Category Archives: Pollination

All pollinators are equal, but some pollinators are more equal than others

The infamous line from George Orwell’s Animal Farm asserting that “All animals are equal, but some animals are more equal than others” nicely captures an ecological view of pollinators and their relationships with plants. “Pollinators” by definition move pollen between flowers, but not all pollinators are equally good at transferring pollen of any particular plant: some are more effective than others. I’ll illustrate this with examples from the urban garden that Karin and I are developing, which I’ve discussed before.

As you can see from that link, the garden is modest in size, but nonetheless this year it contains a significant biodiversity of edible plants that require pollinators for some or all of the fruit and seed set, including: strawberries, apples, greengages, cherries, blackcurrants, squashes, courgettes, blackberries, fennel, runner beans, french beans, passion fruit, tomatoes, raspberries, and radishes.

“Radishes?!” I hear you ask. “But they are grown for their edible swollen roots which don’t require pollination!” True, usually. But we let our radishes flower because we mainly grow them for their seed pods which, picked young, are delicious in salads and stir fries, like mustardy mange tout. They look like this:

The radish flowers are pollinated by a diversity of insects including butterflies, bees, and small flies:

These insects will vary in their effectiveness as pollinators of radish, depending on the frequency of visits, how often they move between flowers, and the amount of pollen on their bodies. This last factor is largely a function of size and hairiness (bigger, hairier insects carry more pollen as a rule), though cleanliness also plays a part: insects often groom the pollen from their bodies and, in the case of bees, may pack it into their pollen baskets where it’s not available for pollination.

The size and behaviour aspect is best illustrated by some recent photos that I took of visitors to the flowers of passion fruit (Passiflora caerulea var.). We have a large, sprawling plant growing up a fence which is currently being visited by honey bees, hoverflies, solitary bees and bumblebees. In comparison to the size of the flower and the position of the anthers (male, pollen producing parts) and stigmas (female, pollen receiving parts), the hoverflies, honey bees and solitary bees are relatively small. These two images are of honey bees (Apis mellifera):

Here’s an unidentified solitary bee:

These bees were occasionally touching the anthers, mainly with their wings, so some pollen will be moved around. But from what I observed it’s likely to be a relatively small amount in comparison to bumblebees, which are usually much larger and hairier, and don’t groom themselves as often as honey bees. Here’s a Buff-tailed bumblebee (Bombus terrestris):

They were actively collecting pollen as well as nectar. Much of this pollen is packed into the pollen baskets on the rear legs and will go back to the nest to feed the developing larvae, but some will be involved in pollination:

So it seems to be the bumblebees we have to mainly thank for the deliciously sweet-sour fruit we will enjoy later in the season. Of course to test this properly we would need to set up an experiment in which we excluded larger bumblebees from the flowers and only allowed smaller bees to forage, with appropriate experimental controls. Would make a great project if any of my students are interested! But it should give you a sense of just how complex the interactions between flowers and their pollinators are: the ecology of pollination is far from simple, despite what some would have us believe.

From Chester to Copenhagen

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It is 6.30am on Sunday morning but I’m wide awake and can hear the hotel in which we are staying stirring into life. Time to reflect on what has been a long and busy week, rather than the start of a long and relaxing summer holiday as some assume academics enjoy. That’s a myth: summers for many of us are at least as busy as the main teaching part of the year, though that’s not to say we don’t teach in the summer – I have final year project students to advise, and for students who did not pass first time round there’s still re-sit exams and assignments to be undertaken.

Of course I’m not complaining and the busyness is part of the fun of my job, which includes opportunities to travel, as I’ve previously described on this blog. Before any travelling this week, however, Monday was taken up listening to my PhD student Kat Harrold give a seminar about the progress of her research on pollinator mapping and habitat modelling in the Nene Valley Nature Improvement Area. This was followed by an hour’s grilling from the supervisory team and an independent colleague, as we drilled down into the research and suggested ways in which Kat could improve on the already excellent work that she’s doing. All of this is a formal part of our PhD programme and Kat aquitted herself very well indeed.

