Although my research is on indefinite/terminal hiaitus, I am happy to see my last seminar remains online. It is certainly far from my best – given under circumstances of COVID lockdown, and after I had left my last academic post – but at least there’s a record for anyone interested in Prostanthera pollination.
The seminar was given as part of the special Prostanthera series of events held by the Australian Plants Society Victoria.
With Melbourne in lockdown and my change in career, time in the bush has been scarce for me this year. So it was with some relish that I recently headed out to Pyrete Range for a therapeutic communion with some native plants and pollinators. Most of Spring flowering had passed already, but I found a large population of Prostanthera saxicola var. bracteolata in peak flower. It’s a pretty restricted and uncommon shrub, so there are not likely to be many other floral visitor observations that have been made on this species.
I spent a relaxed hour or two on a warm day wandering between plants, making informal observations of flower visitors, and photographing interactions. The most frequent floral visitor was a native reed Bee (Exoneura sp.) which was in high numbers and reliably visiting flowers. I must have observed around 50 or more foraging bouts by reed bees, crawling deep into the flowers for nectar and collecting pollen from the anthers in the upper corolla.
I have known reed bees to nest in fern fronds (in wet forest) and in rushes and sedges (in the high country). Quite where they are nesting in this dry sclerophyll habitat, I do not know. But there is obviously a very large population of them.
Honey bees were also somewhat common visitors, as were some striking iridescent blue-green forester moths (family Zygaenidae). I saw six forester moth visits, but am not convinced they could be effective pollinators. They usually perch on the outside of the flowers, extending their proboscis into the base of the corolla for nectar, scarcely contacting the anthers.
One surprise was a single visit observed by this jewel beetle (Castiarina sp.), which was enjoying a feed on Prostanthera pollen.
I recently put together some material on my work for the University of Melbourne open day. As a teaser for the papers in current preparation, here’s an abstract and some visuals on the project.
While we simply do not know what pollinates many of Australia’s plants, there is good evidence emerging showing Australia to be a global hotspot for bird-pollination. This raises questions about what ecological and evolutionary factors might encourage plant lineages to adapt to use birds as couriers for their pollen. As well, we might ask what the outcomes are when a plant species ties its reproductive fortunes to a bird, rather than an insect.
My project employs custom cameras designed for motion-capture data capture of insect visitors to flowers, in order to demonstrate contrasting bird versus insect visitation in pairs of closely related native shrubs. Fine-scale population genetic analysis in these plants is revealing evidence for systemic differences in the movement of pollen under these different pollinator regimes.
Styphelia stomarrhena is pollinated exclusively by birds.
The video below shows bird visitation by a number of honeyeater species, as well as the way in which floral morphology excludes bee pollinators from accessing pollen or nectar in Styphelia stomarrhena.
Styphelia xerophylla is the sister species to S. stomarrhena and has evolved a tight relationship with a single species of native bee: Leioproctus macmillanii.
The videos below show motion-captured footage of the native pollinator of Styphelia xerophyllum, a female native bee (Leioproctus macmillani).
However the flowers are also visited by introduced honeybees (Apis mellifera).
A quick note on plant names: These species recently underwent taxonomic revision, moving them from genus Astroloma to Styphelia. It is rather new, hence the confusion over these shrubs apparently having two names.
The Great Western Woodlands (GWW) form the largest tracts of temperate woodlands left on Earth. They hold approximately 30% of Australia’s Eucalypt species, and close to 20% of Australia’s plant species overall. This is truly an overlooked gem of Australian biodiversity. Last Spring I was lucky enough to visit for my work on pollination in our native plants.
My target there was Eremophila, a genus of approximately 250 species largely confined to arid and semi-arid Australia. The GWW represents one of the centres of diversity for the genus, and so I chose it as a likely spot to set up a new study contrasting bird and insect pollination.
Eremophila alternifolia was one of about 15 Eremophilas I saw flowering despite the drier than average conditions.
