A recent survey on the world’s plants found a shocking number have gone extinct – 571 since 1750. And this is likely to be a stark underestimate. Not all plants have been discovered, so it’s likely other plants have gone extinct before researchers know they’re at risk, or even know they exist.
In Australia, the situation is just as dire. The Threatened Species Recovery Hub recently conducted two evaluations that aren’t yet published of extinct plants in Australia. They found 38 have been lost over the last 170 years, such as the Daintree River banana (Musa fitzalanii) and the fringed spider-orchid (Caladenia thysanochila).
But uncertainty about the number of plant extinctions, in addition to the 38 confirmed, is an ongoing concern.
Both studies pointed out the actual number of extinctions is likely to be far more than those recognised in formal lists produced by the Commonwealth and state and territory agencies.
For example, there is still a high rate of discovery of new plant species in Australia. More than 1,600 plants were discovered between 2009 and 2015, and an estimated 10% are still yet to be discovered.
The extinction of Australian plants is considered most likely to have occurred in areas where there has been major loss and degradation of native bushland. This includes significant areas in southern Australia that have been cleared for agriculture and intensive urbanisation around major cities.
Many of these extinct plants would have had very restricted geographic ranges. And botanical collections were limited across many parts of Australia before broad scale land clearing and habitat change.
There is already one well recognised Australian plant extinction, a shrub in Phillip Island (Streblorrhiza speciosa), which was never formally recognised on any Australian threatened species list.
Researchers also note there are Australian plants that are not listed as extinct, but have not been collected for 50 years or more.
While undocumented extinction is an increasing concern, the recent re-assessment of current lists of extinct plants has provided a more positive outcome.
The re-assessment found a number of plants previously considered to be extinct are not actually extinct. This includes plants that have been re-discovered since 1980, and where there has been confusion over plant names. Diel’s wattle (Acacia prismifolia), for instance, was recently rediscovered in Western Australia.
A significant challenge for accurately assessing plant extinction relates to the difficulties in surveying and detecting them in the Australian landscapes.
Many have histories associated with fire or some other disturbance. For example, a number of plants spend a significant part of their time as long-lived seeds – sometimes for decades – in the soil with nothing visible above ground, and with plants only appearing for a few years after a fire.
But by far, the greatest reason for the lack of information is the shortage of field surveys of the rare plants, and the availability of botanists and qualified biologists to survey suitable habitat and accurately identify the plants.
The continuing decline of Australia’s threatened plants suggests more extinctions are likely. But there have been important achievements and lessons learnt in dealing with the main causes of loss of native vegetation.
Our understanding of plant extinction processes – such as habitat loss, habitat degradation, invasive weeds, urbanisation, disease and climate change – is improving. But there is still a significant way to go.
One challenge in dealing with the causes of Australian plant extinction is how to manage introduced diseases.
Two plant diseases in particular are of major concern: Phytophthora dieback, a soil-borne water mould pathogen, and Myrtle rust, which is spread naturally by wind and water.
Both diseases are increasingly recognised as threats, not only because of the impact they are already having on diverse native plant communities and many rare species, but also because of the difficulties in effective control.
Two Australian rainforest tree species Rhodomyrtus psidioides and Rhodamnia rubescens were recently listed as threatened under the NSW legislation because of myrtle rust.
While extinction associated with disease is often rapid, some individual plants may survive for decades in highly degraded landscapes, such as long-lived woody shrubs and trees. These plants will ultimately go extinct, and this is often difficult to communicate to the public.
While individual species will continue to persist for many years in highly disturbed and fragmented landscapes, there is little or no reproduction. And with their populations restricted to extremely small patches of bush, they’re vulnerable to ongoing degradation.
In many such cases there is an “extinction debt”, where it may take decades for extinction to occur, depending on the longevity of the plants involved.
But it’s not all doom and gloom. A recent study found of the 418 threatened Australian plants showing ongoing decline, 83% were assessed as having medium to high potential for bouncing back.
And with long-term investment and research there are good prospects of saving the majority of these plants.
Every autumn we are treated to one of nature’s finest seasonal annual transitions: leaf colour change and fall.
