Impacts of management intensification on ground-dwelling beetles and spiders in semi-natural mountain grasslands

Agricultural intensification is one of the major threats to the biodiversity of montane and subalpine grasslands. This calls for regional agriculture policies that efficiently protect their flora and fauna without jeopardizing agricultural viability. We experimentally sought a sustainable management, testing the effects of fertilisation (slurry) and aerial irrigation (sprinklers) - separately and in combination (at different levels of intensity) - on the arthropod communities occurring in extensively-managed montane and subalpine meadows in the SW Swiss Alps. Four years after the start of the intensification experiment, we measured the abundance, species richness, community composition and variability (β-diversity) of ground-dwelling beetles and spiders. The abundance of both taxa showed a curvilinear relationship with management intensity. Spider abundance peaked at a moderate level of intensification while ground beetle abundance appeared to be more resilient to intensification, peaking at a high level of intensification. These responses were mainly driven by fertilisation, while irrigation played a minor role. For both taxa, we found no impact of irrigation or fertilisation, either when applied separately or in combination on species richness. Community composition was altered by management intensification in both taxa, but community variability was not. Given these taxon-specific patterns for abundance, applying organic fertiliser and water at levels corresponding to two-thirds of the quantity necessary to achieve local maximum hay yield appears to be compatible with the maintenance of rich ground-dwelling arthropod communities in mountain grasslands.