Tuesday was the start of the travelling, and was spent in Chester helping with filming for an episode of a new four-part BBC2 series provisionally called Plant Odyssey, fronted by Carol Klein, Gardener’s World presenter and Honorary Fellow of the University. The series is being produced by Oxford Scientific Films and will be broadcast in the spring. In the following scene we were making a rose perfume based on an ancient Roman recipe from the writings of Pliny the Elder.

Now, I know very little about how to make perfume, but I do know a bit about flower scents and how they attract pollinators, so my role was to act as both a foil for Carol’s scent experiment and to add some science to the mix. This is not the first filming I’ve done with Carol, having also helped out with her Science in the Garden special edition of Gardener’s World a few years ago. While looking for that last link I discovered that all three episodes of Bees, Butterflies and Blooms is also available on YouTube, which is great to see as the BBC didn’t repeat the series or produce a DVD. I was involved in the making of episode 2, which helped to kick-start the RHS’s Perfect for Pollinators plant labelling campaign. Television work is fun and brings science, and the scientists who do it, to a much wider audience.

Wednesday I prepared my talk for Friday’s lecture in Copenhagen (more of which later) and Thursday involved attending the University of Northampton’s annual postgraduate research conference. This is a highlight of the year for me as it’s an opportunity to see the breadth of postgraduate research going on across the university, something that would be impossible in a larger and more research intensive institution. I was only able to attend the first session, but that alone covered research on the research process itself; feminist cyborg literature; the legality of the World Bank’s scrutiny panel; pollinator conservation (Kat Harrold again); and the experiences of families with children who have difficulties communicating. Questions from the audience tended to be broad and non-specialist, and all the better for that: often it’s the straightforward, naive questions which test specialist knowledge.

The rest of Thursday Karin and I packed and then travelled up to Birmingham International for an early evening flight to Denmark. I’d been invited by my colleague Bo Dalsgaard to present a research seminar at the University of Copenhagen’s Center for Macroecology, Evolution and Climate. Coming from a small and very diverse department, it was great to visit such a large and specialised group of researchers, though over lunch the Center’s Director Carsten Rahbek told me that a common complaint from his staff was: “Why can’t we employ more people doing what I’m going?” Everything’s relative I suppose.

The title of my talk was “Pattern and process in pollination at large geographic scales”, which gave an overview of some of the research I’ve published over the last decade or so, framed around the following questions:

Globally, how many plant species are animal pollinated?
Are plant-pollinator interactions more ecologically specialized in the tropics?
How predictable are pollination syndromes in different parts of the world?
What are the historical and contemporary drivers of the relative importance of animal versus wind pollination? [No link here as that manuscript is currently in review!]

Quite a number of people in the Center were out doing field work or were otherwise engaged so I spoke to a modest-sized audience of some 30 people: certainly not the smallest audience I’ve ever presented to – that was three people, including the two who had invited me to give the talk!

The lecture seemed to be well received and there were some stimulating questions afterwards, though also a couple of challenging ones about statistical analysis. One of these I couldn’t answer until afterwards because I’d forgotten the details of the methods we’d used (note to self: re-read old papers before you present their findings). In answering the other I agreed with the questioner that the data could now be analysed in a more sophisticated way (future task, if I ever get the time). If Kat’s reading this, I hope she takes satisfaction in not being the only person to be asked difficult questions about their research this week!

Afterwards I chatted with Bo and Carsten about the limitations of the current and paleo-climate data sets we’ve been using in some studies, which are indeed very limited. But there are only two options. Do we work with data sets that are flawed, whilst acknowledging that any conclusions are tentative? Or wait until better data become available, which could be a decade in the future? My choice is definitely to go with the former, otherwise we’d never publish anything because there are always limitations to data used in studies of ecology and biodiversity. Personal and public honesty about such limitations, and ideas as to how they can be overcome in the future, are surely preferable to stalling research.

Later that afternoon I discussed science with two of Bo’s collaborators, Pietro Maruyama a Brazilian PhD student whom I’d met last November, and Peter, a Danish undergraduate. Both are doing excellent work on that most charismatic group of pollinators, the hummingbirds.

