Conditions and conservation for biodiversity of the semi^-natural grassland vegetation on rice paddy levees

Abstract

Worldwide, field boundaries are one of the most important habitats for biodiversity. The diversity of these field boundaries is, however, rapidly declining as a consequence of land^-use change. Semi^-natural grassland on rice paddy field levees （Keihan or Aze in Japanese）, which is one of the types of field boundary that contains many grassland plants, is also under serious threat due to farmland consolidation and abandonment. The objectives of the present study are to review the relevant literature, to clarify the factors that affect biodiversity, to present conservation measures, and to suggest the direction of future studies. Before 1990, the vegetation on paddy levees had not been considered as a type of semi^-natural grassland. Thereafter, however, the importance of traditional paddy fields as a habitat for biodiversity was indicated, and the detrimental effects of farmland consolidation and abandonment were reported. After 2000, numerous studies reported on topics such as conservation strategies, comparisons of biodiversity on the lowermost hillside slopes, and soil seed banks. Traditional terraced paddy fields play a significant role in the conservation of biodiversity, pest control, and environmental conservation, and are also of high cultural value. Hypotheses on the diversity of the grasslands on paddy levees differ according to scale. Large^-scale hypothesis, such as those relating to historical factors, have not been well discussed. The vegetation on paddy field levees is affected by the surrounding grasslands, to which they are been historically closely related. Therefore, when attempting to determine the establishment of paddy levee flora, it is important to clarify the history of the paddy fields and their surrounding grasslands. In order to clarify the history of paddy fields, the following approaches can be adopted: researching historical data, surveys of the agricultural landscape, and analyses of pollen, plant opals, charcoal particles, and plant residues from archaeological sites. Vegetational studies, biogeography, and population genetics can also be of value in clarifying the establishment of paddy levee flora. The nature of the vegetation on paddy levees depends on various factors, including habitats, environmental conditions, and management regimes; however, the relationships between the vegetation and these various factors have yet to be clarified. The area of paddy fields is decreasing as a consequence of the abandonment of cultivation. The species richness of the vegetation on paddy levees is rapidly decreasing and its composition is rapidly changing. In order to improve the problems associated with abandonment, it is necessary to identify hotspots that include rare species, to increase the efficiency of cutting machines and methods, to obtain external assistance, and to introduce grazing. Farmland consolidation changes the vegetation of paddy levees. For the improvement of problems resulting from consolidation, the reuse of topsoil and establishment of seed sources are suggested, along with modification of the agricultural engineering of consolidation. Further measures for conservation include the sowing of seeds, transfer of seedcontaining hay, transfer of seed^-containing soil, and removal of exotic plants in the consolidated fields. In future investigations, the compilation of a representative list of grassland species and a comparison of species richness will be necessary. It is also necessary to collect and analyze past information on paddy levee vegetation, to urgently investigate the present vegetation on paddy levees, to establish conservation strategies, and to conserve various semi^-natural grasslands including paddy levees.