A project undertaken at Charles Sturt University, and supervised by Prof David Watson.
Project team: Prof David M Watson, Dr Jodi Price and Dr Adam Frew (all Charles Sturt University), Dr Leonie Valentine (University of Western Australia) and Dr Vanessa Westcott (Bush Heritage Australia)
Keystone resources, including many parasitic plants, ‘punch above their weight’ in their provision of critical ecological functions that are used by other organisms within natural communities. Most research on ecological keystones has been conducted in temperate and/or marine ecosystems, and it is unclear whether they play equivalent roles in arid ecosystems with characteristically extreme temporal and spatial variance in resource availability. Some of the more comprehensive work in terrestrial systems relates to mistletoes, a group of aerial parasitic plants found to boost biodiversity worldwide. What’s missing, however, is counterfactual and comparative data, evaluating how changes in resource provision, canopy complexity, nutrient availability and other factors associated with mistletoes compare with other plant groups. To contextualise this work, and establish the net effect of parasitic plants on ecosystem function, we focused on root parasitic plants, comparing them directly with adjacent non-parasitic but otherwise comparable trees and shrubs to understand the scale, extent and significance of how parasitic plants influence community structure and function. Using Australian Sandalwood (Santalum spicatum) quandong (Santalum acuminatum), and leafless ballart (Exocarpos aphyllus) as our three focal species, we investigated ecological communities at three sites over a 500–kilometre range in Australia’s western rangelands. Our research had three components: documenting visitation frequency and purposes of visitation; measuring the influence on nutrient availability and soil health, and evaluating the impact of these plants on microclimatic diversity.
For fauna visitation, we employed a novel approach, using both remote, motion-sensor camera traps (resulting in 19,077 camera trap nights) and timed observations (total of 970 ‘tree hours’), which provided a record of ~10,000 fauna visitation events by 123 different fauna species over a 24-month period. The primary purposes of visitation of the predominantly avian and reptile fauna species were for foraging or consuming food. While the diversity of visiting fauna was not significantly different to two other Santalaceae species which co-occur in the same communities, sandalwood recorded a significantly higher frequency of visitation than the other root hemiparasites on two of the three conservation reserves. Sandalwood also attracted significantly more frequent visits by fauna species than non-Santalaceae plants within their communities. The data indicate that sandalwood is a keystone resource, used throughout the year by a diversity of fauna species for key essential ecosystem processes. Collectively, the three Santalaceae species experienced elevated levels of fauna diversity and visitation when compared to non-parasitic plants in the same communities, reinforcing previous studies showing the disproportionately high significance of parasitic plants in food webs and ecosystem dynamics. The results of this study indicate that sandalwood, and other root hemiparasites, should be given elevated consideration in rangeland species’ conservation, and in landscape management and restoration planning and intervention.
For litterfall and nutrient dynamics, we installed littertraps beneath six individuals of three parasitic and three non parasitic plant species at each of our three study sites, monthly data on litter mass combined with chemical composition analysis. Initial results indicate that the contribution of these three root parasitic plants is broadly comparable with the relatively well-studied mistletoes, year-round patterns of litterfall combining with elevated concentrations of cations yielding a dramatic facilitative effect. This was most pronounced for the leafless cherry, recalcitrant litter forming deep carbon-rich beds beneath mature plants, supporting a far greater abundance of invertebrates than all other plants surveyed.