Landscape, pollinator and mycorrhizal specifity and their contribution to rarity in Drakaea (Hammer Orchids)

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[Truncated] The question of why some species are rarer than others is fundamental to understanding the ecology and evolution of both communities and lineages. It is predicted that species with specialised ecological interactions will be at greatest risk of species rarity. The Orchidaceae, which is known for specialised mycorrhizal and pollinator relationships, represents a suitable group to test the role of these ecological interactions in species rarity. I investigated the role of landscape characteristics, mycorrhizal specificity and pollinator specificity in the high level of intrinsic rarity in Drakaea (Orchidaceae), a genus well known for its highly modified flowers that achieve pollination through sexual deception of male thynnine wasps.

Using in excess of 13,000 records in the Western Australian Herbarium, I conducted an analysis of the biogeography and factors associated with rarity in the southwestern Australian orchid flora. Highest levels of endemism were in regions characterised by unique edaphic environments and naturally fragmented habitats. Further, a higher incidence of rarity exists in naturally fragmented habitats. The sexual deception strategy for pollination was correlated with high levels of rarity. These results suggest that the use of sexual deception may predispose Drakaea to rarity, though in some species, their occurrence in naturally fragmented environments may also contribute to rarity.

DNA sequencing and field baiting studies were combined to test the role of mycorrhiza in orchid rarity in two communities of Drakaea. Sequencing of the ITS region revealed that all five species studied use a single species of Tulasnella fungus. Further, in vitro germination studies demonstrated that fungi isolated from each Drakaea species can support germination of the other species. A field baiting experiment showed no significant difference in the number of microsites at which common and rare Drakaea germinated. Further, in all species germination occurred at sites where the parent plants do not. As such, differences in mycorrhizal ecology are not contributing to the differences in the abundance of Drakaea species in these communities.

Original languageEnglish
QualificationDoctor of Philosophy
StateUnpublished - 2010


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