Field work in the Biodiversity Exploratories

Recently, I had the pleasure to conduct extensive field work in the field sites of Biodiversity Exploratories located in Hainich-Dün (central Germany), Schorfheide-Chorin (north-east Germany) and Schwäbische Alb (south-west Germany). Together with my team, we measured the ecological processes seed removal, dung removal and predation in forest and grassland sites. Process measurements are part of a time series with the aim to test whether temporal variation in process rates is related to land use intensity. Furthermore, I sampled the newly established grassland experiments with suction sampling (‘biocenometer’) to get a quantitative picture of the arthropod community in the grasslands, and to test whether the reduction in fertilization, mowing and grazing is influencing species communities.

All three process measurements in one shot. Seeds are placed on a plastic tray, dummy caterpillars pinned on the ground and dung is placed on cellulose paper. After 48 h, missing seeds are counted, dummy caterpillars collected and remaining dung retrieved. (picture by Genny Walther)

In some forest sites dung beetles (particularly Anoplotrupes stercorosus shown here) are very common. Hardly any dung is remaining after 48 h. (picture by Robin Fuchs)

Myself sampling an experimental grassland site with the biocenometer in the Hainich region. All arthropods in a defined (and caged) square meter are sucked in our amazing custom-made machine. (picture by Genny Walther)

Phylogenetic diversity paper published

This has been a long way coming. I have been working on this project for almost four years. It has been a bumpy road with several detours, from which I have learned a lot. But it was certainly worth the effort, as the final paper now published in Functional Ecology (read the article here, it is Open Access) went out really well. For this work, I have synthesized a large dataset of arthropods and fungi from plots in the Gutianshan National Nature Reserve. Acquiring this database was a group effort to which very many people have contributed over the years. I have to thank them all.

The data for this study have been collected in the beautiful and diverse forests of Gutianshan.

Ecosystems with more plant species usually harbor more species of other organisms such as insects. However, counting species is just one of many ways to describe the diversity of ecosystems. Many other possibilities exist, among which the present study focuses on ‘phylogenetic diversity’ (PD), a statistical measure that takes account of evolutionary relatedness. In other words, if two ecosystems have the same number of species, the one where the species are more distantly related has the higher PD. Using a large dataset from subtropical Chinese forests, we tested whether the number of tree species present in a forest stand or the PD of those trees is the better predictor of the diversity and species composition of the associated insects, herbaceous plants and microscopic fungi. Following our expectations, we found that tree PD but not the number of tree species relates to the composition of the associated organisms. Also, there were more species of predatory but fewer species of herbivorous insects in forests with higher PD. While our study was necessarily limited in scale and scope, the results point to the important role of plant PD in maintaining biologically diverse species assemblages. As during global change often the evolutionary most distinct (plant) species disappear first, the consequent reduction in PD will likely have far-reaching consequences for entire ecological communities.

Conceptual overview of the core predictions this study is testing.

Exotic garden plants are well-integrated in plant-pollinator interaction networks

Exotic plants are ubiquitous in gardens, where they are usually deliberately planted, for example to achieve a continuous flowering over the entire growing season. As at least in Central Europe, relatively few native plants flower in the end of the season, exotic plants often dominate gardens in late summer and autumn.

In a study freshly published in Oecologia (Open Access Link), we (Michael Staab, Helena Maria Pereira-Peixoto, Alexandra-Maria Klein) investigated if seasonal changes in the availability of flowers from native vs. exotic plant species affect flower visits, visitor diversity and plant-pollinator interaction networks.

While flower visitor species richness decreased over the course of the season, gardens with higher proportions of flowering exotic plants relative to natives partly compensated the seasonal decrease in flower-visitor species richness. Plant-pollinator interaction networks were not influenced by exotic species. Thus, later in the season when only few native plants were flowering, exotic garden plants have at least partly substituted native flower resources without apparent influence on plant-pollinator network structure.

This research suggests that as long as exotic plants are appropriately managed and risk of naturalization is minimized, late-flowering exotic garden plants may provide floral resources to support native pollinators when native plants are scarce.

Seasonal changes in plant-pollinator networks and related quantitative network indices. (a) Exemplary bipartite plant-pollinator interaction networks from a single garden. Width of bars corresponds to flower cover per plant species in the lower and to the number of visits by each flower-visiting species in the higher level (species identities can be resolved with the numerical codes in Table S1 of the publication); width of arrows corresponds to the number of interactions between two species, with the most narrow bars and arrows indicating single interactions each (note that the number of interactions varied: April=229, June=20, August=33, October=13). Arrows narrowing from top to bottom indicate that a plant species was more often visited than expected solely from the cover of this plant among all plants. In turn, arrows that widen from top to bottom indicate relatively less visited plant species. For plants, light grey bars and arrows indicate interactions of native plant species and red of exotic species, respectively. Non-visited plant species are included but do not have any interactions. While in April, most flowers were from native plants, this changed over the season and in October all interactions in the exemplary garden were with exotic plants. Both, (b) the Shannon diversity of species interactions and (c) the linkage density of networks peaked in summer. Regression lines in (b) and (c) indicate the bootstrapped (n=1000) predictions of quadratic lmms (significant at p<0.01) with 95% CI (dashed lines).