Most of the autumn leaf-shedding trees in Australia are not native, and some are declared weeds. Nevertheless, Australia has a spectacular display of trees, from the buttery tresses of Ginkgo biloba to the translucent oaks, elms and maples.
Autumn colour changes are celebrated worldwide and, when the time is right, autumn leaves reconnect us to nature, driving “leaf-peeping” tourist economies worldwide.
However, recent temperature trends and extremes have changed the growing conditions experienced by trees and are placing autumn displays, such as Canberra’s, at risk.
This year, Canberra, like the rest of Australia, endured its hottest summer on record. In NSW and the ACT, the mean temperature in January was 6°C warmer than the long-term average. So far, autumn is following suit.
These extremes can interrupt the ideal synchronisation of seasonal changes in temperature and day length, subduing leaf colours.
In addition, hotter summer temperatures scorch leaves and, when combined with this and the previous years’ low autumn rainfall, cause trees to shed leaves prematurely, dulling their autumn leaf displays.
We learnt in childhood autumn colour change follows the arrival of cooler temperatures. Later we learnt the specifics: seasonal changes in day length and temperature drive the depletion of green chlorophyll in leaves. Temperature can also affect the rate at which it fades.
In the absence of chlorophyll, yellows and oranges generated by antioxidants in the leaf (carotenoids) as well as red through to purples pigments (anthocyanins), synthesised from stored sugars, emerge. Temperature plays a role here too – intensifying colours as overnight temperatures fall.
We’ve also come to understand the role of a leaf’s environment. Anthocyanin production is affected by light intensity, which explains why sunny autumns produce such rich colours and why the canopies of our favourite trees blush red at their edges while glowing golden in their interior.
However, early signs show this year’s autumn tones will be muted. After the record-breaking heat of summer and prolonged heat of March, many trees are shrouded in scorched, faded canopies. The ground is littered with blackened leaves.
Of course, we’ve seen it before.
During the Millennium Drought, urban trees sporadically shed their leaves often without a hint of colour change. Fortunately, that was reversed at the drought’s end.
But we’re kidding ourselves if we believe this last summer was normal or recent temperature trends are just natural variability. If this is a sign of seasons future, we need to prepare to lose some of autumn’s beauty.
Long-term and experimental data show that the sensitivity of autumn colour change to warmer temperatures varies widely between species. While large-scale meta-analyses point to a delay in the arrival of autumn colours of one day per degree of warming, individual genera may be far more sensitve. Colour change in Fagus is delayed by 6-8 days per degree.
Warming temperatures, then, mean the cohesive leaf-colour changes we’re accustomed to will break down at landscape scales.
In addition, as warm weather extends the growing season and deep-rooted trees deplete soil moisture reservoirs, individual trees are driven by stress rather than seasonal temperature change and cut their losses. They shed leaves at the peripheries of their canopies.
The remainder wait – bronzed by summer, but still mostly green – for the right environmental cue.
For years, careful species selection and selective breeding enhanced autumn colour displays. This rich tapestry is now unravelling as hotter summers, longer autumns and drought affect each species differently.
It seems logical warmer temperatures would mean shorter and less severe frost seasons. Paradoxically, observations suggest otherwise – the arrival of frost is unchanged or, worse, occurring earlier.
When not preceded by gradually cooling overnight temperatures, frosts can induce sudden, unceremonious leaf loss. If warm autumn temperatures fail to initiate colour change, autumn displays can be short-circuited entirely.
At the centre of many urban-tree plantings, our long association with elms faces a threat. Loved for the contrast their clear yellow seasonal display creates against pale autumn skies, elm canopies have been ravaged by leaf beetles this year. Stress has made trees susceptible to leaf-eating insects, and our current season delivered an expanse of stressed, and now skeletal, trees.
This dulled image of autumn is far from universal. Climates differ between locations. So too will the climate changes we’ve engineered and their impact on autumn displays.
Increased concentration of anthocyanins associated with warmer summers has, for example, created spectacular leaf displays in Britain’s cooler climates.
Of course, we’ll continue to experience radiant autumn displays too.
In years of plentiful rain, our trees will retain their canopies and then, in the clear skies of autumn, dazzle us with seasonal celebrations. However, that too may be tempered by the increased risk of colour-sapping pathogens, such as poplar rust, favoured by warm, moist conditions. And there are also negative consequences for autumn colour associated with elevated carbon dioxide concentrations.
