Antoine Brin

Species variables or environmental variables as indicators of forest biodiversity: a case study using saproxylic beetles in Maritime pine plantations

Abstract• To assess the sustainability of plantation forest management we compare two types of biodiversity indicators. We used the species richness of saproxylic beetles as a case study to test the “species” and “environmental” indicator approaches. We compared single species abundance or occurrence and deadwood volume or diversity as predictor variables.• Beetles were sampled with flight interception traps in 40 Maritime pine plantation stands. The volume and diversity of deadwood was estimated with line intersect and plot sampling in the same stands. Predictive models of species richness were built with simple linear or Partial Least Square regressions.• Deadwood variables appeared to be good predictors of saproxylic beetle richness at the stand-scale with at least 75% of variance explained. Deadwood diversity variables consistently provided better predictive models than volume variables. The best environmental indicator was the diversity of deadwood elements larger than 15 cm in diameter.• By contrast, the use of “species variables” appeared to be less relevant. To reach the quality of prediction obtained with “environmental variables”, the abundance or occurrence of 6 to 7 species — some of which are difficult to identify — had to be used to build the indicator.Résumé• Pour améliorer le suivi de la gestion durable des forêts de plantation, nous avons cherché à développer des indicateurs de biodiversité en prenant pour exemple la richesse en Coléoptères saproxyliques. Nous avons comparé l’approche directe, basée sur l’abondance ou l’occurrence de certaines espèces de coléoptères saproxyliques, et l’approche indirecte, basée sur les caractéristiques du bois mort comme variables prédictives.• Les Coléoptères ont été inventoriés à l’aide de pièges à interception dans 40 peuplements de pin maritime. Le volume et la diversité des pièces de bois mort ont été estimés à l’aide d’un échantillonnage par transects ou par placettes dans les mêmes plantations. Les modèles prédictifs ont été construits à l’aide de régressions linéaires simples ou PLS.• Les variables locales de bois mort apparaissent comme de bons prédicteurs de la richesse spécifique en coléoptères saproxyliques, avec jusqu’à 75% de la variance expliquée. Les variables de diversité du bois mort semblent de meilleurs prédicteurs que les variables volumiques. Le meilleur indicateur « indirect » pourrait être la diversité des pièces de bois mort de diamètre supérieur à 15 cm.• En revanche, le recours à des indicateurs « directs » semble moins prometteur. Pour obtenir une qualité de prédiction équivalente à l’approche indirecte, il faudrait prendre en compte l’abondance ou l’occurrence d’au moins 6 ou 7 espèces dont certaines sont difficilement identifiables.

Implications from large‐scale spatial diversity patterns of saproxylic beetles for the conservation of European Beech forests

Abstract.  1. European Beech (Fagus sylvatica) is the natural dominant tree species in many forests across Europe. Despite Europe’s global responsibility for these forests, the correct conservation strategies are still a matter of debate. In particular, it remains controversial whether high conservation efforts should be directed towards beech forests, owing to the small number of insects that are Fagus specialists, and at what spatial scale conservation should take place.

Are stumps important for the conservation of saproxylic beetles in managed forests? – Insights from a comparison of assemblages on logs and stumps in oak-dominated forests and pine plantations

Abstract.  1. The expanding interest in harvesting stumps for bioenergy may represent a potential threat to forest biodiversity. Whereas stumps are common in managed stands, knowledge of their associated saproxylic assemblages is still incomplete.

Habitat requirements of the violet click beetle (Limoniscus violaceus), an endangered umbrella species of basal hollow trees

Organisms associated with dead wood (i.e. saproxylic species) are threatened by intensified forest and agricultural management. For a majority of saproxylic beetles the detailed information on their habitat requirements is still lacking, thus hindering efficient conservation information. This also applies to the violet click beetle (Limoniscus violaceus), one of the umbrella species of European saproxylic beetle fauna. This specie needs basal tree hollows for its larval development. We studied habitat preferences of L. violaceus in basal tree hollows using emergence traps installed on 208 trees located in five sites in France. We used logistic regressions and recursive partitioning methods to understand the species–habitat relationships and to identify key environmental variables to predict the occurrence of L. violaceus. The probability of occurrence of L. violaceus in basal hollows increases with increased tree circumference at 30 cm above ground and with increased hollow decay stage. The analysis of threshold values shows that the occurrence of the beetle is relatively high in trees that have a circumference greater than 360 cm, uncertain for those between 235 and 360 cm, and improbable for circumferences lower than 235 cm. Similarly, the occurrence of L. violaceus was probable in advanced stages of hollow decomposition and improbable in early stages. Our results provide non‐specialists with helpful habitat features of an umbrella species which is hard to detect. This will increase the ability of conservation managers to identify sites that deserve protection and better monitor the habitat of many saproxylic species associated with basal hollow trees.

Influence of sampling effort on saproxylic beetle diversity assessment: Implications for insect monitoring studies in European temperate forests

Saproxylic beetle diversity monitoring provides a tool for estimating the efficiency of forest conservation measures. Flight interception traps are commonly employed to monitor beetle assemblages, although little explicit knowledge of the efficiency of this trapping method is available. The present study investigated how slight changes in sampling effort can influence species richness and species composition of assemblages in data sets from standard window‐flight traps. At both trap and plot levels, an additional year or an additional trap provided a 50% increase in the number of species detected (a 75% increase for rare species) and resulted in a different estimated composition of the assemblages. Adding 2 or 3 years of sampling gave twice as many species and resulted in assemblages that were 50% dissimilar. Increases in the detection of species and the dissimilarity of assemblages were similarly affected along a gradient of forest conditions, suggesting that changes in sampling effort were not affected by forest condition. At the forest level, year or trap replication provided smaller increases in species richness (31% and 25%, respectively). Within sites, distance measures in species composition between traps did not differ significantly when based on 1 or 2 years of data. Using two traps per plot compared with one trap influenced comparisons between stand types, based on species richness, in 25% of the cases. Species detection was similarly increased by either year replication or trap replication. The results of the present study highlight the significant role played by finescale patterns of habitat structure and inter‐annual variation with respect to determining catch size and assemblages of saproxylic species.

Biotic Interactions Between Saproxylic Insect Species

A better understanding of biotic interactions in species-rich saproxylic insect communities can provide essential information for biodiversity conservation and ecosystem functioning enhancement. Evidence in the literature mainly relates to beetle species, in particular scolytines, at tree—or even smaller—spatial scales and mostly refers to antagonistic interactions. We here present an overview of competition, predation/parasitism and facilitation among saproxylic insects. We first underline segregation patterns between wood consumers, resulting from competition processes, such as spatial and temporal resource partitioning, competitive displacement via interference and even enemy-mediated “apparent competition.” Considering natural history facts about prey-predator and host-parasitoid relationships, we then emphasize processes regulating the pressure of top-down influences on prey/host population dynamics. Facilitative interactions, including mechanisms of habitat location, creation, and improvement, are thereafter considered. The implications of some findings for pest management strategies (biocontrol, semiochemical-based methods) and for ecosystem functioning (deadwood decomposition) are highlighted meanwhile. Approaches based on life-history traits or indirect mediated interactions finally move the focus from the responses of paired species to multispecific community-level changes. Ecological network analysis should help increase our understanding of biotic interactions and investigate the consequences of environmental changes for those interactions and ecosystem functioning.