New review on the relationship between tree diversity and natural enemies

Recently,  I compiled together with Andreas Schuldt (University of Göttingen) a review on the ‘enemies’ hypothesis for forests ecosystems. Our review paper has now been published open access in Current Forestry Reports (Staab & Schuldt 2020).

The ‘enemies’ hypothesis is a classical ecological concept postulating a positive relationship between plant diversity (and complexity) and natural enemies of herbivores, which, in turn, should increase top-down control in more diverse environments. While the ‘enemies’ hypothesis has abundant support from grasslands and agricultural ecosystems, outcomes from studies in forests are heterogeneous. Our synthesis of the literature suggests that the ‘enemies’ hypothesis does not unambiguously apply to forests. With trees as structurally complex organisms, even low-diversity forests may maintain a high degree of habitat heterogeneity and provide niches for predator and parasitoid species, blurring correlations between tree and natural enemy diversity.

Staab M, Schuldt A (2020) The influence of tree diversity on natural enemies – A review of the ‘enemies’ hypothesis in forests. Current Forestry Reports, online first. >> Open Access

Goodbye Freiburg, hello Darmstadt

After almost 7 years in Freiburg, I have recently moved to the Technical Universit of Darmstadt. I will work as a postdoc in the Ecological Networks group of Nico Blüthgen. There I have the great opportunity to join the Biodiversity Exploratories, a large research platform for functional (bio)diversity research funded by the Germany Research Foundation. At three sites spread over Germany (Schwäbische Alb, Hainich-Dün, Schorfheide) many research groups investigate a broad range taxa and ecosystem functions in forests and grasslands.

I am part of the Arthropod Core team headed by Wolfgang Weisser (Technical University Munich) and Nico Blüthgen (Technical University of Darmstadt) where a substantial part of my research will be dedicated to measuring ecosystem processes and synthesizing data collected in previous phases of the Biodiversity Exploratories.

Nevertheless, I will continue with some of my most exciting old projects and (hopefully) make regular updates here.

Paper recommended by F1000

Our paper ‘A tale of scale: Plot but not neighbourhood tree diversity increases leaf litter ant diversity’ published in the Journal of Animal Ecology has recently been recommended by F1000 as a highly important contribution in ecology. F1000 member Bernhard Schmid states that “This is a very important finding, because it demonstrates that tree diversity beyond the immediate neighbourhood can still affect sampling points within that neighbourhood, and in such cases, biodiversity effect should be evaluated at larger scales.” You can read the full recommendation here.

New PhD Position

I am recruiting a PhD student to work on the exiting project Sex determination and biomass allocation in response to habitat quality. The project is at the interface of ecology, entomology and evolutionary biology (PhD Position Insect Ecology Freiburg). Application deadline is 22 November 2019. The position is anticipated to start on 1 January 2020 (or as soon as possible thereafter) and is fixed-term for 36 months (until 31 December 2022). Payment is subjected to the German standard tariff (50% TVL13).

Project description

Changes in habitats do not only affect species diversity but also population structure within species. Evolutionary theory suggests that the ratio of females to males in offspring of sexually reproducing species responds to resource availability. In resource-poor habitats, it is expected that the share of the less costly sex (in insects usually males) increases, which may skew sex ratios and consequently affect population viability. Hymenoptera are due to their haplodiploid sex determination system exceptionally suitable organisms for studying sex ratios. The PhD student will investigate solitary bees and wasps with trap nests along a habitat gradient near Freiburg and test if local habitat quality influences resource allocation and sex ratios. Food resources will be quantified and foraging efficiencies will be measured. Furthermore, already existing data from a diverse geographical context will be synthesized. Taken together, this project at the interface of ecology, entomology and evolutionary biology will reveal if and how habitat quality influences sex ratios, which could be one mechanism for why insect populations are declining.

Your profile

• Excellent MSc in a relevant field (e.g. biology, ecology, conservation, entomology, evolution)
• Strong experience with statistical data analyses in R
• Interest in entomology and cross-disciplinary research
• Ability to work independently in the field and laboratory
• Experience with scientific writing in English; scientific working language is English; German language skills are required for field work
• Valid driving license class B
• Technical affinity (electronics, computer) is advantageous, as for parts of the experiments custom-made hardware will be build
• Basic knowledge of GIS systems is beneficial

Please submit your application in a single PDF document (maximum 5 MB) no later than 22 November 2019 to Dr. Michael Staab (michael.staab@nature.uni-freiburg.de), who may also be contacted for further information. Your application should include cover letter, short summary of research interests and experiences, CV, certificates, 1 page summary of the MSc thesis, and contacts of two potential referees. Interviews will tentatively be conducted at 9 December 2019. Note that the position is pending final funding approval and that application costs cannot be reimbursed.