Of course, we need to keep it in perspective – the dulling of autumn’s luminescence is far from the worst climate change impacts. Nevetheless, in weakening our link with nature, the human psyche is suffering another self-inflicted cut as collective action on climate change stalls.
The phrase “urban jungle” gets thrown around a lot, but we don’t usually think of cities as places where rare or threatened species live.
Our research, published today in Frontiers in Ecology and the Environment, shows some of Australia’s most endangered plants and animals live entirely within cities and towns.
Australia is home to 39 urban-restricted threatened species, from giant gum trees, to ornate orchids, wonderful wattles, and even a tortoise. Many of these species are critically endangered, right on the brink of extinction. And cities are our last chance to preserve them within their natural range.
Urban environments offer a golden opportunity to preserve species under threat and engage people with nature. But that means we might need to think a little differently about how and where we do conservation, embrace the weird and wonderful spaces that these species call home, and involve urban communities in the process.
When you picture city animals you might think of pigeons, sparrows or rats that like to hang out with humans, or the flying foxes and parrots that are attracted to our flowering gardens.
But that’s not the case here. The threatened species identified in our research didn’t choose the city life, the city life chose them. They’re living where they’ve always lived. As urban areas expand, it just so happens that we now live there too.
The first hurdle that springs to mind when it comes to keeping nature in cities is space: there’s not a lot of it, and it’s quickly disappearing. For example, the magnificent Caley’s Grevillea has lost more than 85% of its habitat in Sydney to urban growth, and many of its remaining haunts are earmarked for future development. Around half of the urban-restricted species on our list are in the same predicament.
It’s especially tough to protect land for conservation in urban environments, where development potential means high competition for valuable land. So when protected land is a luxury that few species can afford, we need to work out other ways to look after species in the city.
Precious endangered species aren’t all tucked away in national parks and conservation reserves. These little battlers are more often found hiding in plain sight, amid the urban hustle and bustle.
Our research found them living along railway lines and roadsides, sewerage treatment plants and cemeteries, schools, airports, and even a hospital garden. While these aren’t the typical places you’d expect to find threatened species, they’re fantastic opportunities for conservation.
The spiked rice flower is a great example. Its largest population is on a golf course in New South Wales, where local managers work to enhance its habitat between the greens, and raise awareness among residents and local golfers. These kinds of good partnerships between local landowners and conservation can find “win-win” situations that benefit people and nature.
It’s no secret that living in the ‘burbs can be risky: a fact best illustrated in the cautionary tale of a roadside population of the endangered Angus’s onion orchid. Construction workers once unwittingly dumped ten tonnes of sand over the patch in the late 1980s, then quickly attempted to fix the problem using a bulldozer and a high-pressure hose. Later, a portaloo was plonked on top of it.
Examples like this show just how important it is for policy makers, land managers and the community to know that these species are there in the first place, and are aware that even scrappy-looking habitats can be important to their survival. Otherwise, species are just one stroke of bad luck away from extinction.
It’s common to think if you want to conserve nature, you need to get as far away from people as you can. After all, we can be a dangerous lot (just ask Angus’s onion orchid). But we also have extraordinary potential to create positive change – and it’s much easier for us to do this if we only have to travel as far as our backyard or a local park.
Many urban-restricted species get support by their local communities. Examples from our research showed communities across Melbourne raising thousands of dollars in conservation crowdfunding, dedicating countless volunteer hours to caring for local habitats, and even setting up neighbourhood watches to combat vandals. This shows a huge opportunity for urban residents to be on the conservation frontline.
Our research focused on 39 species that are restricted to Australian cities and towns today. But that’s not where the opportunity for urban conservation ends.
There are about another 370 threatened species that share their range with urban areas across Australia, as well as countless “common” native species that call cities home. And as cities continue to expand, many other threatened species stand to become urban dwellers. It’s clear that if we only focus conservation efforts in areas far from humans, species like these will be lost forever.
Most citizen science initiatives ask people to record living things, like frogs, wombats, or feral animals. But dead things can also be hugely informative for science. We have just launched a new citizen science project, The Dead Tree Detective, which aims to record where and when trees have died in Australia.
