Does the invasion of the exotic tree Ailanthus altissima affect the soil arthropod community? The case of a riparian forest of the Henares River (Madrid)

Invasive species are a major threat to global biodiversity because they alter the functioning of affected ecosystems by displacing the native vegetation and fauna. This study evaluated the effect of the invasive species Ailanthus altissima on soil properties and the microarthropod community, compared to the native species Populus nigra. Soil samples were taken in riparian forests, an ecosystem with high risk of colonisation by invasive tree species, on a stretch of the Henares River (Madrid, Spain) under the canopy of both tree species. In addition, controlled laboratory experiments were performed to evaluate the effects of litter on the arthropod community. Some groups of springtails (Entomobryomorpha, Poduromorpha and Symphypleona), mites (Gamasida and some Oribatida) and other arthropods groups (spiders, pseudoscorpionids, isopods and insects) were more abundant under the native species, while Actinedida mites were more abundant under ailanthus. Differences in porosity, pH and organic matter content were also observed between soils. In the laboratory experiments, the Arthropleona springtails and a few Oribatida mites preferred poplar leaf extract, and the Actinedida mites preferred ailanthus leaves. The invasion of A. altissima could be influencing soil fauna through their litter and also could be changing several soil properties which consequently affect soil microarthropod communities. This study reveals the clear disturbance of ecosystems that the invasion by exotic species produces at many levels of the soil system.