I was joined by perhaps the best kind of field assistant: a trained and accomplished professional ecologist who also happens to be my beautiful wife. After driving 2800km from Melbourne to field sites near Norseman, Western Australia, we spent a little under two weeks observing pollinators, surveying and mapping populations of plants, and collecting samples for population genetics.
One of the many viewpoints south of the Nullarbor Plain.
I left in awe of the scale of these woodlands, in love with the peace and isolation they offer, and a bit concerned over their insecure future. Fully 60% of the GWW is tenured “unallocated Crown land”, unmanaged and open access. With more visitors, and more appreciation of the value of these vast woodlands, I hope we can find a way to secure more of it into ongoing reserve for future generations.
The bluebush understory contrasts dramatically with red sand in many areas. Front left is one of my study species Eremophila scoparia.
The whole region is dotted with salt-pans.
As predicted from the small, violet flowers, Eremophila scoparia was visited by a host of native bees.
Eremophila decipiens has characteristic bird-adapted flowers.
Camera traps being expertly arranged by Samantha. Footage revealed that E. decipiens was being visited by a range of honeyeater species.
Eremophila calorhabdos
This spectacular Grevillea hid a massive bloom of flowers underneath it
The inflorescences are held on stems that grow along the ground underneath the shrub. The very long style with pollen-presenter is suggestive of adaptation to birds, but mammals might not be out of the question.
Eucalyptus loxophleba with daggy botanist for scale
Majestic Salmon gum (Eucalyptus salmonophloia) with Samantha for scale.
The serenity of wandering amongst giant Salmon gums at dusk was magic.
Gleaming bark on Eucalyptus salubris
Elevating on Lake Cowan. Photo: S. Vertucci.
For the second half of the trip I was joined by collaborator and all-round legend Dr. Renee Catullo. I made us walk 10km to collect camp gear following a single poor decision.
Stay tuned as research results emerge. The study should tell us about the way pollen moves under bee and bird pollination, and how those fine scale patterns play out on a grand landscape level.
Australian flowering plant diversity is legendary. Within an hour trip outside of our major metro centres anyone can quite easily witness unique Australian plant diversity in subtropical forest (Brisbane), grassland (Melbourne), and sandstone heath (Sydney). The diversity close to home is fairly well catalogued, and while it is hard to discover a new plant species, merely spending time around our native plants is very likely to reveal something that has never before been documented.
Something like 90% of our native plants rely on animals for pollination in order to set seed. Despite this, we simply do not know what pollinates most of our Australian native plants. The fact that the private lives for many of our native plants remains mysterious is due to their great diversity and the limited time and resources available to document what’s going on every day in the bush.
Two native bees (Hylaeus (Rhodohylaeus) sp.) visiting flowers of the Broom Bush (Eremophila scoparia) in Western Australia.
And these uncharted interactions are totally critical for the functioning of our native ecosystems. Pollination underpins production of seed for the next generation, builds seed banks for post-fire regeneration, and also produces fruits and seeds that are critical food resources for our native animals.
Our ignorance of native pollination networks is therefore vastly out of step with their importance. This is illustrated in the example of bee declines, where we have all heard about the threats impinging on honeybees and pollination service for food crops, yet when it comes to Australian native bees, we lack the basic benchmark data needed to make a solid judgment about whether they too are declining*. It is therefore imperative that we commit effort to recording native pollination networks now, before they are lost to us. While it is hard for long term ecological monitoring projects to attract funding, ongoing development of automated imaging of flower visitors and large scale citizen science projects offer some promise for increased capability in filling this ecological blind spot.
But our ignorance here can also be thrilling. This means that every time you are in the bush, and witness an insect or bird taking nectar or pollen from a flower, there is a reasonable chance it has never been documented before. In my work with University of Melbourne I have been studying several native shrubs to understand their pollination, and for many of these species, it is gratifying to know that my work will be the first documented evidence of what is visiting them. But you don’t have to be a trained scientist to do this, you just need some patience, luck, and some fine weather. And while discovering and photographing an unusual native bee pollinating one of our native flowers won’t win you a Nobel Prize, I guarantee it will provide any enquiring mind with a hit of electric discovery every single time.