Friday evening I was exhausted, and Karin and I opted for dinner in the hotel restaurant and an early night, as Saturday was to be spent exploring Copenhagen. It’s a great city for wandering around, with fascinating architecture and unexpected additions to buildings, such as bronze dragons:

And parks with statues of artists and writers, such as Hans Christian Andersen:

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After a roundabout wander, via a gallery selling African tribal art (which we couldn’t afford) and a small lunch (which we could only just afford – Copenhagen’s an expensive city!) we eventually ended up at the University’s Botanical Garden, which has a superb living collection of cacti and succulents, orchids and other epiphytes, and alpine plants.

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It beautifully illustrates the huge morphological diversity encompassed within the 352,000 or so species of flowering plants, one of the many reasons why I love visiting botanical gardens: I always see something new. This included two species of bumblebees (Bombus) which I’m sure don’t occur in Britain. I’ll have to look them up when I get back: from Chester to Copenhagen and, tomorrow, back to Northampton.

My Hooper Moment

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Despite the clunkiness of some of the special effects, Jaws is a great movie that influenced a whole generation of organismal biologists into becoming marine ecologists, or at least terrestrial ecologists with a toe in the water. The movie contains some iconic characters and wonderful lines. One of my favourite scenes* is the exchange between Hooper, the shark expert, and Mayor Larry Vaughn, the head-in-the-sand local politician:

Hooper: What we are dealing with here is a perfect engine, an eating machine. It’s really a miracle of evolution. All this machine does is swim. And eat. And make little sharks. That’s all. [Gestures to advertising sign on which a huge shark fin has been drawn] Now why don’t you take a long, close look at this sign. Those proportions are correct….

Mayor Vaughn: Love to prove that wouldn’t you? Get your name into the National Geographic? [Walks away, smiling dismissively]

Hooper: [pause, then slightly maniacally] ….hahahaha….hahahahaha….

Well today I got my Hooper Moment, my name in the National Geographic following an interview about pollination biology with James Owen, one of their writers. It’s the online version, not the printed magazine, but I’m counting it anyway. It’s a nice piece and, for once, doesn’t dwell on honey bees, or even bees at all.

In 1975 I was 10 years old and was accompanied to the cinema to see Jaws by my late parents. Neither were impressed: my mother watched the whole movie with her hands over her face and my father opined that it “was not as good as King Kong in the 1930s”. Nonetheless, I’d like to think that they’d have been proud of my Hooper Moment.

[Thanks to Mark for capturing a moment on the North East coast, some years ago]

*With apologies for the crappy music and dumb repeat-edits – scroll forward to 2:25.

Rational explanations

Nature as gardener (Darwin’s Unrequited Isle part 5)

Guest blogging: Are species interactions stronger and more specialized in the tropics?

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In hushed tones the narrator describes the intricate details of yet another highly specialized relationship between one species of indescribable beauty and a second species with intricate behaviour that is about to eat/infect/cooperate with/exploit it [delete as appropriate].

The camera view pulls back to reveal the green cathedral of a tropical rainforest:

“The tropics” continues the narrator “are special…….…”

Yes, the tropics are special. But how special? Or more to the point, how different are tropical communities to temperate communities? Over at the Dynamic Ecology blog, Jeremy Fox has invited Angela Moles and myself to contribute a guest blog on the subject of whether the idea that species interactions are always stronger and more specialized in the tropics is outmoded and not backed up by the evidence. In Jeremy’s parlance, is it a zombie idea?

The subject of latitudinal variation in species interactions is one that has interested me for a while and I’ve written a few papers on the topic, especially in relation to how plant-pollinator interactions vary with latitude. You’ll find references to some of them in the Dynamic Ecology piece, plus a fuller over view of our arguments.

So what are you doing reading this? Get over to Dynamic Ecology and read that!

“These things aren’t to study. They’re to turn up very loud and say, hey, once upon a time, everything was just as easy as this”

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The title of this brief post is a quote by Led Zeppelin frontman Robert Plant, from an interview that the BBC reported just this morning, regarding the forthcoming release of previously unheard Zeppelin material. You can read the story and hear the interview here.

