Results
The metaweb constructed for the French-Alps region using the taxa
identified in 418 soil samples consisted of 55 trophic groups, three
basal resources and a total of 383 potential interactions (Fig. S1). On
average, local food webs were composed of 41 ± 4 trophic groups (and
resources), with 206 ± 37 interactions. All 10 trophic classes were
present in all local food webs, with a single exception where no group
from the fungivore class was detected. The mean dissimilarity between
local soil food webs was primarily influenced by changes in the
dominance of trophic groups, rather than by changes in the presence or
absence of groups (as indicated by increasing dissimilarity when
increasing parameter q, which represents the weight of trophic group
abundance and interaction, Fig.S2).
Overall, we found that food web composition varied across the
environmental range covered in this study. The habitat type,
particularly when comparing grassland and forest habitats, strongly
structured the food web composition, both in terms of trophic group and
interaction composition (Q1) (Fig.2, Fig.S3). The influence of habitat
type on interaction composition was more pronounced compared to trophic
group composition (Table S1). When examining shrubland areas, which
represent ongoing shifts from grassland to forest, we found highly
variable food web structures (Fig. 2). These structures exhibited
similarities to both forest and grassland food webs, potentially due to
the spatial proximity of shrubland sampling sites to forests or
grasslands, or the specific stage of succession in the shrubland areas.
By contrasting the average food webs of forests and grasslands, we
identified compositional differences in the abundance of trophic groups
and classes between the two habitats (Fig.3). Forest soil food webs were
enriched in ectomycorrhizal fungi, litter and wood saprotrophic fungi,
macro-detritivores like earthworms and diplopods at basal level,
fungivores (particularly eudaphic collembola and protura), soil top
predators including predatory mites, centipedes, predatory coleoptera
and pseudoscorpions, as well as zooparasites. In contrast, grassland
soil food webs had a higher proportion of decomposers and detritivores,
particularly soil saprotrophic fungi, enchytraeids and coprophagous
coleoptera (e.g., dung beetles), arbuscular mycorrhizal fungi and root
endophytes, and most groups of plant phytoparasites and autotrophs.
Grasslands also exhibited a higher proportion of bacterivores and
omnivores compared to forests, notably bacterivorous nematodes, rotifers
and epigeic collembola, at higher trophic levels. We further examined
the ß-diversity of interactions between habitats and compared it to null
expectations derived from randomising trophic group abundances while
preserving trophic group diversity and network structure (see methods).
The observed ß-diversity was significantly higher than expected by
chance (Fig.4), indicating that trophic groups enriched in each habitat
were not randomly distributed in the food webs, but instead formed pairs
of trophically linked groups. This suggests that differences in trophic
group abundance between habitats (Fig.3) are influenced by the structure
of trophic links within the food web (Q2).
The composition of forest and grassland food webs was influenced by
distinct ecological filters (Q3). In both habitats, predictors
associated with these filters accounted for approximately 10% of the
variance in soil food web dissimilarity (Fig.5, Fig.S4). In forests,
spatial and environmental predictors played a more important role,
particularly soil C/N ratio (related to organic matter degradability),
pH, and frozen degree days (FDD) as indicators of abiotic stress
(Fig.5a). Dissimilarities in trophic groups and interactions were more
pronounced between sites separated by large spatial distances and
remained consistent along the gradients of FDD and pH (Fig.5b). Changes
in soil C/N ratio primarily influenced interaction dissimilarity,
particularly at higher values of the C/N ratio gradient
(>20).
In contrast, grassland soil food webs were predominantly driven by plant
taxonomic and functional composition, as well as soil C/N ratio
(Fig.5a). Soil C/N ratio and plant taxonomic/functional composition
better explained interaction dissimilarity (Fig.5c), while plant
taxonomic composition was more effective in predicting trophic group
dissimilarity. Dissimilarities in trophic groups and interactions
remained relatively constant along the plant dissimilarity gradient but
were more pronounced in the lower part of the FDD gradient
(> 40 cumulative degrees) and the upper part of the C/N
ratio gradient (>15).