Changes in community assembly processes after strong disturbance

We examine the effect of forest land use on the structure of and trophic interactions in soil animal communities. The goals of this project are two-fold: First, we characterise the animal communities of the soil and litter layer in differently managed forests and record shifts in abundance, diversity and biomass over time. We then use these data to study trophic relationships of key species, their trophic position and food resources.

Hypotheses:

• Community assembly in both Collembola and Oribatida is based predominantly on environmental filtering but this is more pronounced in forest gaps than consolidated forests.
• Trait-based approaches of Collembola and Oribatida community assembly are more powerful in forests than in (disturbed) gaps, but increase in gaps with time.

Objective:

• To understand how soil animal communities assemble, phylogenies for important species of mesofauna decomposers, the Collembola and Oribatida, will be established. To deepen our understanding of the mechanisms responsible for community assembly, the phylogenetic approach will be combined with trait-based approaches. Traits that will be both drawn from the literature and measured as part of the project include general morphological traits such as body size and sclerotization, physiological traits such as desiccation resistance and mobility, and trophic traits such as trophic position and trophic plasticity (as measured in earlier phases of LitterLinks using stable isotopes). Traits will also be analyzed for phylogenetic signals to determine if trait values can be predicted by relatedness.

Methods:

• Phylogenetic relationships are reconstructed using Bayesian inference as well as the maximum likelihood method. Phylogenetic relatedness of coexisting species in local communities will be analyzed based on the mean pairwise phylogenetic distance (MPD) and mean nearest taxon distance (MNTD) of local communities; this will form the basis for deriving the Net Relatedness Index (NRI) and Nearest Taxon Index (NTI).

Our project LitterLinks examines the community composition and trophic structure of soil animals, and their microbial resources as related to variations in environmental factors, including forest types and land-use intensity.

• We showed that species composition, abundance and biomass of soil animal communities are strongly influenced by abiotic factors such as soil pH or climatic conditions (Bluhm et al., 2016; Pollierer & Scheu, 2017), pointing to the importance of regional environmental conditions, while soil animal communities are surprisingly resistant against changes in forest land-use intensity (Pollierer et al., 2021).
• Using a comprehensive toolbox of methods, such as bulk stable isotope analyses (Erdmann et al., 2012; Klarner et al., 2017), fatty acid analyses (Ferlian & Scheu, 2014; Ferlian et al., 2015), molecular gut content analyses (Günther et al., 2014) and compound-specific isotope analyses of amino acids (Pollierer & Scheu, 2021), we showed changes in energy fluxes from basal resources to animal consumers in different forest types at the level of species.
• Energy fluxes through microbial channels were more pronounced in coniferous compared to beech forests, with most mesofauna species feeding on saprotrophic and not ectomycorrhizal fungi (but see Bluhm et al., 2019). However, despite variations in leaf litter and microbial community composition (Pollierer et al., 2015), trophic niches of soil animal species were consistent across different forest types, presumably due to feeding at specific microsites (Ferlian et al., 2012; Pollierer & Scheu, 2021).
• Deprivation of root-derived resources in a trenching experiment heavily decreased microbial biomass as well as abundance and biomass of soil animals, in particular mesofauna, indicating their major role in fuelling soil webs (Bluhm et al., 2019; 2021). However, only minor changes in community composition of soil microbes and animals suggest non-specific links to root-derived resources.
• Already one year after forest gap formation we could detect changes in the abundance of meso- and macrofauna. While the abundance of macrofauna decomposers such as isopods, diplopods and earthworms was reduced in forest gaps, the effects of forest gap formation on oribatid mites as important mesofauna decomposers were influenced by regional dynamics. In Hainich, there were only minor effects, whereas in the Swabian Alb, abundances were reduced in forest gaps; however, this could be mitigated by addition of deadwood (Junggebauer et al. in prep). This indicates that taxonomic and regional differences must be taken into account when considering the effects of forest gap formation.
• Our long-term studies of soil animal abundance, biomass, and community composition suggest that, unlike in aboveground arthropod communities, there is not a general declining trend in soil animal abundance and biodiversity. Instead, soil animals are more likely to be affected by fluctuations in environmental conditions such as temperature and precipitation (Pollierer et al., Junggebauer et al., Jueds et al. & Klarner et al. in prep).