Parasitic plants, such as mistletoe, are not necessarily the ‘baddies’ that many seem to think they are, according to a Charles Sturt University (CSU) academic and leading ecologist.
Associate Professor David Watson - an expert in the Australian mistletoe “The word parasite has nasty connotations to it but rather than being negative or even neutral, in natural systems parasitic plants have an important role in boosting productivity,” says leading mistletoe researcher Associate Professor David Watson from the University’s Institute of Land, Water and Society.
“They act as a focus.”
A Greek myth in life
Professor Watson has recently published a paper in the preeminent international Journal of Ecology, where he likens parasitic plants to the Dryads in Greek mythology.
A Dryad is a god or tree nymph born attached to particular tree which it looks after and guards. “But rather than just being restricted to the tree itself, the Dryad raises the profile of the whole area into a sacred grove,” explains Professor Watson.
Using his own extensive research on mistletoe in Australia and research by others on an herb found in the sub-Arctic regions that reindeer eat, Professor Watson believes that parasitic plants have a much more distinctive role in their ecosystems than previously recognised.
“Their role is more similar to other groups of plants known as facilitators, such as wattles which are nitrogen fixers and shade plants,” he says. “These plants help other species to grow and create more opportunities for more things.
“Like the Dryad, mistletoe adds additional properties to the neighbourhood. It frees up nutrients that are already there by attracting mobile animals such as possums, insects and birds, which bring in extra nutrients such as phosphorous and magnesium, elements which are often quite limiting, especially in the poor soils found across Australia.
“By adding a couple of these little plants, it’s like adding compost to the garden. It boosts everything along making it more habitable for a range of animals and plants.”
Professor Watson working next to the Panama CanalA worldwide concern
Professor Watson says while parasitic plants are beneficial in natural systems, there were plenty of examples of where things can go wrong, such as when a system is degraded or disturbed, or when a parasitic plant is accidentally moved from one system to another.
“Some of the worst agricultural weeds in the world are parasitic plants such as witchweed - which parasitises wheat - and broomrape,” he says. “The US Government is spending billions on a contingency plan if terrorists spread these weeds throughout the US Mid West.”
Professor Watson has spent the past 15 years researching mistletoe and other parasitic plants in Australia and overseas.
In Australia Professor Watson is coordinating a major study which is looking at how mistletoe affects woodland ecosystems. For that project, he is comparing 20 woodlands near Holbrook, NSW, in the southern Riverina, where all the mistletoe has been removed, with 20 woodlands where it hasn’t been, over a 20 year period. He is also involved in studies of other Australian parasitic plants including sandalwood in the Strzelecki Desert, and cherry ballart (native cherry) in the Barmah-Millewa forest on the Murray River.
Overseas he is working on parasitic plants in collaboration with the Smithsonian Tropical Research Institute in Panama, Northern Arizona University and the University of Oregon, while in Australia he is working with Murdoch University and the University of Queensland.
The good and the bad
Professor Watson acknowledges that mistletoe, which is found throughout the world with Australia having 93 native species, can also be a ‘baddie’ in some circumstances.
“This can happen in disturbed systems where various natural processes, such as fire, possums, and butterflies, that usually keep it in check have been taken away,” he says.
He explains that there is a group of butterflies that are mistletoe ‘specialists’ and will only lay their eggs on mistletoe. So while the resulting caterpillars can feed on the mistletoe, the adult butterflies need to feed on the nectar from other woodland shrubs and flowers to reach maturity.
“It’s one of the indirect effects of livestock grazing,” says Professor Watson. “By taking nectar-bearing plants out of the system through grazing, you remove the butterflies that keep the mistletoe in check. The same goes for possums that go out of their way to eat mistletoe and other parasitic plants such as dwarf cherry.”
A system out of balance
The ubiquitous mistletoe is found in gum trees across Australia. He says an abundance of mistletoe, which can eventually kill the host tree, is an excellent indication that the natural system isn’t working, as is the absence of mistletoe altogether.
“You can spend a lot of money removing mistletoe, but that’s like going to a surgeon and having all your chicken pox lumps cut away,” he says. “The mistletoe will come back as you haven’t attacked the disease. The ‘disease’ here is an overload of nutrients, soil compaction and habitat fragmentation – these make the world a better place for mistletoe which is responding to the imbalance. The reverse has occurred in the wheat belt of Western Australia where the mistletoe has been knocked out of the system and so have all the benefits it provides.”
However, Professor Watson does recommend removing mistletoe from individual trees, such as those standing alone in paddocks or in backyards, if they are “looking worse for wear” because of a mistletoe infestation.
“It works in the short term and it works on a small scale. However, you either have to keep removing it every year as the mistletoe plants will come back, or you need to look at how you are managing the land under the tree so that you allow better water infiltration and less compaction of the soil,” he says.
“The best thing a farmer can do if he wants to save a tree in the middle of a paddock with mistletoe is cut off the mistletoe and then fence the tree off so the stock can’t trample the ground underneath.”
Editor's Note: A story provided by Charles Sturt University. This article is under copyright; permission must be sought from CSU to reproduce it.