‘A tale of scale’ or how ant diversity in the leaf litter is influenced by plot vs. neighborhood tree diversity

Cross-taxon diversity congruence has commonly been found in natural and experimental ecosystems. Usually the diversity of producers (i.e. plants in terrestrial ecosystems) begets diversity in other trophic levels. Yet, at which spatial scale such plant diversity effects are most pronounced has so far only rarely been tested. Tree diversity experiments such as the BEF-China Experiment, where the exact position and surrounding of every tree individual is known, are perfect systems to test for the scale-dependency of biodiversity effects.

In our recent study published in the Journal of Animal Ecology (Skarbek et al. 2019) we investigated whether tree diversity at the larger plot-scale or the smaller neighborhood-scale (i.e. the 8 trees directly surrounding a focal tree) influences different components of leaf litter ant diversity.

Conceptual appraisal of the study design. In tree monocultures (a & b), plot and neighbor tree diversity are identical (and 1) while in more diverse plots (c & d) tree diversity depends on the spatial scale considered. Illustration from Skarbek et al. (2019).

Ants were sampled with Winklers: 1 m² of leaf litter was collected around each focal tree and subsequently sifted and place in Winkler bags. Photograph by Merke Noack.

As leaf litter ants are small organisms with a limited foraging range, we expected that relationships between ant and tree diversity would be strongest at the neighbor scale. However, we found exactly the opposite: more species occurred in plots with higher tree diversity and this result was independent of neighborhood tree diversity. While the exact causal mechanisms for this find remain elusive (have a look at the paper for some possible explanations), we indicate that even for small organisms biodiversity effects really only start to show up at larger spatial scales.

In this picture of a sample (by coincidence from the highest tree diversity level) there are species from the genera Aenictus, Carebara, Plagiolepis, Solenopsis and Temnothorax. Photograph by Merle Noack.

New papers to celebrate

As indicated a few weeks ago, several exiting new papers have just been published. Franca Bongers and coauthors (including me) could show that the influence plant traits have on individual tree growths are stronger (i.e. trees acquire biomass faster) when tree diversity is high. Interestingly, diversity effects where not detectable at the individual neighborhood but only at the larger community scale. The paper is now published in Journal of Ecology (check it out) and was led by Xiaojuan Liu, and I am very happy to contribute to this intriguing research.

The spotlight of the day, however, is on Anna Knuff (who is co-supervised by me): the first two papers from her PhD research conducted in the ConFoBi project are published. Congratulations! In her first manuscript, Anna used plant galls as a study system to study how specialist herbivores are related to the environment. The paper has been published in Ecological Entomology and indicates that the community composition of gall inducing arthropods is mostly influenced by plant composition (check it out).

In here second paper that has been published just now in Methods in Ecology and Evolution (check it out) we systematically tested a modification of conventional flight interception traps (i.e. window traps). The aim was to increase their sampling efficiency for our favorite insect order, the Hymenoptera. Due to their flight behavior Hymenoptera are often not collected quantitatively with flight interception traps. With a simple but effective modification we could dramatically increase sampling efficiency, not only for Hymenoptera but for many other insect groups (e.g. Diptera, Auchenorrhyncha ). These modified window traps are easy to build and cheap, and may have potential for standardized assessment of insect communities in biodiversity monitoring and conservation.

Modified window trap supplemented with top collection unit. Two transparent crossed windows serve as flight barrier. Funnels beneath and on top of the windows connect windows and collectors and direct arthropods into the respective collectors.

Bottom and top collection unit capture distinct insect assemblages. The figure illustrates a NMDS ordination of order composition per trap separated in catches per bottom and top collection unit. Illustrations by Anna Knuff taken from Knuff et al. 2019 Methods in Ecology and Evolution.

 

Impressions from the BEF-China Experiment

I am just back from a two week visit to China. After a few days of meetings in Beijing, I had the chance to see the BEF-China Experiment in Xingangshan, where I had conducted parts of my PhD (e.g. Staab et al. 2015) and postdoc research (e.g. Staab et al. 2017, Schuldt et al. 2017). The trees are so huge now, growth since my last visit in 2015 was immense. A real forest is now standing on the plots, the canopy is closed, trees are interacting and animal life is plentiful. It just looks amazing and I am sure many more exiting findings will arise from this large-scale experiment in the closer future.

I am particularly grateful to Dr. Liu Xiaojuan for her support. We (and Dr. Zhang Naili) have a newly established collaborative project on the effect of trees with extrafloral nectaries, which gave me the chance to come back to Xingangshan. My student Stefanie Pietsch will conduct her thesis research within this project and will collect arthropods from extrafloral nectar trees and control trees in the next weeks. We are all really looking forward to the results, which might help to disentangle the role of trees with extrafloral nectaries in early successional forests.

The trees are really huge now. The last time I saw it, this Choerospondias axillaris was around 3 m tall. Things have changed…

The canopy in many plots is fully closed. Trees are competing and interacting. A real forest, even though the experiment was established only 10 years ago. I definitely look forward to future research in the BEF-China Experiment.

The BEF-China Experiment has also been set in stone.

As always, the food was amazing and the people were incredibly friendly and helpful. I am very grateful to Dr. Liu Xiaojuan (the lady to my left) for her continuous support and the fruitful cooperation.