This is a guest post by Mackenzie Kwak.
Parasitism – the process whereby one organism gains resources by exploiting another without giving anything in return – is the most common form of existence in the living world. Although the term ‘parasite’ often conjures the itchy image of head lice or intestinal worm, there are a myriad of strange and beautiful plants which also share this way of life and many of them can be seen around our city.
Mistletoe is perhaps the most well-known parasitic plant owing to its cosmopolitan distribution and association with Christmas. Melbourne is home to a number of different mistletoe species of the genus Amyema which can only be distinguished when they flower. Despite its commonness around Melbourne, few people recognise the slightly insidious nature of the plant or know of its remarkable life cycle. The mistletoe starts life as a small, juicy fruit hanging on the branch of a mistletoe plant high up in the canopy.
Eventually, the small fruit is spotted by the mistletoebird (Dicaeum hirundinaceum) which is found throughout most of mainland Australia and as its name suggests, is fond of mistletoe fruit. The mistletoebird eats the fruit which can only germinate after its hard coating is dissolved in the stomach of the little bird. The mistletoebird then passes the seeds of the mistletoe out in its scat on the branch of a tree. At this point, the seed germinates and begins growing into the branch of the tree to gain water and minerals from its new host. The mistletoe can live on the tree for many years and will grow and flower over this time as it spreads its seeds through the forest with the help of its ally, the mistletoebird. As well as being an important food source for the mistletoebird, the plant also supports two Melburnian butterflies, the imperial jezebel (Delias harpalyce) and the red-spotted jezebel (Delias aganippe), which feed on nothing else as caterpillars.
The native cherry (Exocarpos cupressiformis) is another common Melburnian plant which few people realise is also a parasite. Native cherry trees superficially resemble small conifers such as cypress but are actually an indigenous species of sandalwood. They grow up to eight metres in height and are completely devoid of leaves; instead, they photosynthesise using a network of long, green stems called phyllodes. Like the mistletoe, the native cherry produces small fruits which are also dispersed by birds. In fact, the small red cherries are edible and were incorporated into the diet of indigenous people as well as some early Europeans. The native cherry may not appear parasitic at first, but in nature it grows its root to the nearest suitable host tree, often a eucalyptus, at which point it joins its root with those of its new host and sequesters water and minerals to aid its growth. Like many parasitic plants though, the native cherry also undertakes photosynthesis to generate sugars to help sustain itself. It is therefore often termed a hemiparasite.
One of the strangest and potentially creepiest parasitic plants in our forests is the native dodder (Cassytha sp.), which grows long winding green vines without leaves. While the genus Cassytha is native to the greater Melbourne region, the invasive golden dodder (Cuscuta campestris) is also found here but is an imported weed only distantly related to our Australian dodders. The native dodder is mainly spread by birds; however, it is also known to be dispersed by native macropods (the family of kangaroos, wallabies and their kin).
Germination experiments suggest that the seeds of this species need to pass through the acrid stomach of a mammal or bird before they will germinate. An interesting aspect of the biology of this species is its ability to act as a vector of microbes between plants. Like mosquitoes that may feed on a number of different animals, passing microbes between them in the process, the native dodder is known to attach itself to multiple plants and feed on them during which microbes are transferred via the dodder between the different hosts.
Perhaps the most beautiful parasitic plants to call Melbourne home are the hyacinth orchids of the genus Dipodium. There has been much controversy regarding how parasitic these plants actually are, as some botanists have suggested they are saprotrophic and feed on decaying organic matter with the help of fungal symbiotes. Others have suggested that they are indirect parasites of eucalyptus, again through the use of a fungal intermediary which helps the orchid feed on the roots of its host. Still others have suggested that the genus may engage in a little of both depending on resource availability. The hyacinth orchids do not produce leaves and can only be seen in summer when they produce tall flower stalks of pink and white-blotched flowers. These emerge from beneath the ground, often close to eucalyptus trees. It is thought that small native bees and wasps are the chief pollinators of these orchids, which the plants may attract by mimicking other native flowers that these pollinators favour.
Unfortunately these spectacular orchids face a number of threats and at least one Victorian member, the yellow hyacinth orchid (Dipodium hamiltonianum), is considered endangered in the state. Grazing by introduced herbivores, particularly the European rabbit, is a serious threat to these orchids. The collection of these plants’ flowers is also a danger and the local extinction of at least one population of yellow hyacinth orchid has been linked to this practice.
Our local forests and wild places host some incredible plants with strange but fascinating ways of life. Whether they provide food for indigenous birds and butterflies, grow fantastic food for us to enjoy, or produce spectacular wildflowers in the middle of our dry Australian summer, our local parasitic plants are a poorly known but incredibly interesting group of flora!
Mackenzie Kwak is a zoologist with a broad interest in Australia's diverse flora and fauna. His research focuses on the biogeography, systematics and ecology of Australasian ectoparasites, particularly ticks, fleas and lice.
All photos sourced from Wikimedia Commons or from the author.