Relative importance of ballistic and ant dispersal in two diplochorous Viola species (Violaceae)

Viola is one of the diplochorous plant genera that disperse their seeds in two ways, ballistic and ant dispersal. We compared the seed dispersal of two major Viola species of northern Japan, V. selkirkii and V. verecunda. The mean weight of seed was less in V. verecunda (0.42 ± SD 0.03 mg) than in V. selkirkii (0.61 ± 0.12 mg). The elaiosome of V. selkirkii (0.02 ± 0.004 mg) was larger than in of V. verecunda (0.006 ± 0.0004 mg), whereas the lipid component of elaiosome was not remarkably different between the two species. In ballistic dispersal, the mean dispersal distance was 56.0 ± 17.5 cm in V. verecunda but only 38.3 ± 5.1 cm in V. selkirkii. In ant dispersal, the mean dispersal distance was 28.1 ± 24.9 cm in V. selkirkii and 36.1 ± 33.7 cm in V. verecunda; however, the seed removal frequency of V. selkirkii (15.5%) was much higher than that of V. verecunda (3.0%). These results suggest that V. selkirkii is more dependent on ant dispersal while V. verecunda is more dependent on ballistic dispersal. The effect of seed predation was very serious in both species. In the quadrat census, 99.0% of V. selkirkii seeds and 99.1% of V. verecunda seeds were damaged by ground beetles, spiders, ticks, and others which frequently devoured diaspores. An experiment with V. verecunda seeds demonstrated that the overdispersion of seeds on the forest floor enhanced the frequency of removal by ants and reduced seed damage by predators.