On the relative importance of resource availability and habitat connectivity as drivers of soil biodiversity in Mediterranean ecosystems

Soil biota influences nutrient cycling and climate regulation and represents an important fraction of global biodiversity, yet we know very little about how this soil biota responds to habitat fragmentation and degradation of habitat quality. We studied the response of different soil trophic groups (microbes and soil fauna), and their trophic structure, to changes in their habitat derived from forest long-term management and extensive tree die-off in a Mediterranean ecosystem. Specifically, we evaluated changes in (i) habitat size, (ii) habitat resource availability and heterogeneity and (iii) habitat connectivity. To do this, we sampled the soil biota of 43 holm oak trees (and five open interspaces) differing in size, quality, heterogeneity, connectivity and the effect of die-off (healthy or affected). We sorted soil biota by trophic group and related their richness to habitat characteristics. Seven of the 12 trophic groups evaluated increased their species richness with soil organic carbon content, which was the most frequently selected driver of soil biota (both microbial and faunal richness). Habitat connectivity positively affected the richness of larger organisms (fauna) and plant attributes (richness, productivity and specific leaf area) also showed significant but contrasting effects depending on the group evaluated. Due to the idiosyncratic responses of different groups, the entire trophic structure (microbes and fauna) was affected by a more complex set of factors than most trophic groups in isolation, including interactions between habitat size and resource availability or connectivity. A major factor influencing habitat resource availability was the die-off of the dominant tree species (drastically altering tree productivity). We found weaker and more negative relationships between trophic groups under trees suffering from die-off than beneath healthy trees, particularly between microbial rather than faunal groups. Synthesis. We provide a comprehensive assessment of the response of key members of the soil food web to habitat fragmentation and tree die-off (landscape-level plant–soil interactions), illustrating the major role of soil carbon, habitat connectivity and tree die-off in driving soil biodiversity and trophic structure.