B3) Diversity and functions of plant-insect interactions along a forest retention gradient
Alexandra Klein Michael Staab & Anna Knuff
University of Freiburg, Faculty of Environment & Natural Resources, Institute of Earth & Environmental Sciences, Chair of Nature Conservation and Landscape Ecology
Insect communities and their interactions with plants are structured by habitat elements of ecosystems and their surrounding landscapes. For example diversity of some functional groups such as predatory ants changes with vegetation diversity and density at the scale of forest plots(1) while the amount of forest in a landscape increases the diversity and functions of insect communities relying on forest(2). Hence the retention of structural elements in forest stands (habitat trees, dead wood) and their surrounding landscapes (amount of forest) shape the diversity of insects and their functions.
From research on the effectiveness of management practices and habitat enhancement to increase insect diversity and ecosystem functions in agricultural landscapes(3) general hypotheses were formulated(2): for example the intermediate landscape complexity hypothesis describes that management at the habitat scale aiming to enhance biodiversity and ecosystem functions are most effective in landscapes of medium complexity. How landscape complexity and plot-scale habitat elements interact and influence insect diversity and related functions in forest landscapes is not known.
Study questions and hypotheses
We study insect diversity (in particular the order Hymenoptera) and different forms of herbivory (chewing, mining, leaf-sucking, gall-building and fruit removal) along the two non-confounding gradients of the amount of habitat trees and dead wood at plot scale and the amount of forest cover at landscape scale of the ConFoBi research platform to test the following main hypotheses:
- Diversity of parasitic Hymenoptera, especially of wasps laying their eggs in old and dead wood will increase with increasing amount of habitat trees and dead wood.
- Overall herbivory will not be influenced by the amount of habitat trees and dead wood or the amount of forests in the landscape as functional groups of plant-herbivore interactions will respond differently: Fruit/Seed removal by birds will be positively influenced by both the amount of habitat and dead trees and the amount of forest in the landscape while gall-forming herbivory will be negatively influenced through the better control provided by parasitic Hymenoptera.
- Enhancement of biodiversity and insect-related functions by retention management at the forest plot scale will be influenced by the complexity of the surrounding landscapes according to the “intermediate landscape complexity hypothesis(2)“.
Approach and methods
Two flight interception traps are exposed in all 135 forest plots for 8 weeks in spring to summer and emptied and refilled at fortnightly intervals (sum of 1080 catches) (4). Insects are identified to species/morphospecies level to analyse diversity patterns. Herbivory is assessed to the functional types of herbivory (leaf-chewing, mining and galling, sucking) of ten randomly selected tree individuals of tree species in all plots in spring and summer using defined leaf damage categories (4).
Fruit/seed removal by birds is estimated by exposing artificial fruits made from clay for 4 days to calculate the proportion of fruits with peck marks (5). Plant galls are collected in all 135 forest plots in spring to summer from the understory and the trees to rear the gall-forming insects and their parasitoids in the lab to build up knowledge of the gall-forming community and their parasitoids (Hymenoptera) for further investigations in the second phase.
We deliver comprehensive insect diversity and functions assessments to study the effectiveness of forest retention practices in relation to landscape complexity. Data on insect diversity is collected jointly with B4. We will deliver data on moth abundance to B5 and seed predation by birds and its relation to bird communities is jointly analysed with B6. The biodiversity data is delivered to C1 for economic evaluations.
- Staab, M., Schuldt, A., Assmann, T., Klein, A.M. 2014. Tree diversity promotes predatory but not opportunistic ants in a subtropical Chinese forest. Forest Entomology 39:637-647.
- Tscharntke et al. 2012. Landscape moderation of biodiversity patterns and processes – eight hypotheses. Biological Reviews 87:661-685.
- Scheper, J., Holzschuh, A., Kuussaari, M., Potts,S.G., Rundlof.M., Smith, H.G., Kleijn, D. 2013. Environmental factors driving the effectiveness of European agri-environmental measures in mitigating pollinator loss – a meta-analysis Ecology Letters 16:912–920.
- Schuldt, A., M. Baruffol, M. Böhnke, H. Bruelheide, W. Härdtle, A.C. Lang, K. Nadrowski, G. von Oheimb, W. Voigt, H. Zhou, and T. Assmann. 2010. Tree diversity promotes insect herbivory in subtropical forests of south-east China. Journal of Ecology 98:917-926.
- Galetti, M., Alves-Costa, C.P. & Cazetta, E. 2003. Effects of forest fragmentation, anthropogenic edges and fruit colour on the consumption of ornithocoric fruits. Biological Conservation 111:269-273.