title: Saproxylic insects in tree hollows : drivers of diversity, dispersal and food web structure
Creators: name: Schauer, Bastian
Publisher: name: University of Bayreuth
Year: 2019
Subjects: subject: Forest ecology
subject: dead wood
subject: forest management
subject: forest entomology
subject: 500
subject: 570

description: During the last centuries, intensive forest management has changed the characteristics of forests worldwide. This has caused a worldwide decline of old trees and deadwood structures, leading to a threat for saproxylic species, i.e. those species depending on deadwood at some point in their life cycle. Saproxylic species comprise about 25% of forest biodiversity and are important for various ecosystem functions e.g. wood decomposition and nutrient cycling. A very important key structure for saproxylic species and their diversity are tree hollows, contributing stable and long-lasting habitats for many threatened saproxylic insect species. Tree hollows enable undisrupted development for individuals of species with long developmental times and special habitat preferences. To develop effective management strategies maintaining the diversity of tree hollow associated insect species, evidence-based knowledge on environmental characteristics influencing their diversity, dispersal abilities, and food web structures have to be well understood. For this purpose, I collected all emerging arthropods from hollows with wood mould in beech trees over two consecutive years in a managed forest using emergence traps. 
In the first manuscript, I investigated characteristics of tree hollows and the immediate environment influencing the diversity of saproxylic beetles inside. Overall diversity was influenced by the degree of decomposition of the wood mould and increasing hollow entrance area. Additionally, tree hollow volume, height of the hollow above ground, surrounding deadwood structures and surrounding tree hollows, solar irradiation, temperature inside tree hollows and diameter of the tree hollows were influencing the biodiversity of saproxylic beetles, but their importance differed between the two years. Furthermore, tree hollow characteristics influencing the diversity of Red List and non-Red List species were different, as degree of decomposition of the wood mould, height of the hollow above ground and surrounding tree hollows had no influence on Red List species but did have on non-Red List species. This study showed that the diversity of saproxylic beetles inhabiting tree hollows can be supported by maintaining a high diversity of differently structured and sized tree hollows.
In the second manuscript, I investigated the genetic population structure of three saproxylic insect species (Anaspis ruficollis, Coleoptera; Criorhina floccosa, Diperta; Xylomya maculata, Diptera) inhabiting tree hollows to draw conclusions on their potential dispersal within the study area, using microsatellite analysis. Dispersal is a key trait for species in a changing environment, like managed forests. It allows colonization, gene flow among populations and thus ensures viable populations. These threatened species were chosen based on their relatively high abundance to allow for population genetic analyses. In contrast to the existing assumption, that saproxylic species inhabiting long lasting habitats have limited dispersal, we neither found genetic population substructure nor indications for restricted gene flow in all species investigated indicating dispersal over the whole study area. However, I studied relatively abundant species and our study site may not have been large enough to detect genetic substructure. 
The third chapter is based on a book chapter where we reviewed the current knowledge on the dispersal of saproxylic insects. We presented an overview of the studies that have to date tried to measure dispersal distances or dispersal abilities of different saproxylic taxa, methods to measure dispersal, factors influencing dispersal and highlighted open research questions. Dispersal abilities vary a lot between different saproxylic species and long-distance dispersal is not uncommon. We could stress that dispersal is not only a matter of the physical ability of a species but also of habitat quality and availability, feeding strategies, competition and sex-specific dispersal. 
In the fourth manuscript I investigated the food web structure of saproxylic beetle communities in tree hollows using stable isotope analysis of nitrogen and carbon isotopes. In this study, the trophic position (feeding guild) of most species is congruent between the classification based on their stable isotope signatures and literature derived classifications. Further, the isotopic signature of mycetophagous species suggests a highly omnivorous feeding behavior compared to other feeding guilds. Niche redundancy was high in the xylophagous and zoo-xylophagous guilds and niche conservation of saproxylic beetle families was common. Additionally, saproxylic beetle communities followed inverted Eltonian pyramids with zoophagous species contributing the highest biomass. However, considering all saproxylic arthropods in tree hollows and not only saproxylic beetles, no inverted Eltonian pyramids might be present anymore as organisms of lower trophic levels like collembolans comprise a high proportion of total biomass in tree hollow communities. Higher species richness was related to longer trophic food chains, highlighting the importance of high diversity of saproxylic species for ecosystem functioning. 
The recommendations based on the results of this thesis can help foresters, political decision makers and conservationists to apply effective management strategies for the conservation of saproxylic diversity.