Abstract
Riparian habitats are important for rare grassland butterflies. However, exotic plants have recently invaded these habitats, and have often spread rapidly and widely. Indigofera pseudotinctoria Matsum. is a shrub in the Leguminosae, and is the host plant of the threatened butterfly Lycaeides argyrognomon praeterinsularis. An exotic invasive grass, Eragrostis curvula (Schrad.) Nees, has recently populated the floodplains of the Abe River in Shizuoka, Japan, and its ecological impact on I. pseudotinctoria and therefore on L. argyrognomon is of increasing concern. We examined the occurrence of L. argyrognomon, the size of I. pseudotinctoria populations, and the invasion status of E. curvula at 32 sites along a river where L. argyrognomon had previously been recorded. Then, we conducted a three-year removal experiment at Kadoya to assess the effects of removal treatment on I. pseudotinctoria. Adult L. argyrognomon were observed at seven sites (21.9%) that were all located in the middle reach (21〜30km from the mouth) of the river. E. curvula was observed at 20 sites (62.5%), including five sites with L. argyrognomon. The removal experiment confirmed that this treatment has positive effects on both the flower and seed production of I. pseudotinctoria. In the third year of the experiment, the total number of flowers per plant was 6,965.5±2,308.9 (mean±SE) in the removal plot and 2,467.7±658.6 SE in the invaded one (i.e., the control). The total number of seeds per plant was 573.2±156.2 and 300.7±29.2, respectively. Contrary to our expectations, the recovery of seed production was rather small due to the extremely low fruit set (4.3±0.9 and 3.2±0.6, respectively). These findings suggest that special effort is needed to prevent E. curvula invasion of the existing L. argyrognomon habitat, and for restoring these sites properly to increase habitat quality. At the landscape level, designing and restoring the habitat networks connecting the current habitat (core sites) and Kadoya (the demonstration site) are necessary for the conservation of L. argyrognomon along the river.
