Forests cover almost one-third of the global land area and host most of Earth’s terrestrial biodiversity. In Europe, approx. 65% of plant and animal species are located in forest habitats [1]. In particular, arthropods comprise 70%–90% of taxa and dominate the animal biomass in forest ecosystems [2]. Forest biodiversity not only responds to environmental changes but also promotes various ecosystem functions and has a positive relationship with most ecosystem services, which are essential for sustaining human welfare ^[3][4][3,4]. Hence, forest biodiversity can be considered a key forest resource. Despite the fact that old-growth and natural forests are the most valuable for biodiversity [5]—especially for insect biodiversity ^[6][7][6,7]—only a few are left standing, unfragmented, and unaltered ^[8][9][8,9]. On the other hand, it is clear that strictly protected areas alone are insufficient for the achievement of global biodiversity conservation targets ^[10][11][10,11], and conservation management practices must be considered in managed forests as well.

One-third of the world’s forest area is used primarily for timber production [12], and forest management regimes have altered natural forest ecosystems over time at varying intensities, altering forests’ biodiversity. Unlike natural forests, commercial forests—which make up most forests in Europe—are characterized by low biodiversity [13]. Managed commercial forests do not grow as old as natural forests, as they are generally harvested at the point of economic maturity, with yield and market-based criteria as the main considerations. Consequently, later phases of forest succession, with characteristics of late development, degradation, or stand breakdown—development phases that hold a rich diversity of rare niches and species—are either lacking or rare. Therefore, lower biodiversity of canopy trees and other species reduces the ability to provide certain ecosystem services.

Biodiversity in managed forests depends on the continuity of habitats, the variability of site conditions and tree species, and forestry that mimics natural dynamics. Even though forest cover in the European Union has increased in recent decades, major pressures represent a risk to vulnerable forest species and habitats. Pressures from human activities leading to fragmentation and degradation have already caused much decline and homogenization in biodiversity ^[14][15][14,15].

Saproxylic beetles—insects that range widely in size (from <1 mm for families such as Ptiliidae and Ciidae to >150 mm for certain Scarabaeidae and Cerambycidae) and depend on dead and decaying wood, wood-decaying fungi, or other saproxylic organisms for at least a part of their lifecycle [16]—are a classical example of a group that has been adversely affected by forest management practices ^[17][18][17,18]. Saproxylic beetles are common in all types of forests and woodlands worldwide ^[19][20][19,20], but fragmented and degraded habitats ^[21][22][21,22], along with the decline of deadwood in managed forests, have led their populations to a decline in recent years. Saproxylic beetles are considered to be bioindicators for the maturity and ecological stage of their characteristic habitats ^{[23][24][25][26]}[23,24,25,26], as they are involved in decomposition processes and the recycling of nutrients in natural ecosystems ^[27][28][27,28], and they interact with other organisms by providing an important food source for birds and mammals [29] and contribute to forest pest control through the action of saproxylic predators on primary xylophagous beetles (i.e., Scolytinae) ^[30][31][30,31]. However, they also include several economically important groups such as bark and ambrosia beetles or wood-boring beetles, which can be major forest pests [32].

Saproxylic beetles can be considered either as beneficial for the forest ecosystem, or as forest pests. Conservation of saproxylic organisms usually requires the designation of protected areas or, at least, substantial changes in forest management practices that result in reduced logging and the preservation of large numbers of dying trees and large amounts of deadwood in forests [24], as well as a conflict with timber production [17]. Furthermore, these conservation measures are not always properly implemented, as can be easily seen from the distribution of rare and threatened deadwood-related species, which are usually absent from managed stands. On the other hand, several species of saproxylic beetles are referred to as “pests” [33]. These species are generally considered to be harmful to trees, as their outbreaks weaken the trees and lead to the development of diseases (e.g., fungal infections), usually resulting in the death of infested trees and/or loss of large forest areas. This group is considered to be one of the most serious problems in timber production and is the reason for many studies aimed at finding methods to prevent or mitigate such outbreaks [34]. Forest management neglects the natural role of these common saproxylic species in forest ecosystems. These organisms are natural elements in the life of trees [35].

