Land abandonment and intensification diminish spatial and temporal β-diversity of grassland plants and herbivorous insects: Within site scale study in agricultural lands

Background/Question/Methods

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 β-diversity losses (biotic homogenisation). However, patterns and causes of β-diversity loss, especially at a within-field scale, have remained unexplored. We examined β-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 8 abandoned, 13 traditional and 10 intensive paddy terraces. Three indices, the additive partitioning of species richness (β_add) and the turnover (β_tu) and nestedness (β_ne) components of Jaccard dissimilarity of plants and herbivores, were used to evaluate different aspects of β-diversity. We hypothesized that biotic homogenisation due to abandonment and intensification would be expressed as low spatiotemporal β_add and β_tu, low spatial β_ne and high temporal β_ne. We examined relationship between β-diversity and land-use type, relationships between plant β-diversity indices and disturbance and landscape variables and relationships between β-diversity of herbivorous insects and plant β-diversity, disturbance and landscape variables.

Results/Conclusions

The spatial and temporal β_add of plants and herbivores were higher in traditional terraces than in others. Changes in mowing frequency caused direct decline in butterfly β_add and indirectly decreased herbivore β_add through diminished plant β_add in abandoned and intensified terraces. Neighbouring forests increased plant spatial and butterfly temporal β_add and β_tu. Abandoned terraces had higher orthopteran spatial and plant temporal β_tu than traditional terraces. Mowing frequency generally did not influence spatial and temporal β_tu of plants and herbivores. In intensive terraces, temporal β_ne of plants were higher than in traditional terraces, suggesting dominance by plant species sharing similar phenologies. These results suggest that traditional mowing practices are essential for maintaining plant and herbivore β_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.