Effects of grassland duration on butterfly communities and its relevance to grassland area

Semi-natural grasslands, renowned for their rich biodiversity, are experiencing global decline, necessitating urgent conservation prioritization. Recently, researches have established that the vegetation history significantly influences grassland biodiversity. This study focuses on three vegetation types in and around ski-run grasslands of central Japan: (i) ancient or old grasslands persisting for 300 to several millennia, (ii) new grasslands recently converted from forests 43 to 72 years ago, and adjacent forests serving as control vegetation type. We examined the effect of grassland duration, in relation to its spatial area, on butterfly communities. Transects of 5 × 20 m were established at six to seven sites for each vegetation type to survey butterfly communities. In 2017, plant communities were similarly surveyed along the same transects (Inoue, Yaida et al., 2021). Species composition, species count, and overall butterfly abundance were not affected by grassland duration. Nevertheless, the species composition was affected by the spatial area of old grasslands within a 100 m radius of each transect, with an increase in the number of rare grassland species in extensive old grasslands. Minois dryas and Eurema mandarina were identified as indicator species of extensive old grasslands (>8000 m2 within 100 m radius of each transect), whereas no indicator species were detected in the other grassland sites. The abundances of M. dryas, Brenthis daphne, Melitaea ambigua, Curetis acuta, and Zophoessa callipteris were greater in areas where the areas of old grasslands within 100 m of each transect were greater. The previous study at these sites have shown that Sanguisorba officinalis (Rosaceae), the host plant of B. daphne, and Lespedeza bicolor (Fabaceae), the main host plant of E. mandarina and C. acuta, are indicators of old grasslands, explaining the dependency of these three butterfly species on old grasslands. Our findings suggest that extensive old grasslands should be accorded high conservation priority to preserve diversity of grassland butterflies.