2. Major Threats

The main reasons that have led to the decline of saproxylic beetles’ populations are reduction in their habitats’ area [21] are fragmentation and the loss of connectivity between habitats ^[22][36][22,137], along with the decline of deadwood in forests, in terms of both quantity ^[37][138] and quality ^[38][139]. In particular, the absence of large-diameter deadwood in managed forests and changes in disturbance dynamics (such as the prevention of forest fires and the removal of trees from storm-damaged areas) can be seen as major factors driving many saproxylic species to the edge of extinction. Generally, for forest insects, several studies indicate that the number of species does not differ much between managed and unmanaged forest stands, whereas the population levels of certain species differ markedly ^[39][140]. Saproxylic beetles are less abundant in mature stands ^[25][40][41][25,141,142] versus much older ones ^[42][143], and early successional stands are richer in primary saproxylics and/or bark dwellers ^[43][144]. Hence, saproxylic beetles can be naturally abundant in unmanaged forests, but are mostly rare in long-managed forests and, especially, in monocultures. Despite the fact that some intensively managed forests (e.g., coppice forests) can have high conservation value ^[44][145], forest management is the most important driver of saproxylic beetles’ diversity and is considered to be their main threat, since they demonstrate sensitivity to timber-harvest practices ^[45][46][146,147]. Forest management practices that drastically reduce the quantity and quality of deadwood available, or alter the habitat structure—usually based on clear felling with site preparation and subsequent planting or seeding, followed by a few thinning operations during the rotation cycle—can be a major threat to these communities ^{[18][37][47][48]}[18,138,148,149]. Deadwood plays a major role in forest ecosystems, as it stores carbon, nutrients, and water, influences soil’s development and regeneration by reducing erosion, and acts as a reservoir of biodiversity by retaining complex trophic chains and providing microhabitats that host a broad diversity of organisms, including saproxylic species ^{[16][20][29][37][49][50][51]}[16,20,29,44,138,150,151]. It has been estimated that deadwood-related biodiversity alone represents about 30% of the global forest biodiversity ^[52][152], reaching 50% in groups such as beetles ^[53][153]. Depending on the forest type, deadwood quantities ranging from 20 m³/ha to 50 m³/ha have been identified as a threshold to maintain the majority of saproxylic species, while very demanding species require more than 100 m³/ha ^[54][154]. Even though all of the deadwood is important, the size of the deadwood seems to be relevant, since the larger the size of the debris, the higher the environmental suitability for saproxylic insects [20]. This is because a larger diameter and, therefore, a greater volume—or a combination of a large diameter with a significant length—can increase the heterogeneity of available microhabitats and, therefore, the number of potential ecological niches, allowing the more specialized organisms to occupy the same space at the same time [29]. In addition, large fragments take longer to decompose and maintain a more stable microclimate within them, while fragments with greater surface area and volume can support more diversified and consistent fungal communities, to which numerous species of saproxylic insects are linked ^[20][29][20,29]. However, studies have evidenced that high-quality and abundant decaying parts of still-living trees, such as relatively small wood from the dead branches of still-standing trees, can also host peculiarly rich saproxylic fauna—sometimes even richer than that of large fallen trees and logs [29]. Sun exposure is known as an important factor for saproxylic insect species, and many species are primarily associated with Sun-exposed environments ^[55][56][57][155,156,157]. However, the importance of Sun-exposed sites may have been overestimated, since invertebrates tend to be more active in Sun-exposed conditions ^[58][158]. Species adapted to Sun-exposed deadwood may be particularly vulnerable ^[46][147]. Thus, for saproxylic beetles, high stem densities in managed forests can be inappropriate, while shade and low temperatures may render deadwood unsuitable as a habitat ^[59][159]. Given the fact that most of these species have low dispersal capability ^[50][60][150,160], spatial and temporal breaks in habitat continuity can lead to population declines and extinctions. Habitat heterogeneity is widely recognized as an important factor driving saproxylic beetles’ assemblage, and maintaining their habitats’ continuity is crucial ^[61][161]. The prevention of forest fires can also affect saproxylic beetles’ populations. Fire rapidly changes the species composition ^[62][162] and functional diversity of saproxylic beetles ^[63][163] and increases their species richness ^[64][164]. Fire also supports open-habitat-associated species, including most rare saproxylic species ^[65][66][165,166], as it contributes to the increasing amount of deadwood ^[67][167]. However, fire itself, e.g., the burning of rough hillsides to refresh the pastures for grazing and to suppress scrub development, can result in the early death of trees and suppress natural regeneration, which can pose a threat to isolated and vulnerable populations of beetles such as B. splendens in the Mediterranean, while fire suppression is a major threat to many boreal beetles, which need the resulting burnt wood ^[68][53].