Regular readers of this blog will know that I’m not averse to sprinkling musical references into my posts, and this was a great quote that seemed to chime with something else I read this morning. Over at the Small Pond Science blog, evolutionary ecologist Amy Parachnowitsch has a thought provoking post entitled “Save the bees, but maybe not this way“. I’ll let you read it for yourself, but in a nutshell Amy is concerned about the scientific legitimacy of a “Save the Bees” campaign being crowd funded by online activist network Avaaz.org.

I share this concern and it worries me that whoever is organising the campaign is exploiting the genuine desire by people to “do something for the bees” without any regard for what exactly it is that’s “being done”. It seems to me to be purely a campaign fund-raiser by people who don’t understand the issues or how science works, the message being: “These things aren’t to study. They’re to turn up very loud and say, hey, once upon a time, everything was just as easy as this”.

The organisers promise “the world’s first large scale, grass-roots supported, totally independent study of what’s killing our bees that decisively challenges the junk science of big pharma”. As Amy notes, this is hugely offensive to independent scientists who are working on bee conservation issues (such as myself). But without ever actually saying what they are going to do with the money, they’ve already had pledges of money from over 78,000 people! If only raising funds for real research was that easy!

To reiterate what I said in the comments to Amy’s post, something that really worries me is that over-emphasis on pesticides and honey bees as single issues affecting “pollinator conservation” deflects attention from other factors which are at least as important, such as habitat loss. Colleagues and I have a manuscript in preparation at the moment showing that native bee and flower-visiting wasp extinctions in Britain began in the mid-19th century and reached their highest rate during the period 1929-1959, during a time of rapid agricultural intensification (but prior to the introduction of neonicotinoid pesticides that is currently exercising many people). Loss of pollinator diversity is an issue that has deep roots.

In actual fact, although wild bee diversity is declining in the UK, overall abundance seems to be stable as some species are doing extremely well, including a new natural colonist, the Tree Bumblebee (Bombus hypnorum) which is spreading fast and is locally common. But clearly greater diversity provides us with future insurance against losses of other species.

There are positive things that can be done for pollinator populations by every citizen, beyond giving money to crappy, pseudo-scientific campaigns, as I talked about in a recent post of mine. So please don’t contribute to this Avaaz.org request, and use the money you save to buy some wild flower seeds and/or the Led Zeppelin reissues. It will make the world a better place.

Book review: “Pollination and Floral Ecology” by Pat Willmer

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Review of “Pollination and Floral Ecology” by Pat Willmer (2011) Princeton University Press. £65. pp. 832.

Some backstory: In early 2012 I was asked by the review editor of the journal Annals of Botany to review this book, and I jumped at the chance as it’s the first major single-author overview of the field of pollination ecology for a number of years, by a well respected academic in the field. Unfortunately the review took a lot longer than I expected, in part because I was also coordinating my department’s Research Excellence Framework submission, on top of other teaching, research and admin duties, and it was taking up quite a bit of my time.

In addition I had mixed feelings about the book and wanted my review to do it justice, not be over-critical but at the same time highlight what I saw as flaws. In the words of the Fairport Convention song, Who Knows Where the Time Goes? – my review was only completed last Christmas and duly submitted. Turns out that the journal has a backlog of book reviews to publish and the editor asked that, given it’s been a couple of years since the book was published, would I mind if the review was posted on the Annals of Botany blog rather than in the printed journal. I happily agreed as it’s likely to get more readers on the blog, and said I’d also post it on my own blog. So here it is:

Any text book that tries to assess and summarise the whole of a multidisciplinary research field such as pollination ecology and floral biology is required to be four things: (1) comprehensive in its scope; (2) up to date in its coverage of the literature; (3) accurate in its assessment of the current state of the field; and (4) authoritative in the conclusions it presents.

This volume by Professor Pat Willmer of the University of St Andrews certainly ticks the first box. It’s a huge book, and covers everything relating to the evolution of flower attraction and reward systems, ecological interactions with pollinators, biochemistry, physiology, agriculture and conservation; all in 29 chapters split into three sections, with 87 pages of references. The literature extends to 2010, which is impressive for a book published in 2011 (though see my comments below about completeness of the literature). Specialist terms are highlighted in bold to direct the reader to the glossary at the back, a useful device even if there are a few inaccuracies, which I’ll mention later.

