Impact of ecological filters within habitats
Our study design aimed to investigate the impact of ecological processes
on soil food web structure across altitudinal gradients and diverse
environmental conditions. Although the GDM explained a relatively low
percentage of variance when quantifying changes in the structure of
local food webs (Fig. 5a), it’s important to note that typical deviance
explained for models of compositional dissimilarity ranges from 20% to
50%, and this percentage can decrease with an increasing number of
sites, which was relatively high in our study (Ferrier et al.2007; Mokany et al. 2022). Furthermore, the resolution of our
soil food webs using trophic groups instead of species led to low
average dissimilarities (0.27 and 0.48 in forests, and 0.30 and 0.53 in
grasslands for groups and interactions respectively), reducing model
predictability (Mokany et al. 2022). Importantly, we found that
trophic link structure significantly influenced changes in trophic
groups abundance composition (Fig.4). However, current models cannot
account for this factor, and incorporating network structure in
diversity models may enhance predictability but requires further
implementation (Poggiato et al. 2022).
Spatial processes, environmental filtering, and changes in plant
community composition explained variation in food web structure within
habitats. In forests, spatial distance played a significant role,
explaining a substantial portion of trophic group (31%) and interaction
dissimilarity (58%), even when accounting for confounding environmental
changes. Limited dispersal opportunities in closed forest habitats,
particularly for organisms dispersed through wind (anemochory), might
influence soil food web structure in mountain forests. Conversely, open
grassland habitats subject to strong winds (Tackenberg & Stöcklin 2008)
and high levels of herbivory, both domestic and wild, can facilitate the
dispersal of soil organisms. Furthermore, mountain massifs and large
distances between forested areas (up to 250 km) can impact meso and
macro fauna, susceptible to ecological drift due to their limited
dispersal capacity or smaller population sizes (Arribas et al.2021; Kang et al. 2022). This is in line with our results showing
that food web dissimilarity increased with greater spatial distances
(Fig.5b). Yet, the spatial distance used in the model may not fully
capture the complexity of spatial, and further investigation using
experimental setups to clarify spatial connectivity and dispersal
limitation effects is required.
Environmental filtering played a significant role in both habitats,
primarily influenced by soil edaphic properties. In the French Alps, the
soil C/N ratio emerged as the most influential abiotic factor in shaping
soil food web composition. It reflects organic matter decomposability
and resource availability, as discussed above, impacting decomposer
groups, their consumers, and the entire feeding channel Conversely,
variables related to total energy or resources, such as the NDVI and
SOM, had a smaller impact on soil food web dissimilarity. While this
contrasts with previous studies emphasizing the role of these variables
in diversity patterns of soil groups in mountains (Zinger et al.2011; Calderón‐Sanou et al. 2022), it highlights the emergence of
different ecological drivers at different levels of biodiversity
organization. In our study, resource quality, particularly litter
quality, seemed to dominate food web composition rather than quantity.
Additionally, environmental factors associated with harsh conditions,
such as pH and frozen degree days, known drivers of soil taxonomic
diversity and composition (Fierer & Jackson 2006; Decaëns 2010), also
played a major role in shaping soil food webs.
Plant-soil interactions likely play a crucial role in shaping grasslands
food webs in the French Alps. Although we did not directly measure these
interactions, the taxonomic and/or functional composition of plant
communities had a major impact on soil food web structure. Mountain
grasslands exhibit high variability in plant species richness and the
presence of functionally important taxa such as Fabaceae, Brassicaceae
or Poaceae across different altitudes and massifs. Changes in plant
diversity can influence the composition of soil food webs (Eisenhaueret al. 2013) , and altering plant identity can affect different
soil taxa (Zinger et al. 2011). Interestingly, taxonomic
dissimilarity had a larger effect than plant functional dissimilarity,
possibly because above-ground traits used in our study may have a lesser
role in plant-soil interactions compared to root traits (Kardol & De
Long 2018; Wilschut et al. 2019). Surprisingly, in forests, plant
composition had a minor impact on food web structure. Soil variables
like pH and C/N ratio may better capture variations in forest type
(e.g., broadleaf vs coniferous forests), directly influencing soil food
webs.