Land abandonment and intensification diminish spatial and temporal -diversity of grassland plants and herbivorous insects within paddy terraces

Although traditional land-use and management practices are known to enhance environmental heterogeneity in agricultural lands, loss of heterogeneity resulting from recent land-use changes can lead to spatiotemporal -diversity losses (biotic homogenization). However, patterns and causes of -diversity loss, especially at a within-field scale, have remained unexplored. We examined -diversity declines in plants, butterflies and orthopterans resulting from land abandonment and intensification in semi-natural grasslands on paddy margins at a within-field scale in relation to changes in disturbance frequency and surrounding landscapes by surveying eight abandoned, thirteen traditional and ten intensive paddy terraces. Three indices, the additive partitioning of species richness ((add)) and the turnover ((tu)) and nestedness ((ne)) components of Jaccard dissimilarity of plants and herbivores, were used to evaluate different aspects of -diversity. We hypothesized that biotic homogenization due to abandonment and intensification would be expressed as low spatiotemporal (add) and (tu), low spatial (ne) and high temporal (ne). The spatial and temporal (add) of plants and herbivores were higher in traditional terraces than in others. Changes in mowing frequency caused direct declines in butterfly (add) and indirectly decreased herbivore (add) through diminished plant (add) in abandoned and intensified terraces. Neighbouring forests increased plant spatial and butterfly temporal (add) and (tu). Abandoned terraces had higher orthopteran spatial and plant temporal (tu) than traditional terraces. Mowing frequency generally did not influence spatial and temporal (tu) of plants and herbivores. In intensive terraces, temporal (ne) of plants were higher than in traditional terraces, suggesting dominance by plant species sharing similar phenologies.Synthesis and applications. These results suggest that traditional mowing practices are essential for maintaining plant and herbivore (add), and consequently, species pools within terraces. The higher spatial or temporal species turnover of plants and orthopterans in abandoned terraces and higher temporal plant nestedness in intensive than in traditional terraces were not due to increases in species pools within terraces. Because within-terrace environmental heterogeneity is thought to remain in abandoned and intensive terraces, we recommend reduction in mowing frequency in intensive terraces and reintroduction of mowing in abandoned terraces for biodiversity restoration. These results suggest that traditional mowing practices are essential for maintaining plant and herbivore (add), and consequently, species pools within terraces. The higher spatial or temporal species turnover of plants and orthopterans in abandoned terraces and higher temporal plant nestedness in intensive than in traditional terraces were not due to increases in species pools within terraces. Because within-terrace environmental heterogeneity is thought to remain in abandoned and intensive terraces, we recommend reduction in mowing frequency in intensive terraces and reintroduction of mowing in abandoned terraces for biodiversity restoration.