Keystone species ‘punch above their weight’, to use a popular metaphor. They contribute more to their ecosystems than their numbers would indicate. Charismatic top predators such as wolves and dingoes are great examples of keystone species. They generate the trophic cascades that enhance whole systems of life including the geophysical foundations (discussed here). But as the fascinating ecologist Stephan Harding tells us:
‘You never know who the big players are in the wild world.’
To my mind one of the least likely ‘big players’ is mistletoe. Can a parasite actually be a keystone? Surprisingly, the answer is ‘yes’. Not only is mistletoe good for kissing, this great cohort is a ‘keystone resource’.
Let us enter the entrancing world of mistletoe through symbiotic mutualism. A relatively non-technical definition is ‘two or more species that live together to their mutual benefit’. Although the idea of symbiosis was not the dominant paradigm for much of the 20th century, a growing body of research is showing that it complements competition and is utterly fundamental to life on earth and is part of how every creature lives. The great biologist Lynn Margulis declares:
‘We are symbionts on a symbiotic planet.’
Mistletoe, it turns out, is a highly eclectic and inclusive symbiotic mutualist. One of the main families all around the world, and a prominent player in Australia, is Loranthaceae – a family of mistletoe with about 1,000 member species. Most of them are ‘obligate, stem hemiparasites’. This means that they can only live by being attached to another plant (obligate), that they attach to stems (not roots), and that while they get water and some nutrients from their host, they are also able to photosynthesise.
The story of mistletoe mutualisms is all about entanglements of interdependencies, nutrient cycles, and seductions. Loranthaceae are themselves deeply dependent. First there is dependence on the tree or shrub on which they grow. No host, no parasite. Next, there is dependence on birds and bees to pollinate. No pollination, no seeds, no future generations. Then there is dependence on birds, in particular, to eat the fruits and disperse the seeds. No dispersal, very little chance of germination and growth. And there is dependence on the leaf-eaters: no browsing means too much mistletoe growth leading to multiple deaths and disasters.
If mistletoes are to survive they have to entice and nourish their mutualists. The brightly coloured flowers are powerful attractors of pollinators, and the nectar is not only high in sugars, but also fats. Some of the Australian Loranthaceae produce nectar containing droplets of pure fat. The berries are highly visible, abundant and full of nutrition. Worldwide, many ‘folivores’ eat the nutritious leaves: deer, camels, rhinoceroses, gorillas and possums, amongst many others.
Their adaptive edge goes beyond mere provisioning and involves dazzling abundance.
The most awesome interdependence is between mistletoes and their mutualist mistletoe birds. ABC Science journalist Abbie Thomas wrote a delightful account:
Many mistletoes continue to flower in drought or during winter, when few other blossoms are available. Indeed, they are often the only local source of nectar and pollen during hard times. Packed with sugar and carbs, mistletoe fruits are good tucker, not just for the ubiquitous mistletoe bird, but also for cuckoo-shrikes, ravens, cockatoos, shrike-thrushes, woodswallows, bowerbirds, and even emus and cassowaries.
The mistletoe bird plays an important role in the mistletoe plant’s life cycle. The life of most mistletoes begins when a viscous, gluey seed drops onto a branch from the rear end of the brilliantly coloured black, red and white Mistletoe bird. Found throughout Australia, these birds are highly mobile and go wherever mistletoe is in fruit. Once eaten, the seed of the fruit quickly passes through the bird, emerging just 10-15 minutes later. The sticky seed fastens onto the branch, although many seeds fail to adhere, and are lost.
Within days, a tiny tendril emerges from the seed, growing quickly and secreting a cocktail of enzymes directly onto the corky outer protection of the branch. Unable to resist the onslaught, the bark yields a small ulcer-like hole into which the tendril probes, seeking its way down into the sappy tree tissue until it hits paydirt: the water and mineral-rich plumbing of the tree.’
Mutualisms are entanglements of interdependencies. The host tree supports its mistletoes physically and nutritionally, and it also buffers them against the vicissitudes of climate uncertainty. So, too, mistletoes support other species and provide a buffer against fluctuations and uncertainties. A study from Australia shows that mistletoes have extended nectar and seed producing periods, and that within a given region nectar and fruit are available from one or another mistletoe species all year round. In addition, as mistletoes are host to so many insect species, the insect-eating birds also get the benefit. Mammals join the feast, eating leaves, seeds and flowers. Possums are amongst the main leaf eaters, and are seasonally dependent on mistletoe.
Along with all the creatures who consume mistletoes, there is yet another entourage that benefits. Some animals build their nests in the mistletoe where they get some protection from the elements and predators. The action of the mistletoe itself increases hollows in trees, and so all the creatures that nest in hollows get the benefit. A further benefit is that their presence in trees alters the forest canopy and reduces the severity of bushfires.