So far so good, and the author is to be congratulated on putting together such a comprehensive, not to mention timely, single-author book. It’s clearly the summation of a career devoted to studying pollinators and flowers, and the author’s passion for her subject is apparent throughout.

However when we come to points 3 and 4, things are less straightforward. There are some issues with accuracy that are troubling in a book aimed at newcomers to the field as well as established researchers. To give just a few examples:

– on p.18 we are told that asclepiads have “one stamen” (they have five); on p.169 and in the glossary that asclepiad pollinia are the pollen grains from one anther (they are the contents of half an anther); and on p.170 that the pollinaria are “glued” to pollinators (they actually clip on).

– in the glossary, tree ferns are referred to as “cycads”, an error that is repeated on p.89.

– on p.88 there is a statement suggesting that tree fern spores were dispersed by “animal fur” 300 million years ago, long before the evolution of mammals, and that this (and dispersal of spores of fungi and mosses) is the equivalent of pollination: it is not, it equates to seed dispersal.

These are troubling errors of basic botany that are forgivable in an early draft of the book (everyone makes mistakes) but not in the final published version, after it’s been read, reviewed, checked and edited. If the book goes to a second edition I hope that these (and other) mistakes will be fixed. But they do hint at a fundamental problem with a book (and a field) as large and complex as this: a single author is arguably unlikely to be able to do justice to all of the subject matter.

There are parts of the book where it is unclear (to me at least) what the author is actually saying. For example, on p.96 there is a graph which, it is suggested, demonstrates that pollination by animals is “technically uncommon when assessed in terms of the numbers of broad taxonomic groups that use it”, though the legend to the figure claims that “most orders of plants have no families” that possess wind pollination. This is confusing: what is to be concluded by someone new to the field? Is animal pollination common or rare? Likewise, on p.91 we are told that the “first angiosperms…would probably have had their pollen moved mainly by wind…”, but then on p.92 that “an element of insect pollination could be regarded as almost ancestral”. Which is correct?

There are other aspects to the book that are simply out of date; for example the linear, rather deterministic schemes set out in Figures 4.6 and 4.8 showing that Cretaceous flowers were open and radially symmetrical, and only later evolved into complex, bilateral flowers in the Tertiary, ignores fossil discoveries showing that orchids evolved in the Cretaceous (Ramírez et al., 2007). Likewise, discussion of “counterproductive” crypsis in flowers (p.124) neglects recent findings of cryptic, wasp-pollinated plants in South Africa (e.g. Shuttleworth & Johnson, 2009).

There is a theme emerging here: some of the botany that the book presents is inaccurate, confused or out-dated. Fortunately the zoological aspects of the book are much better, as one might hope from a Professor of Zoology.

The final criterion, that the book should be “authoritative in the conclusions it presents”, is however, in my view, the main weakness of this volume. The author is unhappy with recent developments in the field, particularly as they relate to community-scale assessments of plant–pollinator interactions, in terms of network analyses and predictive utility of pollination syndromes. Clearly Professor Willmer is on a mission to rebalance what she perceives as failings within some of the current trends in studying pollination. A book review is not the place for a technical dissection of the author’s arguments, which is best left to the peer-reviewed literature (though I would argue that that’s also the place to present some of the criticisms the author introduces, rather than into a text book such as this). I could focus the whole of this review on these topics because: (a) they take up a large proportion of the book, about one-third of the text pages; and (b) they are highlighted on the cover as being one of the main contributions of the book; specifically, that the author provides a critique of previous work that does not distinguish between “casual visitors and true pollinators” that can in turn result in “misleading conclusions about flower evolution and animal-flower mutualism”. Unfortunately her targets are straw men, and one – I believe quite telling – example will suffice.

On p.447 there is a criticism of the use by Waser et al. (1996) of Charles Robertson’s historical data set, and specifically that the analyses they present “…did not distinguish visitors from pollinators even though Robertson’s database did include information on this”. However Waser et al. clearly state (p.1045 of their paper) that only pollinators were included in the analyses, not all flower visitors, and that “visitation is not a synonym for pollination…non-pollinating visitors are excluded (as in Robertson 1928)” (p.1048).

