B3) Plant-Insect Interactions

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

Alexandra Klein
Doctoral researcher: Nolan James Rappa (since 2019) & Riko Fardiansah (since 2022)

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

Background

Declines of insect diversity and biomass have recently become increasingly documented phenomena with strong declines in agricultural landscapes but also in forest ecosystems. While some work has focused on climate change as a driver of forest insect decline, relatively little has focused on how forest management practices influence insects. Hence, our subproject focuses on the linkages between forest structures and diversity, biomass and trophic interactions of insects at the stand and landscape scales.

In the first phase our PhD student Anna Knuff deployed modified flight interception/window traps to research plots and showed that (1) our modified traps sample a high diversity of insect taxa in forests and that (2) structural forest elements such as multi-layered forest vegetation can have strong effects on insects.

Our second PhD student Nolan Rappa used the insects sampled by Anna Knuff to analyse beetles at a higher taxonomic resolution. We found that alpha diversity and biomass of beetle functional groups responded differently to forest structural elements, and that structural elements characteristic of older forests greatly benefit both. Additionally, Nolan Rappa exposed standardized reed trap nests for cavity-nesting Hymenoptera to study trophic interactions of bees, wasps and their natural enemies. He also installed new wooden trap nests to test the differences of nesting rates by bees and wasps when different deadwood species are offered as nesting substrates. With these wooden nests, we aim to establish a trap-nest system for forests that can be used to study early wood decomposition processes by deadwood nesting insects.

The third PhD student will analyse inter- and intra-specific trait responses to forest retention at the stand and landscape scales using the existing flight interception and trap nest data. The PhD student will also collect additional data on multi-trophic bee/wasp networks in standing dead wood to analyse the food items collected by cavity-nesting bees and wasps.

Research questions and hypotheses

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 (1) Biomass, diversity, and functional diversity of insects at the scale of forest stands (plots) are explained by local forest structure and the surrounding landscape; (2) Biomass and functional diversity will increase with structural heterogeneity at the plot scale and be highest in connected forests; (3) 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; (4) Communities of cavity-nesting Hymenoptera will be different between stands with varying amounts and species of deadwood.

We will particularly test in the third phase the hypotheses that (5) The effects of stand-scale retention measures for insect communities and their trophic interactions are driven by their feeding habits/specialisation and by their inter- and intra-specific traits; (5) Cavity-nesting herbivore-hunting wasps are mainly feeding on forest herbivores/pests and are therefore strongly related to stand-scale retention measure; (6) Cavity-nesting bees feed on forest trees and herbs in the surrounding landscapes and are therefore more strongly related to open areas in the surrounding landscape than to plot-scale retention measures.

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.

In particular, in the third phase, we will:

1. Measure individual-based traits of bees and wasps to understand intra- and inter-specific responses to forest retention measures of reproductive traits such as number of brood cells and parasitized brood cells, but also body size, other morphological traits and sex ratio. With these data, we will determine how reproductive traits, overall trait diversity, offspring production and sensitivity to parasitism and predation respond to forest retention measures. We will also determine how reproductive trait variation explains the offspring production of individual bee and wasp nests, by reflecting the different ways bees and wasps navigate trade-offs between collecting food resources and protecting their larvae from parasitoids and predators. To do this, we will take advantage of trait diversity metrics specifically designed to accommodate individual-based data and reflect different mechanisms shaping diversity patterns through abiotic and biotic interactions (e.g. niche partitioning).
2. Continue the surveys of cavity-nesting bees, wasps and their natural enemies in our new woody trap nests representing standing dead wood. This will allow to analyse temporal and spatial dynamics of the insect trophic networks and community changes in regard to forest retention, habitat trees and landscape features.
3. Use a subset of freshly occupied wasp nests to identify the food items of cavity-nesting wasps feeding on herbivorous insects. With these samples, we will analyse trophic networks beyond bi-partite interactions using traditional DNA barcoding. Preliminary analyses shows that many of the prey items used by cavity-nesting wasps are forest pests. This will allow us to understand top-down/bottom up regulation processes to control forest pest insect populations.

B3 collaborates with the other B projects (B2-B6, B10) and D2 by identifying insects (abundance, biomass, diversity, trophic links). B3 will also contribute to analyse the soil seed bank of the understory vegetation (in cooperation with B2). Additionally, B3 aims to collaborate with C2 to understand the knowledge about the value of bees and wasps for forests from the perspective of forest managers and forest users. B3 also cooperate with B4 (functional connectivity) in the analysis of genetic connectivity of saproxylic Hymenoptera.