The current drought across southeastern Australia has been so severe that native trees have begun to perish, and we need people to send in photographs tracking what has died. These records will be valuable for scientists trying to understand and predict how native forests and woodlands are vulnerable to climate extremes.
Understanding where trees are most at risk is becoming urgent because it’s increasingly clear that climate change is already underway. On average, temperatures across Australia have risen more than 1℃ since 1910, and winter rainfall in southern Australia has declined. Further increases in temperature, and increasing time spent in drought, are forecast.
How our native plants cope with these changes will affect (among other things) biodiversity, water supplies, fire risk, and carbon storage. Unfortunately, how climate change is likely to affect Australian vegetation is a complex problem, and one we don’t yet have a good handle on.
All plants have a preferred average climate where they grow best (their “climatic niche”). Many Australian tree species have small climatic niches.
It’s been estimated an increase of 2℃ would see 40% of eucalypt species stranded in climate conditions to which they are not adapted.
But what happens if species move out of their climatic niche? It’s possible there will be a gradual migration across the landscape as plants move to keep up with the climate.
It’s also possible that plants will generally grow better, if carbon dioxide rises and frosts become less common (although this is a complicated and disputed claim.
However, a third possibility is that increasing climate extremes will lead to mass tree deaths, with severe consequences.
There are examples of all three possibilities in the scientific literature, but reports of widespread tree death are becoming increasingly commonplace.
Many scientists, including ourselves, are now trying to identify the circumstances under which we may see trees die from climate stress. Quantifying these thresholds is going to be key for working out where vegetation may be headed.
Australian plants must deal with the most variable rainfall in the world. Only trees adapted to prolonged drought can survive. However, drought severity is forecast to increase, and rising heat extremes will exacerbate drought stress past their tolerance.
To explain why droughts overwhelm trees, we need to look at the water transport system that keeps them alive. Essentially, trees draw water from the soil through their roots and up to their leaves. Plants do not have a pump (like our hearts) to move water – instead, water is pulled up under tension using energy from sunlight. Our research illustrates how this transport system breaks down during droughts.
In hot weather, more moisture evaporates from trees’ leaves, putting more pressure on their water transport system. This evaporation can actually be useful, because it keeps the trees’ leaves cool during heatwaves. However if there is not enough water available, leaf temperatures can become lethally high, scorching the tree canopy.
We’ve also identified how drought tolerance varies among native tree species. Species growing in low-rainfall areas are better equipped to handle drought, showing they are finely tuned to their climate niche and suggesting many species will be vulnerable if climate change increases drought severity.
Based on all of these data, we hope to be able to predict where and when trees will be vulnerable to death from drought and heat stress. The problem lies in testing our predictions – and that’s where citizen science comes in. Satellite remote sensing can help us track overall greenness of ecosystems, but it can’t detect individual tree death. Observation on the ground is needed.
However, there is no system in place to record tree death from drought in Australia. For example, during the Millennium Drought, the most severe and extended drought for a century in southern Australia, there are almost no records of native tree death (other than along the rivers, where over-extraction of water was also an issue). Were there no deaths? Or were they simply not recorded?
The current drought gripping the southeast has not been as long as the Millennium Drought, but it does appear to be more intense, with some places receiving almost no rain for two years. We’ve also had a summer of repeated heatwaves, which will have intensified the stress.
We’re hearing anecdotal reports of tree death in the news and on twitter. We’re aiming to capture these anecdotal reports, and back them up with information including photographs, locations, numbers and species of trees affected, on the Dead Tree Detective.
We encourage anyone who sees dead trees around them to hop online and contribute. The Detective also allows people to record tree deaths from other causes – and trees that have come back to life again (sometimes dead isn’t dead). It can be depressing to see trees die – but recording their deaths for science helps to ensure they won’t have died in vain.
This is part of a major series called Advancing Australia, in which leading academics examine the key issues facing Australia in the lead-up to the 2019 federal election and beyond. Read the other pieces in the series here.
We need nature. It gives us inspiration, health, resources, life. But we are losing it. Extinction is the most acute and irreversible manifestation of this loss.
Australian species have suffered at a disproportionate rate. Far more mammal species have become extinct in Australia than in any other country over the past 200 years.