Why should Professor Willmer make a statement to the contrary? Evidently she wishes to impress upon her readers that (in her opinion) there are fundamental problems in current approaches to studying pollination at a community level. But even if that were the case (and I don’t believe it is) misrepresenting previous studies to suit an argument is poor scholarship at best.

Regardless of whether some of her criticism is well founded, the author does not seem to appreciate that plant–flower visitor interaction networks are ecologically important regardless of whether or not a flower visitor acts as a pollinator. More fundamentally, true pollination networks possess similar attributes to flower visitor networks, for example a nested pattern of interactions, and arguments about level of generalisation of species are a matter of scale, not category (Ollerton et al., 2003).

At the end of her Preface, Professor Willmer reveals to us quite a lot about her personal attitude to research when she states that some readers might find her approach “too traditional” in an “era where ecological modelers [might be claimed to] have more to tell us than old-style field workers”. What the author fails to appreciate is that this is a grossly false dichotomy and that most of the pollination ecologists who have embraced new analytical methodologies for understanding plant–pollinator interactions are also “old-style field workers” with considerable experience of studying the ecology of flowers and their pollinators beyond the computer screen.

In summary this is a book that, for all its good qualities of comprehensiveness and (mostly) up to date coverage, should be read with caution: parts of it are neither as accurate nor as authorative as the field of pollination and floral ecology deserves.

LITERATURE CITED

Ollerton J, Johnson SD, Cranmer L, Kellie, S. 2003. The pollination ecology of an assemblage of grassland asclepiads in South Africa. Annals of Botany 92: 807-834.

Ramírez SR, Gravendeel B, Singer RB, Marshall CR, Pierce NE. 2007. Dating the origin of the Orchidaceae from a fossil orchid with its pollinator. Nature 448: 1042-1045.

Shuttleworth A, Johnson SD. 2009. The importance of scent and nectar filters in a specialized wasp-pollination system. Functional Ecology 23: 931-940.

Waser NM, Chittka L, Price MV, Williams N, Ollerton J. 1996. Generalization in pollination systems, and why it matters. Ecology 77: 1043-1060.

What are YOU doing for our pollinators this year? (reduce, reuse, recycle part 6)

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Earlier this year I was asked to write a short article by my former PhD student, and still a current collaborator, Dr Sam Tarrant. Sam works for the RSPB as the CEMEX UK-funded Biodiversity Advisor, and wanted something on pollinator conservation that could be circulated in the CEMEX company’s e-newsletter. In the spirit of reworking and reusing odd bits of writing, I thought I’d post it here too.

Insects are vital for our country’s economy. Don’t believe me? Then read on….

Beneath a large black mulberry tree near the University of Northampton’s Newton Building there is a plaque that commemorates its planting “On Shakespeare Commemoration Day, 3^rd May 1916”. Despite its age this tree annually produces large crops of succulent berries, aided by the fact that wind eddies are sufficient to disperse its pollen, ensuring pollination and fruit set. Each year it’s a scramble between students, lecturers and birds, to see who can eat the most.

In contrast, the old apple trees in the grounds possess a different strategy – pollination by insects that move from flower to flower each spring. This form of pollination is both more sophisticated and less reliable than wind pollination, and is currently under considerable threat: whilst there will never be a shortage of wind currents in Britain, insect pollinators are in decline.

The apples trees are not alone in requiring insects to pollinate them, so to do other farm and garden crops, including oil seed rape, field beans, courgettes, runner beans, and strawberries and other soft fruit. It’s worth at least £440 million annually to the British economy, and most of it is done by wild bees and hoverflies, rather than managed hives of honey bees.

But all is not well with these insects in Britain – they are in decline. Although the extent of the “pollination crisis” is debated by scientists, long term records show us that these insects are under pressure: 23 species of bee and flower-visiting wasp have gone extinct since the mid 1800s, as have 18 species of butterflies. Less obviously, other species have considerably reduced in abundance so that they are now found in only a small part of their previous distribution.

There are lots of gardeners who want to “do something” for the pollinators, and keeping honey bees is often mentioned. By all means, if you wish to help the honey bees (which are suffering their own problems) then keep a hive or two. That will not, however, help our wild, native pollinators; the analogy I use is that it’s the equivalent of trying to help our declining songbirds by opening a chicken farm!

