Land use intensity in grasslands: Changes in biodiversity, species composition and specialisation in flower visitor networks

The relationship between resource availability and biodiversity of consumers has gained particular attention with the increasing loss of biodiversity. We evaluated resource availability on meadows of low intensity (low/unfertilised, mown once or twice per year) and meadows of high-intensity land use (high fertilisation, mown twice or thrice) before and after the first mowing in relation to network specialisation, species richness and composition of flower visitors. We studied 40 meadows, simultaneously sampling one meadow of low intensity and one meadow of high-intensity land use. Each survey yielded a separate interaction network, comprising all flower visitors found during 6 h in an area of 1000 m(2). In total we recorded 105 plant species and 586 flower visitor species. Species composition of plants and flower visitors differed considerably between the two management regimes, with species overlaps of 43% in plants and 42% in flower visitors. Complementary specialisation of flower visitors differed between taxa: dipterans were significantly less specialised than beetles and butterflies, and bees had the highest degree of specialisation. Earlier in the season (before mowing), meadows of low-intensity land use were significantly richer in flowering plant, bee and butterfly species. They also showed more plant flower visitor interactions, greater flowering areas and higher individual numbers of butterflies than meadows of high-intensity land use. However, later in the season (after mowing) management types differed only in plant species richness, being higher on meadows of low-intensity land use. We conclude that variations in plant species composition resulting from differences in grassland management may alter plant flower visitor interactions. Moreover, an impoverishment of flower diversity has stronger effects on the diversity and species composition of specialised flower visitor groups than on little or unspecialised flower visitors.