The thylacine is the most recognised and mourned of our lost species, but the lesser bilby has gone, so too the pig-footed bandicoot, the Toolache wallaby, the white-footed rabbit-rat, along with many other mammals that lived only in Australia. The paradise parrot has joined them, the robust white-eye, the King Island emu, the Christmas Island forest skink, the southern gastric-brooding frog, the Phillip Island glory pea, and at least another 100 species that were part of the fabric of this land, part of what made Australia distinctive.
And that’s just the tally for known extinctions. Many more have been lost without ever being named. Still others hover in the graveyard – we’re not sure whether they linger or are gone.
The losses continue: three Australian vertebrate species became extinct in the past decade. Most of the factors that caused the losses remain unchecked, and new threats are appearing, intensifying, expanding. Many species persist only in slivers of their former range and in a fraction of their previous abundance, and the long-established momentum of their decline will soon take them over the brink.
These losses need not have happened. Almost all were predictable and preventable. They represent failures in our duty of care, legislation, policy and management. They give witness to, and warn us about, the malaise of our land and waters.
How do we staunch the wound and maintain Australia’s wildlife? It’s a problem with many facets and no single solution. Here we provide ten recommendations, based on an underlying recognition that more extinctions will be inevitable unless we treat nature as part of the essence of this country, rather than as a dispensable tangent, an economic externality.
We should commit to preventing any more extinctions. As a society, we need to treat our nature with more respect – our plants and animals have lived in this place for hundreds of thousands, often millions, of years. They are integral to this country. We should not deny them their existence.
We should craft an intergenerational social contract. We have been gifted an extraordinary nature. We have an obligation to pass to following generations a world as full of wonder, beauty and diversity as our generation has inherited.
We should highlight our respect for, and obligation to, nature in our constitution, just as that fusty document could be refreshed and some of its deficiencies redressed through the Uluru Statement from the Heart. Those drafting the blueprint for the way our country is governed gave little or no heed to its nature. A constitution is more than a simple administrative rule book. Countries such as Ecuador, Palau and Bhutan have constitutions that commit to caring for their natural legacy and recognise that society and nature are interdependent.
We should build a generation-scale funding commitment and long-term vision to escape the fickle, futile, three-year cycle of contested government funding. Environmental challenges in Australia are deeply ingrained and longstanding, and the conservation response and its resourcing need to be implemented on a scale of decades.
As Paul Keating stated in his landmark Redfern speech, we should all see Australia through Aboriginal eyes – more deeply feel the way the country’s heart beats; become part of the land; fit into the landscape. This can happen through teaching curricula, through reverting to Indigenous names for landmarks, through reinvigorating Indigenous land management, and through pervasive cultural respect.
We need to live within our environmental limits – constraining the use of water, soil and other natural resources to levels that are sustainable, restraining population growth and setting a positive example to the world in our efforts to minimise climate change.
We need to celebrate and learn from our successes. There are now many examples of how good management and investments can help threatened species recover. We are capable of reversing our mismanagement.
Funding to prevent extinctions is woefully inadequate, of course, and needs to be increased. The budgeting is opaque, but the Australian government spends about A$200 million a year on the conservation of threatened species, about 10% of what the US government outlays for its own threatened species. Understandably, our American counterparts are more successful. For context, Australians spend about A$4 billion a year caring for pet cats.
Environmental law needs strengthening. Too much is discretionary and enforcement is patchy. We suggest tightening the accountability for environmental failures, including extinction. Should species die out, formal inquests should be mandatory to learn the necessary lessons and make systemic improvements.
We need to enhance our environmental research, management and monitoring capability. Many threatened species remain poorly known and most are not adequately monitored. This makes it is hard to measure progress in response to management, or the speed of their collapse towards extinction.
Extinction is not inevitable. It is a failure, potentially even a crime – a theft from the future that is entirely preventable. We can and should prevent extinctions, and safeguard and celebrate the diversity of Australian life.
John Woinarski, Professor (conservation biology), Charles Darwin University; Sarah Legge, Professor, Australian National University, and Stephen Garnett, Professor of Conservation and Sustainable Livelihoods, Charles Darwin University
People don’t like parasites. But there’s a local Aussie tree that’s only a little bit parasitic: the native cherry, or cherry ballart.
