Taux de mortalité et causes de disparition de Apetahia raiateensis (Campanulaceae), une plante endémique de l’île de Raiatea (Polynésie française)

Abstract

Mortality rate and extinction causes of Apetahia raiateensis (Campanulaceae), an endemic plant to the island of Raiatea (French Polynesia). — Identifying the causes of extinction and their intensity is a prerequisite to define the status of species conservation and to set up efficient and effective conservation plans. Apetahia raiateensis H. Baillon (Campanulaceae), an endemic shrub on three trachytic plateaus of the island of Raiatea (Society Islands), is a legendary plant in Polynesian culture and a legally protected species in French Polynesia. Systematic inventories were conducted on the plateaus Te mehani ‘ ute’ute (94 ha), Te mehani rahi (202 ha) and Te vaihue (12 ha) to assess populations (882 plants in 2009), to map the species, and to compare with previous inventories performed in 1995. 81 accessible plants and 820 of their stems were monitored on Te mehani rahi between 2009 and 2011, in order to quantify and understand the reasons of the species rapid extinction. The inventories revealed a rate of disappearance to be respectively of 74 % on Te mehani ‘ ute’ute (1995-2006) and of 76 % on Te mehani rahi (1995-2009). The plants are currently found in subpopulations restricted on the borders of the plateaus and on cliffs. After 18 months of follow-up, 41 out of 57 reproductive plants (72 %) and 219 stems were attacked by rats (Rattus spp.) and 9 reproductive plants and 133 stems were found dead. In addition, 31 % of these reproductive plants had 29 stems broken by people. Among the wilted stems with unknown causes, half (55 stems on 15 plants) might be related to the cyclone that hit Raiatea in 2010. The species habitat is also threatened by feral pigs and by the invasion of twelve naturalized alien plants. Our results support the classification of Apetahia raiateensis in the «critically endangered» category (CR) of IUCN. Rat control trials using baits around plants are currently conducted on Te mehani rahi, together with in situ propagation from seeds, for the recovery of this species of scientific and cultural heritage values.