Rhizanthella gardneri (Rogers) is a critically endangered orchid restricted to two isolated regions of south-western Australia. Rhizanthella gardneri is an entirely subterranean mycoheterotrophic species that purportedly forms a tripartite relationship with a mycorrhizal fungus (Ceratobasidiales) that links with an autotrophic shrub of the Melaleuca uncinata complex to acquire nutrients. Whether the rarity of R. gardneri is intrinsic is overshadowed by the recent effect of extrinsic factors that means R. gardneri requires some form of conservation and may also be a viable candidate for restoration. To create an integrated conservation strategy for R. gardneri, reasons for its decline and knowledge of its biological and ecological functioning must be elucidated. This thesis focuses on three key questions; 1) what are the habitat requirements and limitations to R. gardneri survival; 2) what is the identity and specificity of the fungus R. gardneri forms mycorrhizas with; and 3) does R. gardneri form a nutrient-sharing tripartite relationship with a mycorrhizal fungus and autotrophic shrub. Key climate, soil and vegetation characteristics of known R. gardneri habitats were quantified to provide baseline data for monitoring known R. gardneri populations, to better understand how R. gardneri interacts with its habitat, and to identify possible new sites for R. gardneri introduction. Habitats of the two known R. gardneri populations differed considerably in soil chemistry, Melaleuca structure and Melaleuca productivity. Individual sites within populations were relatively similar in all attributes measured while overall Northern and Southern habitats were distinct from each other. These results suggest that R. gardneri can tolerate a range of conditions and may be more widespread than previously thought, given that there are extensive areas of Melaleuca thickets with similar habitat characteristics across south-western Australia. The fungus forming mycorrhizas with R. gardneri was identified, using nuclear ribosomal DNA sequences, as a Rhizoctonia-type fungus within the Ceratobasidiales. All fungi isolated from R. gardneri individuals representative of its currently known distribution were genetically similar, suggesting R. gardneri is highly dependent on this specific fungal species. Given that R. gardneri appears to exclusively associate with a specific fungal species, species-specific molecular primers were designed and used to analyse the fungi’s presence in known and potential R. gardneri habitats. These results 6 suggest that the fungus exists beyond the known R. gardneri habitats and gives hope to finding new populations.

Rhizanthella gardneri (Rogers) is a critically endangered orchid restricted to two isolated regions of south-western Australia. Rhizanthella gardneri is an entirely subterranean mycoheterotrophic species that purportedly forms a tripartite relationship with a mycorrhizal fungus (Ceratobasidiales) that links with an autotrophic shrub of the Melaleuca uncinata complex to acquire nutrients. Whether the rarity of R. gardneri is intrinsic is overshadowed by the recent effect of extrinsic factors that means R. gardneri requires some form of conservation and may also be a viable candidate for restoration. To create an integrated conservation strategy for R. gardneri, reasons for its decline and knowledge of its biological and ecological functioning must be elucidated. This thesis focuses on three key questions; 1) what are the habitat requirements and limitations to R. gardneri survival; 2) what is the identity and specificity of the fungus R. gardneri forms mycorrhizas with; and 3) does R. gardneri form a nutrient-sharing tripartite relationship with a mycorrhizal fungus and autotrophic shrub. Key climate, soil and vegetation characteristics of known R. gardneri habitats were quantified to provide baseline data for monitoring known R. gardneri populations, to better understand how R. gardneri interacts with its habitat, and to identify possible new sites for R. gardneri introduction. Habitats of the two known R. gardneri populations differed considerably in soil chemistry, Melaleuca structure and Melaleuca productivity. Individual sites within populations were relatively similar in all attributes measured while overall Northern and Southern habitats were distinct from each other. These results suggest that R. gardneri can tolerate a range of conditions and may be more widespread than previously thought, given that there are extensive areas of Melaleuca thickets with similar habitat characteristics across south-western Australia. The fungus forming mycorrhizas with R. gardneri was identified, using nuclear ribosomal DNA sequences, as a Rhizoctonia-type fungus within the Ceratobasidiales. All fungi isolated from R. gardneri individuals representative of its currently known distribution were genetically similar, suggesting R. gardneri is highly dependent on this specific fungal species. Given that R. gardneri appears to exclusively associate with a specific fungal species, species-specific molecular primers were designed and used to analyse the fungi’s presence in known and potential R. gardneri habitats. These results 6 suggest that the fungus exists beyond the known R. gardneri habitats and gives hope to finding new populations.