It’s what we call hemiparasitic. It can photosynthesise, but gains extra nutrients by attaching its roots to host plants.
The native cherry, Exocarpos cupressiformis, might be our most widespread root hemiparasite tree, but we’re not quite sure – root-parasitic shrubs and trees are a bit of a research blank spot. We are not even really sure who all the hosts of cherry ballart are.
Warty hammer orchids are sexual deceivers
Although other parasites – like mistletoes – have a more direct Christmas association, cherry ballart does have an Australian Yuletide connection: their conifer-like appearance (the species name cupressiformis means “cypress-like”) was noted by homesick European settlers, who chopped them down for Christmas trees.
Cherry ballart grows from the Atherton Tablelands in Queensland to southern Tasmania, and across to the Eyre Peninsula in South Australia.
The first European to record it was Jacques-Julien Houtou de Labillardière, the botanist on d’Entrecasteaux’s expedition in search of La Perouse. He formally described the species in 1800, but we have no physical type specimen – the botanical type is his illustration and description. Maybe he lost his specimen, or disposed of it, or thought a picture would do; Jacques seems to have been a bit cavalier with his record-keeping.
Or perhaps it was stolen or misplaced after all his specimens were seized in an overlapping series of defections, wars, defeats and revolution as the expedition tried to return to Europe. The collection was eventually returned after the intercession of English botanist Joseph Banks – but no cherry ballart.
Its distinctive shape led to native cherry being marked on early Australian orienteering maps, since they are in a cartographic Goldilocks zone: obvious, just numerous enough to make them useful, but not so many as to clutter the map.
That was until Australia held the World Orienteering Championships in the mid-1980s, when the standardisation of Australian orienteering maps for overseas competitors led to the cherry ballart becoming an early victim of internationalisation – at least cartographically speaking.
Its utility also extended to the timber. Among the uses of its “close-grained and handsome wood” are tool handles, gun stocks and map rollers (although the last is probably a niche market these days).
Indigenous Australians ate the fruit, used the wood for spear throwers and reportedly used the sap as a treatment for snakebite. They called it Tchimmi-dillen (Queensland), Palatt or Ballot (Lake Condah, Victoria) and Ballee (Yarra).
Despite producing large quantities of fruit and seed, no one seems to be able to get native cherry to germinate reliably. There are anecdotal reports that feeding the seed to chooks works, but other growers dismiss this approach.
The edible fruit isn’t actually a true fruit: it’s a swollen stem. It’s reported to have the highest sugar level of any native fruit in the forests of southern Victoria and is much tastier than you’d think a stem would be. (It’s also probably an important nutrient supply for some birds, but that’s yet another thing we are yet to prove.)
This odd “fruit” gives rise to the genus name (exo = outside, carpos = fruit,) and was often touted by early European writers as another example of the topsy-turvy nature of Australia – “cherries” with the pit on the outside went along with “duck-billed playtpus”, animals with pouches, trees that shed bark rather than leaves, and Christmas in the middle of summer.
Despite their oddness, native cherries in the bush are biodiversity hotspots. My camera trap data show they preferentially attract echidnas, possums, foxes, swamp wallabies, white-winged choughs and bronzewing pigeons.
This might be because they modify their immediate environment. My research shows they create moderate micro-climates in their foliage, reduce soil temperatures, increase soil water retention, concentrate nutrients in the soil beneath their canopies, and alter the understorey vegetation. They also kill some of their host trees, creating patches with higher concentrations of dead timber. All these probably have something to do with their animal attraction, but exactly how is a mystery yet to be solved.
In addition to their attractiveness to vertebrates, native cherries are required hosts for some striking moths and share specialist host duties with mistletoe for some of our most beautiful butterflies (although mistletoes take most of the glory in the scientific literature).
My research into our cherry ballart hopes in part to correct these historical slights. I want to set the record straight on this overlooked widespread and attractive little tree, which has a long indigenous use and was one of the first of our native flora to be described by Europeans.
Orchids are famed for their beautiful and alluring flowers – and the great lengths to which people will go to experience them in the wild. Among Australian orchids, evocative names such as The Butterfly Orchid, The Queen of Sheeba, and Cleopatra’s Needles conjure up images of rare and beautiful flowers.
