umbrella species” (Pagola-Carte, 2016), since they act as indicators of the state of conservation of forests (by Rosa Maldonado, 2014).

Some species also play a role as pollinators during their adult phase. In addition, saproxyl beetles serve as food for many other arthropods, birds, and even mammals.

Below are some images of examples of trees and habitats where saproxyl beetles can develop and where it is possible to find them.

What are their threats?

As early as 1992, Kirby and Drake found that:”Throughout Europe, saproxylic species have been identified as the most threatened community of invertebrates.” (“Across Europe, saproxyl species have been identified as the most threatened invertebrate community.”)

The disappearance of mature forests and dead wood, together with felling, loss of trees and intensive wood harvesting, are some of the main reasons why saproxyl beetles are among the most threatened invertebrates in the forest environment. This loss of trees represents a threat both because of the disappearance of veteran specimens and because of the alteration in the age structure of trees in a forest.

The inclusion of species linked to this habitat in national and international protection lists and catalogues reflects the situation of vulnerability faced by this group (Martínez de Murguia et al., 2007). In Europe, 17.9% (21.7% in Member States) of the saproxyl beetles evaluated (693 species) are classified as threatened (Critically Endangered, Endangered or Vulnerable). However, for almost a quarter of the species (168 species = 24.4%), there is not enough data and scientific information available for proper evaluation, which could considerably increase the real percentage of threatened species (Calix et al., 2018).

Given the close relationship between these insects and wood, changes in forests are directly reflected in this community (Alonso Román & Bahillo de la Puebla, 2022). If a veteran tree disappears, along with all the microhabitats it houses, life support for saproxyl insects is lost... and also for many other associated species! The presence of old trees is essential where the process of wood death occurs gradually, generating cavities, mulch, dry dead parts, etc. (Pagola-Carte, 2016).

The richness of saproxyl beetles depends on the quantity and quality of available dead or decaying wood, as well as on factors such as the age structure of the trees, the total number of specimens, their variable density and the continuity of the habitat (Alexander, 2008; Mendez Iglesias, 2009). This wealth decreases markedly in managed forests compared to pristine forests.

Pagola-Carte (2016)) points out other reasons why there are not as many threatened species in the canopy as in the saproxyl complex: on the one hand, extreme specialization, and on the other, the scarce dispersion capacity of many species of saproxyl beetles.

A representative example of this problem is the hermit beetle (Osmoderma eremita Scopoli, 1763), the largest ketonide in the Iberian Peninsula. Some individuals may not come out, even in an adult state, from the hollow where they developed (hence their common name, “hermit”). Their dispersion capacity is very limited: adults fly only a few hundred meters (100—300 m) (Micó et al., 2012), so that a population can be maintained for long periods in the same area, or even on a single tree! (Pagola-Carte, 2016). Therefore, their main requirement is a continuous or, at least, moderately fragmented environment.

Finally, it is worth noting the longevity of many species of saproxyl beetles. How is it possible to talk about longevity in insects? The question makes sense, considering the usual shortness of their adult life—a phase known as imago—which is also the most visible stage of their life cycle. In some groups, such as ephemeroptera (Ephemeroptera, from the Greek ephemeros = ephemeral, pteron = wing), the adult phase can last only a few hours or a few days.

However, in the case of some saproxyl beetles such as the flying deer (Lucanus cervus), the short life as an imago, of just a few weeks, contrasts with the prolonged duration of their larval stage, which can reach up to seven years. Although the hatching takes place in autumn, the imago does not emerge until the summer of the following year (López-Colón, 2000; Cortes-Fossati & Méndez Iglesias, 2021). During these years, the larva remains hidden inside old or felled trees, feeding on wood in an advanced state of decay. This is possible thanks to a symbiosis with bacteria capable of degrading cellulose, which inhabit a dilated posterior intestine of the larva (Mendez Iglesias, 2012).

In the case of rosalía alpina (Rosalia alpina), the complete biological cycle lasts between two and three years (Vives, 2000). This longevity is a key factor that increases the vulnerability of these species to “cleaning” or forest management tasks in forests and parks, when veteran trees are eliminated. Many of these interventions are likely to destroy trees that harbor larvae that are still developing.

The fact that many saproxyl species rely strictly on aging trees, with rotting parts, and on microhabitats such as decaying heartwood or tree humus deposits, further accentuates their fragility. This is also highlighted by the Red List of Saproxyl Beetles in Europe, which expressly recommends the conservation of veteran trees throughout the continent:
“Veteran trees should be preserved throughout Europe” (Calix et al., 2018).

Conclusions

In this article, we have briefly explored the fascinating world of saproxyl beetles, decomposing organisms that, although they live hidden, are home to some of the most beautiful and iconic species in our entomofauna. We have addressed their fundamental role in forest ecosystems, as well as the main threats they face.

One of the most significant threats is the current model of forestry and forest management, both in forests and in urban parks. It is not simply a matter of preserving uniform tree masses, but of understanding that a forest is a dynamic system, with different successive and structural phases. It should not only extend in space, but also extend over time. The organic matter contained in wood should ideally result in its natural decay.

A well-developed saproxyl complex requires large volumes of dead wood — fallen trunks, dead standing trees, stumps — as well as the presence of veteran trees, in which the decomposition process occurs slowly and gradually, generating microhabitats such as hollows, mulch and dry parts. Forest management based exclusively on production, without considering the spatio-temporal continuity of forest elements, leads to a significant reduction in biodiversity, especially with regard to saproxyl species.

Valuing wooded masses, especially outside protected areas, should not respond solely to an economic-extractive logic. It is essential to integrate into forest planning and management an ecological vision that recognizes the essential role that these beetles play in the recycling of organic matter and in the balance of forest ecosystems.

In addition, even today there is a clear lack of recognition of the importance of saproxyl species and their role in biodiversity and ecosystem functioning. Available information about their distribution, ecology and other characteristics remains scarce and fragmented. As Méndez Iglesias (2009) points out:

“The role of saproxylic agents in Mediterranean terrestrial ecosystems, and the details of the degradative succession of wood in those ecosystems, are equally unknown. This makes it impossible to assess the consequences for ecosystem functioning of the loss of saproxyl diversity. This gap should be filled to give a stronger basis to all saproxylic conservation efforts.”

The greater the number of unknowns surrounding a faunal group, the more difficult it is to evaluate, monitor and preserve it. There is still a long way to go to learn more about this fascinating group, which is key to the recycling of organic matter and to the stability of forest ecosystems.

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