Urbanization and urban habitat fragmentation do not compromise soil food web complexity and conservation value

Abstract

Urbanization-induced environmental changes such as habitat fragmentation impacts arthropod assemblages and food web-related ecosystem functions, such as nutrient cycling, carbon storage and energy fluxes. Yet, we lack insight into how arthropod food webs are structured along urban fragmentation gradients. Here, we investigated the community composition and food web structure of litter-dwelling arthropods along fragmentation gradients (green median strip, urban park, urban forest and natural forest). We found the density of litter-dwelling arthropods in median strip and urban park to be two to four times higher than in urban and natural forests, with, as indicated by literature-based stable isotope values, 67% - 68% of the individuals comprising primary consumers (trophic level I) in median strip and urban park. Urban forests, reserved for biodiversity conservation, hosted the least arthropod density, taxa richness, biomass and body mass, but were colonized by a high number of specialist arthropods, e.g. Archaeognatha and Isoptera. Food webs were most simple in urban forest, but more complex in median strip and urban parks, i.e. open fragments, with abundant primary and secondary consumers including decomposers. Chilopoda and Araneae formed the apex predators in fragments and mostly consumed other predators of trophic level III. The biomass of decomposed litter on the soil surface as major resource of the soil animal community significantly correlated positively with the density of arthropods of trophic levels I, III and IV. Supporting the dominance of bottom-up forces, the density of adjacent trophic levels consistently correlated positively. Overall, our results suggest that small size urban fragments maintain a diverse community of arthropods forming complex food webs and thereby may contribute to conserving biodiversity and providing important ecosystem functions.