Yet there is a rich diversity of our orchids. Some are diminutive, warty, and unpleasant-smelling, bearing little resemblance to a typical flower.
While many orchid enthusiasts have a soft spot for these quirky members of the Australian flora, what has brought them international recognition is their flair for using some of the most bizarre reproductive strategies on Earth.
From the very beginnings of pollination research in Australia there were signs that something unusual was going on in the Australian orchid flora.
In the 1920s Edith Coleman from Victoria made the sensational discovery that the Australian tongue and bonnet orchids (Cryptostylis) were pollinated by males of a particular species of ichneumonid wasp attempting to mate with the flower.
But this was just the beginning.
We now know that while the insect species involved may vary, many of our orchid species use this strategy. Australia is the world centre for sexual deception in plants.
Perhaps the most sophisticated flower of all sexually deceptive plants is seen in the hammer orchids, a diminutive genus that only grows in southwestern Australia. Their solitary stem reaches a height of around 40cm, and each stem produces a single flower no more than 4cm in length.
Even among sexually deceptive orchids, hammer orchids stand out from the crowd. They have a single heart-shaped leaf that sits flush with the soil surface, and grow in areas of dry inhospitable sand – an unusual choice for an orchid.
And then there is the flower. Not only does the lip of the flower more closely resemble an insect than a petal, but it is hinged partway along. All of which starts to makes sense once you see the pollinators in action.
Like many other Australian sexually deceptive orchids, they are pollinated by thynnine wasps – a unique group in which the male picks up the flightless female and they mate in flight.
In the case of hammer orchids, the male grasps the insect-like lip and attempts to fly off with “her”. The combination of his momentum and the hinge mechanism swings him upside down and onto the orchid’s reproductive structures.
So, how do you trick a wasp?
Accurate visual mimicry of the female insect does not appear to be essential, as there are some sexually deceptive orchids that are brightly coloured like a regular flower.
Instead, the key ingredient for attracting pollinators to the flower is mimicking the sex pheromone of the female insect. And boy, is this pheromone potent.
Indeed, one of the strangest fieldwork experiences I’ve had was wasps flying through my open car window while stopped at traffic lights, irresistibly drawn to make love to the hammer orchids sitting on the passenger seat!
While determining the chemicals responsible for attraction of sexually deceived pollinators is a laborious process, we now know that multiple classes of chemicals are involved, several of which were new to science or had no previously known function in plants.
What’s more, we are still discovering new and unexpected cases of sexual deception in orchids that don’t conform to the insect-like appearance of many sexually deceptive orchids.
A classic example is the case of the Warty hammer orchid and the Kings spider orchid – these two species have totally different-looking flowers, yet both are pollinated by the same wasp species through sexual deception.
While the ability to attract sexually excited males without closely resembling a female insect may partly explain the evolution of sexual deception, it does not explain the benefit of evolving this strategy in the first place.
A leading hypothesis for the evolution of sexual deception is that mate-seeking males be more efficient at finding orchid flowers than food-foraging pollinators – but this remains a work in progress.
From a conservation point of view, pollination by sexual deception has some interesting challenges. Female animals produce sex pheromones that only attract males of their own species. This means an orchid that mimics a sex pheromone typically relies on a single pollinator species. As such, conservation of any given orchid species requires the presence of a viable population of a particular pollinator.
Further, an interesting quirk of these sexually deceptive systems is the potential for cryptic forms of the orchid: where populations of orchids that appear identical to human observers actually attract different pollinator species through shifts in pheromone chemistry. Indeed, of the ten known species of hammer orchid, three contain cryptic forms.
Australia’s unusual species
Not only does this create a major challenge for managing rare species, it raises the possibility that – should these forms prove to be separate species – the true diversity of sexually deceptive orchids could be greatly underestimated.
Sign up to the Beating Around the Bush newsletter here, and suggest a plant we should cover at email@example.com.
The 1800s was a time of colonial expansion across the globe. During this time the great and the good of Britain filled their grand gardens with exotic novelties from all corners of the world.
Amongst these were many species of Asian rhododendron, a diverse and colourful genus of shrubs and small trees, whose high altitude origins made them well suited to the cool temperate climate of England and Scotland.