In life systems, what goes around comes around. The host tree or shrub gets a steady rain of litter, droppings, and other organic matter that become part of the nutrient cycle, benefiting both the host and other plants in the area. In short, the benefits of mistletoes pass through the lives and bodies of many species before turning into nutrients to be drawn up by hosts and tapped into by mistletoes.
The relationships work because of the extravagant generosity of interdependence: highly nutritious nectar produced by bright showy flowers; shiny seeds loaded with carbs and sugars; mistletoe birds with their gorgeous red feathers, lovely song, and fertile poop; gliders and possums; butterflies who visit, eat, and reproduce.
There is an association between songbirds and mistletoe, and as new evidence is showing that both groups have their origins in ancient Gondwanaland, perhaps there is more to this old and beautiful alliance than is yet properly understood. I found myself totally captivated by a story shared by Andrew Skeoch, a sound recordist specialising in the sounds of nature. He recorded a mistletoe bird in full song, and inadvertently also recorded the fact that this talented little creature was singing and pooping at the same time. Something about this bright little bird creating and performing musically, while depositing mistletoe seeds securely wrapped in glue and fertiliser seems almost magical in its joyfulness (listen to the birdsong here).
It is good to recall that there is an old European history of respect. Mistletoe is sacred to Druids (contemporary and ancient), and it is still a customary Christmas decoration. Hung over the threshold, it invites people to kiss. In earlier days it was said to be able to find buried treasure, keep witches away and prevent trolls from souring milk! It would be good also to recall that Aboriginal Australians respect mistletoe as a food for humans and for many other creatures. In North Australia, where so much of my learning has taken place, people give berries to children, but adults avoid them. Perhaps they are aware that growing children have a particular need for the high nutritional value of mistletoe.
At this time, many people think mistletoe is a pest. The term ‘parasite’ conjures negative imagery, but the larger issue, at least in Australia, is that in some areas mistletoes are over-abundant. Trees are dying, and something has gone askew because mistletoe cannot thrive if the host dies. The renowned science writer Tim Low tells us that the loss of possums, those folivores who love their mistletoe, is a key. “Foxes, by preying on mistletoe-munching possums,” set up conditions where mistletoes can grow out of control. Possums are only prey to foxes when they come down out of the trees. Along roadsides and on farms, they are at risk. Within forests where they can remain up in the trees possums thrive and mistletoe is contained.
So, what would partnership rewilding be like if the focus were on mistletoes and their ‘ground up’ trophic dynamics?
First, it would involve fewer foxes and more possums. Here the answer is readily to hand in the form of the dingo. As I have been reporting in other essays, the evidence is overwhelmingly clear that dingoes reduce the numbers of invasive species such as foxes and cats, and promote the viability of smaller native marsupials such as possums.
Second, it would involve on-going health and reproductive capacity of more extensive stands of trees. Here the answer is readily to hand in the form of flying-foxes. Their pollination is utterly crucial to the future of forests and woodlands in Australia, and their lives and livelihoods are central to partnership rewilding.
Third, it would involve changes in human thought and action. Not everyone thinks mistletoes are innate pests, but, as the great mistletoe scientist David Watson indicates, “pretty much all of the public’s perceptions about Mistletoe are fundamentally incorrect.” I want to be clear that Aboriginal people are not likely to hold these misperceptions. Here, as with other matters, the limitations of the mainstream public cannot readily be attributed to everyone. Having said that ~~
I want to set up camp, metaphorically at least, under the mistletoe. Here the kiss of life is sensuous, continuous, and diverse.
I hope others will join me, and I rather hope we won’t get pooped on! Let us open our lives to the great, complex, on-going, joyful, benefit-rich, exuberant and dazzling generosity that holds entangled interdependencies together. A camp in the midst of all these mutualisms is place of coming-forth for those whose flows of life and death are achieved together. These entangled partnerships have co-evolved over millions of years, and if the human newcomer can partner in with them, we may yet become part of ecosystems that will hold together in this time of flux and uncertainty.
© Deborah Bird Rose (2014)
This is the third in a series of essays on partnership rewilding. The others include: Partnership Rewilding with Flying-Foxes, and Partnership Rewilding with Predators.
Most of the scientific information in this essay is drawn from David Watson’s outstanding work. One of his main articles is free online: http://rspb.royalsocietypublishing.org/content/early/2012/07/03/rspb.2012.0856.full
Another is not open access except for the abstract: http://www.annualreviews.org/doi/pdf/10.1146/annurev.ecolsys.32.081501.114024
Abbie Thomas’s article is available online: http://www.abc.net.au/science/articles/2004/03/05/2044992.htm
Lynn Margulis’s book is Symbionic Planet (a New Look at Evolution).
The book by Tim Low mentioned in this essay is New Nature.
Information on dingoes as top predators is available in previous essays, and is the subject of a recent article by Arian Wallach, published in The Conversation. (read here)
My essay on flying-foxes and the kiss of life is not freely available online but I am happy to share copies if asked.