If you want to make a real difference for pollinators in your own garden, here are a few ideas:

start by planting nectar and pollen rich flowers; there’s a useful list on the Royal Horticultural Society’s website (see below).
allow plants such as clover and dandelion to flower in your lawn, bees love them.
as well as food, pollinators also need nest and egg laying sites, so you could help by allowing some of the far corners of your plot to run a little wild.
wait until late Spring to cut back hollow stemmed perennials as they are used as hibernating places by some of our bees.
allow mason bees to nest in old walls and don’t worry about them, the wall won’t fall down.
And finally, stop using pesticides!

Changing some of our gardening habits can help a group of insects on which we rely and which supports our economy in a very real way.

Further reading and information:

Bees Wasps and Ants Recording Society: http://www.bwars.com/

Bumblebee Conservation Trust: http://www.bumblebeeconservation.org.uk/

Butterfly Conservation: http://www.butterfly-conservation.org/

Hoverfly Recording Scheme: http://www.hoverfly.org.uk/

Royal Horticultural Society’s list of plants for pollinators: http://www.rhs.org.uk/Gardening/Sustainable-gardening/Plants-for-pollinators

Ménage à trois mutualism

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Relationships involving a “household of three” hold a fascination that is part prurient and part wonderment: prurient for perhaps obvious reasons, and wonderment as it’s sometimes hard enough to make a ménage à deux work! Historically this domestic arrangement has been the lifestyle of choice of a surprisingly large and diverse set of influential thinkers and creative individuals, including Aldous Huxley, Lord Nelson, Carl Jung, Erwin Schrödinger, and Hattie Jacques. Indeed, one of my favourite musicians, David Crosby, wrote a song about such relationships (Triad) which got him kicked out of The Byrds.

In nature, ménage à trois are occasionally encountered and may be more common than we think, and have been on my mind because this week I’ve been talking about mutualistic relationships with my first year undergraduates. Mutualisms are interactions between species in which both benefit, as opposed to exploitative relationships such as predation or parasitism in which one of the partners is at a disadvantage (being eaten is a great disadvantage….) Mutualistic interactions are common and important, and include many (but not all) plant-pollinator interactions, seed dispersal by birds and other animals, mycorrhizal relationships between plants and fungi, and many more. As well as studying plant-pollinator interactions, I’ve a long-standing interest in the full breadth of these examples of “biological barter“, in all their varied forms.

In most cases mutualistic relationships involve pairs of species (for example a plant and a pollinator) although these species pairs are embedded within a larger network of interactions: that plant may have many pollinators, and those pollinators may service other plants. In this sense it requires just two partners to make the interaction work – a “household of two”. More rarely, research on the biodiversity of species interactions throws up examples of “households” involving three species, and a fascinating case has recently been worked out and published by Jonathan Pauli and colleagues. This involves three-toed sloths and their relationship with the algae and moths that colonise the sloth’s fur – you can read the abstract here. In summary, the algae benefit from nutrients provided by the moths living in the fur; the sloths eat the algae to supplement a restricted diet of leaves; the moths benefit from the sloths transporting them to defecation sites where they lay their eggs, then recolonise the sloths. This slothy ménage à trois is a wonderful instance of interdependency within nature.

The other case of a three-part mutualism with which I’m familiar is that between anemonefish, and sea anemones and the algae which are housed in their tentacles. The fish and the anemones provide mutual defence of one another, whilst the algae photosynthesise and provide carbohydrates to the anemone, and benefit from the nitrogenous waste produced by the fish. It’s a system that I’ve done a little work on with marine biologist colleagues, specifically the broad scale biogeography of the interaction and its local assemblage structure, but we’ve not studied the whole three-part system.

What other three-part examples are there in nature? I’d be very interested to hear about any of which you’re aware.

It begs a question as to whether three is an upper limit to the number of species that can engage in such relationships? Are there any four- or five-part mutualisms? Or are these too unstable over evolutionary time, because if one species goes extinct it could cause the extinction of other species? Interesting questions about fascinating interactions!