Grass-tree (Xanthorrhoea semiplana, Liliaceae) facilitation of the endangered pink-lipped spider orchid (Caladenia syn. Arachnorchis behrii, Orchidaceae) varies in South Australia
Sophie Petit A C and Catherine Ruth Dickson A BA School of Natural and Built Environments, University of South Australia, Mawson Lakes, SA 5095, Australia.
B Department of Environment and Heritage, PO Box 134, Naracoorte, SA 5271, Australia.
C Corresponding author. Email: sophie.petit@unisa.edu.au
Australian Journal of Botany 53(5) 455-464 https://doi.org/10.1071/BT04034
Submitted: 2 March 2004 Accepted: 4 April 2005 Published: 11 August 2005
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.
Acknowledgments
We thank Doug Bickerton for his encouragement throughout the years. David Wilson assisted with the field work in 2001, John Weber helped with soil identification, Peter Canty processed National Parks permits (E 24836 series), Andrew Moylan assisted with Forestry Reserve permits, Paul Smith and Erik Dahl facilitated access to the reserves, the Bureau of Meteorology (Mount Crawford Headquarters) provided us with meteorological data. We also thank Manfred Jusaitis of the Plant Biodiversity Centre and two anonymous referees for editing the manuscript of this paper. This research was sponsored by Early Career Researcher grants from the Division of Information Technology, Engineering, and the Environment, University of South Australia to S. Petit, a Summer Scholarhip to C. Dickson from the same Division, and an Australian Technology Network grant to S. Petit. This is contribution 135 from the Program of Biodiversity, Environmental and Park Management, University of South Australia.
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