Phytophagous insects and web-building spiders in relation to pasture vegetation complexity

We investigate the potential use of structural characteristics of vegetation as predictors of the densities of pasture arthropods in three Azorean islands. Two types of upland pastures, recent sown pastures (3-4 yr old) and wet semi-natural old pastures (> 35 yr old), were studied in three Azorean islands (S. Maria, Terceira and Pico). Three arthropod assemblages. of particular importance in pasture habitats, were assessed: insect forb-feeders, insect grass-feeders and web-building spiders. These are numerically abundant and represent a range of feeding strategies. Point quadrats for plants and suction (Vortis) for arthropods were used as sampling methods. Several vegetation indices were investigated, but only three (cover abundance of perennial forbs, cover abundance of perennial grasses and the total vegetation alpha-diversity) were good predictors of invertebrate abundance. Vegetation structure, defined by a Height Index, was of minor predictive value. In sown and semi-natural pastureland. diverse and structurally complex pasture sites support more individual insect forb-feeders. grass-feeders and web-building spiders per unit area than less complex ones.