Throughout the 19th century, commercial collectors and field naturalists discovered rhododendron species in southern China, the Himalayas, on the high peaks of Borneo, Java and especially New Guinea.
These finds lead Victoria’s government botanist of the time, Ferdinand von Mueller, to speculate about finding rhododendrons on the high tropical mountains on the northeast coast of Queensland. He wrote:
When in 1855 [I] saw… the bold outlines of Mount Bellenden-Ker, the highest mount of tropical Australia, towering to 5,000 feet, [I] was led to think, that the upper region might prove to be the home of species of Rhododendron… forms of plants characteristic of cool Malayan sylvan regions.
But the lofty heights of Mt Bellenden Ker were unknown to European Australians. It would be another 32 years before an expedition led by naturalist W.A. Sayer reached its central peak.
Sayer’s expedition, accompanied by two indigenous assistants, reached the mountain’s high ridge after several mishap-filled attempts. It was here they confirmed Mueller’s suspicions. Sayer’s account of its discovery is interesting:
The top of the range is razor-backed, and on travelling along the range beyond the spur by which we ascended, I could not see the sides, they being, if anything, hanging over. We tumbled rocks over, but could not hear them fall.
It was here that I observed the Rhodendron Lochae growing, and asked the Kanaka to get it; but he remarked, ‘S’pose I fall, I no see daylight any more; I go bung altogether;’ so I had to get it myself.
Mueller received the hard-won specimens and named the species Rhododendron lochae (later corrected to R. lochiae) after Lady Loch, the wife of the Victorian Governor.
Since then, rhododendron plants have been found on nine peaks and tablelands in the Wet Tropics region of north Queensland. Populations on peaks south of Cairns are called Rhododendron lochiae, whilst plants growing on mountains to the north of Cairns are considered by some to be a distinct species: Rhododendron viriosum.
Both northern and southern plants are straggly shrubs that grow in thin soils or rock cracks, sometimes in open cloud-swept boulder fields, sometimes in deep shade along creeks, or rarely as epiphytes on moss-covered trees. They produce bunches of gloriously red, bell-shaped flowers, followed by dry brown capsules filled with small winged seeds that are apparently spread by wind.
They grow slowly but with relative ease from cuttings, and are often cultivated in gardens and nurseries in temperate Australia. However, over time knowledge of the precise origin of these cultivated plants has been lost, which means they are unsuitable for detailed scientific investigations.
All of Australia’s rhododendron populations are located at altitudes above 950m in National Parks within the Wet Tropics World Heritage Area. Most are difficult to access, requiring arduous climbs on rough foot tracks through leech-infested rainforest. And yet, although isolated in protected areas, they are threatened by human activities: loss of habitat due to climate change.
Recent climate modelling research published by scientists from James Cook University and the CSIRO predicts significant reductions in suitable habitat for a suite of mountaintop flora species in Australia’s tropics (our rhododendrons were not included in the analysis, but occupy the habitats assessed).
The habitat of many of these species is predicted to disappear altogether well before the end of the century.
Using rhododendron as a model, the Australian Tropical Herbarium at James Cook University is working to save these threatened species through “ex situ” conservation – cultivation in temperate zone public gardens, well outside their natural range. Because the threatening process – climate change – is not readily mitigated, establishing precautionary ex situ collections is the only viable conservation intervention for these plants.
With funding from the Australian Rhododendron Society Victoria Branch and the Ian Potter Foundation, and the support of traditional owners, Queensland National Parks and the Wet Tropics Management Authority, we have mounted expeditions to collect samples from most of the known populations.
These expeditions have put expert naturalists into rarely visited and challenging environments. Beyond gathering rhododendron samples, new moss species have been discovered and are being named, a fern previously thought extinct was rediscovered, and beautiful little epiphytic orchids have been found on a mountain where they’d not previously been recorded. Golden bower-bird bowers have been mapped in remote mountain rainforests, and a likely new species of snail has been discovered.
Australia now has a well-documented and genetically diverse collection of native rhododendron plants thriving in the Dandenong Ranges Botanic Garden.
We plan to expand this work, ensuring the preservation and public display of rhododendron and many other mountain species threatened by climate change.