Photographed on Mount Buffalo, Ken Walker (Victoria Museum) later identified this bee as the very rare Lasioglossum (Callalictus) callomelittinum. Few photos of it exist. This individual is buzz-pollinating a Fringe Lily (Thysanotus tuberosus).
Links for pollinator observations:
Bowerbird: Nature observations database
Government pollinators repository
*But given native bees need native habitat, and native habitat is being cleared at astonishing levels, we can, with a high degree of confidence, say that native bees are declining too.
Plants mate. In a manner far more elegant than our own mammalian shenanigans, and far more important for the ongoing survival of the Earth’s ecosystems, plants are out there constantly having sex. And it is a deeply interesting thing involving insects, and wind, and stigmas, and stamens, and enough puzzling evolutionary biology to fill journal articles and occupy a small handful of academic careers.
My latest paper is about plant mating, and rather than wade through meadows of blooms to collect the data for this effort, I remained desk-bound in Milwaukee, wading through 30 years of plant mating literature with the intention of assembling a dataset to help us see plant mating in a new light.
A mint-bush (Prostanthera lasianthos), caught in the act of mating.
As you may know, the flowers of many plant species are hermaphrodite, i.e. they bear both male and female sexual organs. This means plants can have sex with themselves. It’s called self-pollination, or “selfing”, and it can happen in two ways. First, pollen can move from the male parts of a flower to the female parts of the same flower. Second, on a single plant, pollen can move from the male parts of one flower to the female parts of a different flower.
“But isn’t this inbreeding??” I hear you ask, recoiling in horror at the moral and population genetic gutter down which I have so suddenly led you. Yes! Seed fertilized in a selfed flower is about as inbred as it gets, even more so than sibling mating. But you see, inbreeding is not always a bad thing—in fact inbreeding can actually be beneficial in some circumstances—and here’s where things get messy.
Genes are selfish. As such, they will do whatever it takes to get themselves propagated into another body. Consider then the selfish genes of a mother plant, which want to get into the next generation, and into as many individuals as possible. If a mother plant mates with a genetically different plant (an “outcross” mating event), its seeds each inherit approximately 50% of the mother’s genetic variation. This is because they combine with (and are diluted by) the genes of the pollen donor during pollination and sexual reproduction. On the other hand, if a mother self-fertilizes its own seeds, it transmits closer to 100% of its genetic legacy into each offspring. In the eyes of natural selection, this is a huge difference, and offers a great advantage to selfing over outcrossed reproduction.
So why aren’t all plants selfing all the time? This is because the huge advantage in transmitting genes via selfing is offset by the drawbacks of inbreeding. You can’t often get away with inbreeding without cost, and the cost is that inbred offspring are commonly less fit than their “outbred” siblings. Called “inbreeding depression”, this occurs because breeding from related individuals vastly raises the probability of combining rare genetic variants that harm the individual they are in. In the long run, inbreeding also reduces genetic variation that is essential for adapting to changing environments.
Evolutionary biologists have been pondering this tension for decades, and it led to the hypothesis that plants should be driven by natural selection into two polar opposite strategies: selfing plants should be favoured by natural selection when inbreeding depression is weak, and outcrossing plants should be favoured by natural selection when inbreeding depression is strong. Anything in the middle should not be adaptive, and should therefore be rare in nature. Sounds sensible, except numerous studies have now measured selfing/outcrossing across hundreds of plant species, and there is a curious and difficult-to-explain preponderance of plants that are neither exclusively selfing or outcrossing. These plants are having it both ways, seemingly hedging between strategies, and we call them the “mixed maters”.
Now for the problem my study addresses… Over the last 30 years, loads of studies have been measuring the selfing/outcrossing rate of plants in the wild. There have also been important studies which collect all these outcrossing estimates into big global datasets and generate observations like the one above: that mixed-mating is inexplicably common. For 30 years however, much of this discussion on global patterns was centred around average outcrossing per species, and using this to classify a species as a “selfer”, “outcrosser”, or “mixed mater”. We know evolution doesn’t work on a “species level” though. What is more appropriate is what is happening in a population context. And perhaps by averaging away all the variation within each species, my co-authors and I thought we might be missing an important perspective on the data.
