Unexpected sensitivity of the highly invasive spider Mermessus trilobatus to soil disturbance in grasslands.

The dwarf spider Mermessus trilobatus (Araneae: Linyphiidae), native to North America, has expanded its range over large parts of Europe within less than fifty years. It is notable for occurring in a wide range of mostly agricultural habitats, while most other invasive spiders in Europe are associated with human buildings. As in other invasive invertebrates and plants, the tremendous colonisation success of Mermessus trilobatus might be related to anthropogenic habitat disturbance. Here we aim to test if the invasion success of Mermessus trilobatus in Europe is associated with high tolerance towards soil disturbance. We sampled spiders from eight grasslands experimentally disturbed with superficial soil tillage and eight undisturbed grasslands without tillage. Opposite to our expectation, Mermessus trilobatus densities decrease sharply with soil disturbance. This is in contrast to several native species such as Oedothorax apicatus, which becomes more abundant in the fields after superficial soil tillage. Our study suggests that invasion success of Mermessus trilobatus is not connected to a ruderal strategy. The ecological and evolutionary processes behind colonisation success of Mermessus trilobatus need to be further investigated.