Extinction threats of a narrowly endemic shrub, Stachyurus macrocarpus (Stachyuraceae) in the Ogasawara Islands

Stachyurus macrocarpus is a narrowly endemic endangered shrub of which only one population remains on Chichi-jima Island (Ogasawara). We surveyed its population dynamics and reproductive states for 4 years and analyzed the population viability. In a survey of its entire distribution range, a total of 87 S. macrocarpus individuals have been found and 68 individuals have been confirmed in 2007. Thus, the actual population size is estimated to be less than 100 individuals. Environmental conditions and the results of a hand-pollination experiment suggest that low fruit setting in both sexes (female: 4.9–11.2%; hermaphrodite: 0.0%) may be caused by resource limitation. In addition, fruit predation by alien Rattus rattus was observed despite the rare fruit setting. The small effective population size (23.1–24.6 individuals) and hermaphrodite-biased sex ratio (46 hermaphrodites:12 females) would increase the risk of extinction. The habitat of S. macrocarpus was limited to dense scrubland and forest understory, and seedling regeneration was very scarce. During the survey period, 19 individuals (21.8%) were found dead, there were only two seedling recruitments, and the annual population growth rate was 0.979. The lack of occurrence on the south and west slopes and the shortness of shrubs in open spaces suggests that S. macrocarpus suffers stress from both dryness and frequent typhoons. However, S. macrocarpus also exhibited high mortality of shoot in forest understories. These findings suggest that the suitable habitat of S. macrocarpus is likely to be narrow in Ogasawara. While the recent increase of goat grazing has not affected individual mortality, 58.6% of shoots that had been grazed by goats were dead within 2 years. As elasticity analysis had shown that larger individuals make a greater contribution to the population growth rate, repeated goat grazing would impact the S. macrocarpus population in the near future by decreasing the vitality of larger individuals. Emergency measures for protecting the shrub from goat grazing and reinforcing the population through nursery cultivation were proposed.