What my co-authors and I did then was to collect all the outcrossing estimates from published papers containing three or more population outcrossing estimates. Going back 30 years, we collected data for 105 species and measured the variation in outcrossing within a species, among populations.
What we found was huge variation! There was commonly so much variation in mating within a species, that species averages felt inadequate for expressing where mating system variation lies. The data also showed that variation was difficult to predict. For example, there was an old hypothesis floating about that wind-pollinated plants were less variable in their outcrossing/selfing rate than animal pollinated species. The reason being that animals were thought to fluctuate more in their abundance and service between sites and seasons, while wind was a more consistent, reliable force for pollination. For the first time we were able to test this, and our analysis did not support the hypothesis. We also tested whether mating variation was evolving in a predictable fashion, but found that the relationship of plant species had no bearing on the variation we found in those species’ mating patterns.
Arabis alpina: revealed in our study as a species with one of the most variable mating systems measured. By Hedwig Storch CC BY-SA 3.0, from Wikimedia Commons.
Ultimately, our study can’t solve the mystery of why we have so many mixed-mating plants. What it does do though is present a new way to look at the known range of variation in plant mating. There are other analyses waiting to be done with the data collected here, and we have highlighted where more studies are needed to answer interesting questions. For example, do different kinds of animal pollinators result in more or less variable outcrossing? Or do populations on the fringe of a species’ range experience more or less variable outcrossing?
As the body of knowledge on plant mating systems continues to grow, this dataset will grow too, and this population-level perspective on plant mating will hopefully provide the basis for the next insights into evolution’s influence on plant mating.
Full reference:
Further reading:
Goodwillie C, Kalisz S, Eckert CG. (2005) The evolutionary enigma of mixed mating systems in plants: occurrence, theoretical explanations, and empirical evidence. Annu. Rev. Ecol. Evol. Syst., 36:47-79.
Schemske, DW and Lande, R. (1985) The evolution of self‐fertilization and inbreeding depression in plants. II. Empirical observations. Evolution, 39:41-52.
Barrett, SC and Harder, LD. (2017) The Ecology of Mating and Its Evolutionary Consequences in Seed Plants. Annual Review of Ecology, Evolution, and Systematics, 48:135-157.
I’m thrilled to share this never-before seen sequence of birds feeding on Astroloma stomarrhena, a winter-flowering shrub endemic to Western Australia.
Earlier this year, I decided A. stomarrhena looked like a perfect candidate for my new study on pollinators and gene flow. What I needed was a bird-pollinated species of plant, closely related to an insect-pollinated species. This one seemed to match all the criteria I needed, except there was no evidence that it was bird-pollinated. But with those long, tapered corolla tubes, and that pink-red coloration, I believed that birds absolutely had to be the pollinator.
The danger was, that while birds might be visitors, the plant could be somewhat “generalized”, and also use insects. This is pretty common, especially in places like Australia where European Honeybees (Apis mellifera) have invaded ecosystems that evolved in their absence, and honeybees will visit absolutely everything whether the plants are adapted to bees or not.
By deploying a new camera-trapping method that I am developing to record insect visitation, I was able to gather several days of pollinator observations, despite some very bad weather. After initially being baffled as to what honeyeater might visit such a low ground-hugging shrub, I got my answer after day one, when I captured video of my new favourite bird: the Tawny-crowned Honeyeater (Gliciphila melanops) feeding on the flowers. Furthermore, the recordings of honeybee fly-bys are sufficient to rule them out as pollinators.
This little result is a win on two fronts: a successful trial of new pollinator-monitoring cameras, and vindication of predicting pollinators from flower morphology.
Click here for the full HD video.
