Grass-tree (Xanthorrhoea semiplana, Liliaceae) facilitation of the endangered pink-lipped spider orchid (Caladenia syn. Arachnorchis behrii, Orchidaceae) varies in South Australia

Abstract

We examined the role of the grass-tree Xanthorrhoea semiplana F.Muell. canopy in the survival of the nationally endangered orchid Caladenia (syn. Arachnorchis) behrii Schltdl. at three sites in the northern Adelaide region. We compared grazing and pollination of the flowering orchids within and away from the grass-tree canopy. Grass-trees generally provided significant protection from grazing, but orchids protected by grass-trees experienced reduced pollination for 2 out of 5 years. The net effect of these interactions, as measured by seed set, varied across years. When grazing pressure is high, orchids may benefit from grass-tree protection (facilitation), but under low grazing pressure, it is more probable that orchids set seeds away from grass-trees than under their canopy. Grazing pressure probably does not decrease in view of habitat fragmentation the importance of fragments as wildlife refuges. Therefore, factors affecting the survival of grass-trees, such as Phytophthora cinnamomi, may also affect orchid survival. Kangaroos which are often assumed to be responsible for most grazing in the Adelaide Hills are not the only predators of orchids, and a culling program has so far not resulted in a decrease of grazing pressure for the orchids. Translocation and fencing programs should examine the role of facilitative plants and grazers before spending precious conservation resources.