Findings

So far, we demonstrated that flight interception/window traps can be modified in a simple way to broaden the range of flying insect taxa to be captured. The shortcoming of most conventional trap types is their rather narrow taxonomic cover, making the use of several complementary collection methods necessary for comprehensive coverage of the insect fauna. This initial testing of our modified traps was necessary to justify the completeness of our sampling. We then used the overall abundance of trapped insects to show that it is positively related to multi-layered forest vegetation. At the community level, mean tree diameter and share of deciduous trees influence composition of insect taxa. Hence, increased habitat heterogeneity as in multi-layered, mixed-species stands increases total insect abundance. Tree species composition is of particular importance. We complemented this study by an exemplary investigation of the ecological mechanisms responsible for abundance and species richness of highly specialized invertebrate herbivores, i.e. gall-inducing arthropods to test whether abundance and richness of gall-inducing arthropods increases with plant richness. However, neither abundance nor species richness of gall-inducing arthropods, but the number of gall species found per plant species was related to plant richness. So, although in general higher insect abundance is positively associated to high stand-structural diversity of forest vegetation, it seems that no single structural element will be beneficial for all insect taxa simultaneously in managed forests due to varying habitat requirements among and within taxa. Hence in the second phase we analyzed specific taxa e.g. beetles at a higher taxonomic level and showed also for this insect order that different functional groups respond differently to stand-level forest structural elements related to their feeding habits. We are currently still in the process of preparing the cavity-nesting trophic interaction data to analyse how herbivore-hunting wasps, spider-hunting wasps and pollen-foraging bees respond to structural retention at the stand level.

ConFoBi-Publication by B3

Augustynczik, Andrey L. D.; Asbeck, Thomas; Basile, Marco; Jonker, Marlotte; Knuff, Anna & Yousefpour, Rasoul et al. (2020). Reconciling forest profitability and biodiversity conservation under disturbance risk: the role of forest management and salvage logging. Environ. Res. Lett., 15, 0940a3. www.doi.org/10.1088/1748-9326/abad5a.

Basile, Marco; Asbeck, Thomas; Jonker Marlotte; Knuff, Anna K.; Bauhus, Jürgen & Braunisch, Veronika et al. (2020). What do tree-related microhabitats tell us about the abundance of forest-dwelling bats, birds, and insects? Journal of environmental management, 264, 110401. www.doi.org/10.1016/j.jenvman.2020.110401.

Gustafsson, Lena; Bauhus, Jürgen; Asbeck, Thomas; Augustynczik, Andrey Lessa Derci; Basile, Marco & Frey, Julian et al. (2020). Retention as an integrated biodiversity conservation approach for continuous-cover forestry in Europe. Ambio, 49, 85–97. www.doi.org/10.1007/s13280-019-01190-1.

Knuff, Anna K.; Staab, Michael; Frey, Julian; Helbach, Jan & Klein, Alexandra‐Maria (2019). Plant composition, not richness, drives occurrence of specialist herbivores. Ecol Entomol, 44, 833–843. www.doi.org/10.1111/een.12767.

Knuff, Anna K.; Winiger, Nathalie; Klein, Alexandra‐Maria; Segelbacher, Gernot & Staab, Michael (2019). Optimizing sampling of flying insects using a modified window trap. Methods Ecol Evol, 10, 1820–1825. www.doi.org/10.1111/2041-210X.13258.

Knuff, Anna Katharina; Staab, Michael; Frey, Julian; Dormann, Carsten F.; Asbeck, Thomas & Klein, Alexandra-Maria (2020). Insect abundance in managed forests benefits from multi-layered vegetation. Basic and Applied Ecology, 48, 124–135. www.doi.org/10.1016/j.baae.2020.09.002.

Storch, Ilse; Penner, Johannes; Asbeck, Thomas; Basile, Marco; Bauhus, Jürgen & Braunisch, Veronika et al. (2020). Evaluating the effectiveness of retention forestry to enhance biodiversity in production forests of Central Europe using an interdisciplinary, multi-scale approach. Ecology and evolution, 10, 1489–1509. www.doi.org/10.1002/ece3.6003.