Rare plants nurseries are like second hand bookshops. It’s always so tempting to browse on the off chance you find that little treasure. I recently visited a charming rare plants nursery in Mt Macedon (boutique-y town outside Melbourne, Australia) where I discovered these for sale:
Hello old friend! (Hesperantha coccinea)
The last time I saw this elegant iris, it was flowering on stream banks 10,000 km away in the Drakensberg Mountain range in South Africa. There in its natural habitat, it is pollinated in some areas by a very special butterfly: the Mountain Pride (Aeropetes tulbhagia). In other places, it is pollinated by the amazing long-tongue fly (Prosoeca ganglbaueri). The two forms are a wonderful example of “pollination ecotypes”, where different populations are undergoing adaptation to their unique pollinators. The fly-serviced ones are a pink hue with narrow petals, while the butterfly-pollinated ones are much redder with broader petals.
Hesperantha coccinea at home in South Africa with its pollinator (Prosoeca ganglbaueri).
Fast forward two weeks, and I’m home walking the dog in my quite unremarkable Melbourne suburb, when who should I see?
Hello old friend! (Diascia sp.)
It’s winter here, with very little in flower, but these brilliant little pink blooms volunteering themselves from underneath a fence in suburban Melbourne really made my day. The last time I saw a Diascia, it was growing amongst the boulders on creek beds and on cliffs in the Drakensberg Mountains. These are Diascia, or “twinspur” and its this common name that alludes to their fascinating pollination story.
Hug-pollination by oil-collecting bee (Rediviva sp.) in Diascia.
Diascia have two spurs on the back of the flower, which is distinct from the usual arrangement of a single nectar-spur. The difference is that these flowers don’t reward pollinators with sugary secretions, instead they provide oil to specialised oil-collecting bees in the genus Rediviva. The bees use this oil to line their nests and provision their young. In order to collect the nectar, they must reach deep into the twin spurs with their lanky forelimbs, and comb it out. In so doing, they effectively hug the reproductive parts of the Diascia flower and effect pollination.
In Spring, I plan to take some cuttings from this little Diascia. Keeping species with special personal significance is a deeply satisfying part of cultivating plants. A plant can be kept like a souvenir or memento marking a time in one’s life, just like a photo or trinket. But plants have an advantage over these inanimate reminders. Because biological reproduction requires the physical donation of part of the mother’s cells to the daughter cells, my keepsake plant can be viewed as a physical part of the plant that appears in my fond memory. If I could see in four dimensions, I could literally look down the line of cell-divisions all the way back to where the Hesperantha in the nursery physically intersects as the same individual with the Hesperantha I observed flowering in the Autumn sun of the Drakensberg Mountains in South Africa.
The Drakensberg Mountains, South Africa, Autumn 2014.
I recently began a brand new project with the University of Melbourne. The beginning of a new project is filled with equal parts excitement and trepidation—excitement at the novelty, the blank canvas, the potential, and trepidation at not wanting to put a foot wrong in critical early decisions that will affect the outcome of a career-defining opportunity.
Here the photos from a first foray into East Gippsland, surveying sites for bird-pollinated Prostanthera walteri.
Mt. Elizabeth
Snowy River National Park
Prostanthera walteri
Prostanthera hirtula
McKillops Bridge
The Snowy River
The Snowy River
Prostanthera walteri
Snowy River National Park
Gippsland waratah – Telopea oreades
Floral diversity in Prostanthera
Seduction. Pollination. Deception.
I recently wrote an article for Wildlife Australia about Australian sexually deceptive orchids, their evolutionary biology, and historical and current research about them. You can download and read the article here: PDF. Thanks to Carol Booth for her collaboration and editorial guidance.
The latest of Australia’s sexually deceptive orchids that I have seen (below) are Caleana major, the Flying Duck orchid (left), and a spider orchid Caladenia clavigera (right). Both were photographed last week in Brisbane Ranges NP, Victoria.
Flowering this year is one of the best seasons of recent times both east and west of the country. So if you’re in Australia, don’t miss the chance to get out bush and enjoy it.
Michael Whitehead 