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B3) Plant-Insect Interactions

Diversity and functions of plant-insect interactions along a forest retention gradient

Alexandra Klein & Michael Staab
Doctoral researchers: Anna Knuff (2016 - 2020) & Nolan James Rappa (since 2019)

University of Freiburg, Faculty of Environment & Natural Resources, Institute of Earth & Environmental Sciences,
Chair of Nature Conservation and Landscape Ecology


In forests, arthropod communities and their diversity and trophic interactions are structured by various habitat elements and the surrounding landscape. For example, the diversity of most functional groups, including many predatory and parasitoid Hymenoptera, changes with vegetation diversity and forest structural diversity while forest cover and connectivity in a landscape stabilize the diversity and functions of insect communities. Thus, the retention of structural elements in forest stands (e.g. habitat trees, dead wood, microhabitats) and the composition and configuration of forests shape the diversity of insects and their functions.


Research questions and hypotheses

Project B3 addresses the overall hypothesis that stand-scale retention measures influence the diversity and trophic interactions of Hymenoptera (and other arthropods), which are each mediated by forest composition and configuration. In particular, we expect that:

  • Biomass, diversity, and functional diversity of insects at the scale of forest stands (plots) are explained by local forest structure and the surrounding landscape;
  • Biomass and functional diversity will increase with structural heterogeneity at the plot scale and be highest in connected forests; and
  • Trophic interaction networks of cavity-nesting Hymenoptera and their natural enemies will be more stable and redundant in forest stands with high amounts of standing deadwood;
  • Communities of cavity nesting Hymenoptera will be different between stands with varying amounts and species of deadwood


Approach, methods and linkages

B3 will analyze the relationship between components of Insect diversity (e.g. biomass, functional diversity) and environmental variables related to forest retention (e.g. deadwood, microhabitats). To quantify biomass and functional diversity of Insects, we will analyze already available specimens that were collected with flight interception traps. To quantify trophic interactions and food webs, trap nests for cavity-nesting Hymenoptera will be installed and monitored in all 135 ConFoBi plots. Using a suite of statistical techniques (e.g. mixed-effect models, network analyses), the data will allow us to evaluate how various components of tree retention and forest structure relate to Hymenoptera diversity and multi-trophic food webs, which are each mediated by the landscape context.

B3 provides data on arthropods and their trophic interaction for the ConFoBi data pool. B3 uses structural data from A1 and A2 and analyses biodiversity data jointly with all other B-projects. In particular, B3 will cooperate with B4 (functional connectivity) in the analysis of genetic connectivity of saproxylic Hymenoptera.



During the first three years, B3 has collected comprehensive data on all plots for all flying arthropods with flight interception traps, quantified plant-insect networks, and bird-fruit interactions. B3 has established a reference collection for Hymenoptera; analyses of associations between the diversity of selected Hymenoptera taxa and forest structural variables are ongoing.

Nolan Rappa currently focuses on biomass, functional diversity, and food webs of Hymenoptera and potentially further arthropod taxa. In his project, B3 will:

  • Extend the analysis of the Insect data collected so far by measuring biomass and functional diversity of specimens collected by his predecessor Anna Knuff;
  • Collect field data of arthropod-based food webs;
  • Assess interaction networks of cavity-nesting Hymenoptera with trap nests as well as deadwood nesting resources; and
  • Analyze all data in the context of forest structure and landscape variables, particularly deadwood availability.



In the future (years 7-9), B3 plans to synthesize the comprehensive Hymenoptera and general arthropod data. Also, we plan to extend the food web analyses to further systems and species communities. A promising study system is the brown food web in the leaf-litter, which is expected to be sensitive to a variety of retention measures but virtually unstudied.