Abstract
More than 30 per cent of the world’s trees are facing extinction, with climate change already contributing to many species’ declines. The loss of these plants will have far-reaching effects, particularly on organisms directly linked through food webs and interaction networks. This particularly applies to parasitic plants––which comprise just one per cent of the world’s flowering plants, but provide disproportionate resources used by co-occurring organisms. Parasitic plants provide food and nutrient-rich litter, enhance nutrient-cycling, and have structural attributes used by fauna for a variety of purposes, but the combined influence of these contributions have rarely been quantified.
Australian sandalwood (Santalum spicatum) is highly valued economically, which has resulted in massive overexploitation. Its population has been substantially reduced, being simultaneously impacted by over-harvesting and other threats including land clearing, grazing, and loss of seed-dispersers. To ascertain its ecological value, I investigated the demographic status and key ecological functional parameters of sandalwood in Australia’s arid and semi-arid western rangelands, and compared the latter with two other Santalaceae root hemiparasites––quandong (S. acuminatum) and leafless ballart (Exocarpos aphyllus)––and a suite of co-occurring non-parasitic plants. I conducted more than two years of field studies investigating the diversity, and visitation frequency and purpose of native fauna visiting the plants. I also investigated whether the plants have microclimatic refuge attributes––a function which will become increasingly important under warming climatic conditions. Key questions that I set out to test included: (i) what is the status of sandalwood in the wild?; (ii) what is its current demographic profile?; (iii) what fauna species visit sandalwood communities and for what purpose?; and (iv) does sandalwood, and co-occurring Santalaceae species provide a microclimate refuge function for visiting fauna?
My research indicates that sandalwood populations are declining and are composed almost entirely of very old trees in advanced states of senescence. Populations have negligible recruitment, and, with ongoing natural mortality, the species is on a trajectory towards extinction in the wild. My results also indicate that sandalwood, and the two co-occurring hemiparasites, exhibit attributes of keystone species––supporting a greater diversity of fauna for multiple purposes than non-parasitic plants in the same communities at the majority of my study sites. A total of 123 fauna species were observed visiting the sandalwood communities, primarily to forage for food, but also to shelter, including from direct insolation during periods of extreme high temperatures. Of the three study species, the sandalwood trees recorded the highest diversity and frequency of fauna visits at most of the study sites. Sandalwood, and the combined Santalaceae species, recorded significantly more diversity and frequency of fauna visitors than non-parasitic plants at most of the study sites. Camera trap recordings of fauna species sheltering under the study trees during periods of high temperatures stimulated a subsequent microclimate comparison which showed that the shaded canopy interiors of sandalwood and the two co-occurring hemiparasites were significantly cooler (on average ~5–7ºC cooler) than the canopy interiors of adjacent non-parasitic plants during extreme heat conditions. My results indicate that the loss of sandalwood from rangeland ecosystems will potentially have a significant impact on native fauna species currently using the plant’s ecological functions.
This research adds to the growing body of literature on the functional importance of parasitic plants. It also provides critical information about the demography and ecological importance of sandalwood in situ, and the need for effective conservation and land management interventions if the species is to be retained in the wild. My research has already been used to assess sandalwood for listing as a threatened species at global, national, and state levels; and for re-assessing wild sandalwood exploitation and conservation by government and industry stakeholders.
Australian sandalwood (Santalum spicatum) is highly valued economically, which has resulted in massive overexploitation. Its population has been substantially reduced, being simultaneously impacted by over-harvesting and other threats including land clearing, grazing, and loss of seed-dispersers. To ascertain its ecological value, I investigated the demographic status and key ecological functional parameters of sandalwood in Australia’s arid and semi-arid western rangelands, and compared the latter with two other Santalaceae root hemiparasites––quandong (S. acuminatum) and leafless ballart (Exocarpos aphyllus)––and a suite of co-occurring non-parasitic plants. I conducted more than two years of field studies investigating the diversity, and visitation frequency and purpose of native fauna visiting the plants. I also investigated whether the plants have microclimatic refuge attributes––a function which will become increasingly important under warming climatic conditions. Key questions that I set out to test included: (i) what is the status of sandalwood in the wild?; (ii) what is its current demographic profile?; (iii) what fauna species visit sandalwood communities and for what purpose?; and (iv) does sandalwood, and co-occurring Santalaceae species provide a microclimate refuge function for visiting fauna?
My research indicates that sandalwood populations are declining and are composed almost entirely of very old trees in advanced states of senescence. Populations have negligible recruitment, and, with ongoing natural mortality, the species is on a trajectory towards extinction in the wild. My results also indicate that sandalwood, and the two co-occurring hemiparasites, exhibit attributes of keystone species––supporting a greater diversity of fauna for multiple purposes than non-parasitic plants in the same communities at the majority of my study sites. A total of 123 fauna species were observed visiting the sandalwood communities, primarily to forage for food, but also to shelter, including from direct insolation during periods of extreme high temperatures. Of the three study species, the sandalwood trees recorded the highest diversity and frequency of fauna visits at most of the study sites. Sandalwood, and the combined Santalaceae species, recorded significantly more diversity and frequency of fauna visitors than non-parasitic plants at most of the study sites. Camera trap recordings of fauna species sheltering under the study trees during periods of high temperatures stimulated a subsequent microclimate comparison which showed that the shaded canopy interiors of sandalwood and the two co-occurring hemiparasites were significantly cooler (on average ~5–7ºC cooler) than the canopy interiors of adjacent non-parasitic plants during extreme heat conditions. My results indicate that the loss of sandalwood from rangeland ecosystems will potentially have a significant impact on native fauna species currently using the plant’s ecological functions.
This research adds to the growing body of literature on the functional importance of parasitic plants. It also provides critical information about the demography and ecological importance of sandalwood in situ, and the need for effective conservation and land management interventions if the species is to be retained in the wild. My research has already been used to assess sandalwood for listing as a threatened species at global, national, and state levels; and for re-assessing wild sandalwood exploitation and conservation by government and industry stakeholders.
Original language | English |
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Qualification | Doctor of Philosophy |
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Place of Publication | Australia |
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Publication status | Published - 2023 |