2 More than just bees: the diversity of pollinators
3 To be a flower
4 Fidelity and promiscuity in Darwin’s entangled bank
5 The evolution of pollination strategies
6 A matter of time: from daily cycles to climate change
7 Agricultural perspectives
8 Urban environments
9 The significance of gardens
10 The shifting fates of pollinators
11 New bees on the block
12 Managing, restoring and connecting habitats
13 The politics of pollination
14 Studying pollinators and pollination
As you can see it’s a very wide-ranging overview of the subject, and written to be accessible to both specialists and non-specialists alike. To quote what I wrote in the Preface:
“While the book is aimed at a very broad audience, and is intended to be comprehensible to anyone with an interest in science and the environment, and their intersection with human societies, I hope it will also be of interest to those dealing professionally with plants and pollinators. The subject is vast, and those working on bee or hoverfly biology, for example, or plant reproductive ecology, may learn something new about topics adjacent to their specialisms. I certainly learned a lot from writing the book.”
The book is about 100,000 words in length, lots of illustrations, and there will be an index. My copy editor reckons there’s 450 references cited, though I haven’t counted. I do know that they run to 28 pages in the manuscript, and that’s with 11pt text. All going well it will be published before Christmas.
The Wildlife Trust for Bedfordshire, Cambridgeshire and Northamptonshire has invited me to run my Introduction to Pollinators and Pollination workshop again this year, but of course it will all be online. Details for signing up are on the images, or you can follow this link.
Here’s a description of the workshop:
Pollination of flowers ensures the reproduction of most British wild plants and many of our agricultural crops. This session will provide an introduction to the natural history of pollinators and how they interact with the flowers that they pollinate. The main groups of pollinators will be introduced, with guidance on how to identify them, and their ecology and behaviour will be explored. The session will also consider why conserving these species is so important, followed by a Q and A discussion showing what individuals can do to help ensure their future diversity and abundance.
UPDATE: turns out the figure I cited for number of bee species is out of date so I’ve corrected it below. Thanks to John Ascher for pointing this out.
Publication of my book Pollinators & Pollination: Nature and Society by Pelagic Publishing has been pushed back until the end of this year or early in 2021. The current pandemic has created problems for the printing and distribution sectors, as it has for so many industries. Therefore, to celebrate World Bee Day, here’s a preview of the bee section from Chapter 2 which is entitled (ironically enough) “More than just bees – the diversity of pollinators”.
2.3 Bees, wasps and sawflies (Hymenoptera)
The bees and their relatives rank only third in terms of overall pollinator diversity. Within this taxonomic Order, bees are not especially species rich (17,000 or so described species, perhaps 20,000 in total) – over 20,400 (see: https://www.catalogueoflife.org/col/details/database/id/67) compared with the other 50,000 social and solitary wasps, sawflies, and so forth. But what they lack in diversity the bees make up for in importance as pollinators of both wild and agricultural plants, and in their cultural significance. The general notion of what a bee is, and how it behaves, looks to the honeybee (Apis mellifera) as a model: social, with a hierarchy, a queen, and a large nest (termed a hive for colonies in captivity). In fact, this view of bee-ness, though long embedded within our psyche, is far removed from the biology of the average bee: most of them have no social structure at all, and a fair proportion of those are parasitic. In Britain we have about 270 species of bees, give or take (Falk 2015) though there have been extinctions and additions to this fauna (see Chapters 10 and 11). These species provide a reasonable sample of the different lifestyles adopted by bees globally. They can be divided into four broad groups.
Honeybees include several highly social species and subspecies of Apis, of which the ubiquitous western honeybee (A. mellifera) is the most familiar. Most colonies are found in managed hives, though persistent feral colonies can be found in hollow trees, wall cavities, and other suitable spaces. They are widely introduced into parts of the world where they are not native (e.g. the Americas, Australia, New Zealand) and there is some debate as to whether they are truly native to Britain and northern Europe, with supporting evidence and arguments on both sides. Colonies can be enormous and contain thousands of individuals, mostly female workers, with a single queen. Unmated queens and males (drones) are produced by the colony later in the season.
Bumblebees (Bombus spp.) are typically also social, though their nests are much smaller (tens to hundreds of individuals). Depending upon the species these nests can be in long grass, rodent holes, or cavities in buildings and trees. Twenty-seven of the more than 250 species have been recorded in the UK, but six of these are not strictly social; they are parasitic and belong to the subgenus Psithyrus which will be described below.
The so-called solitary bees are by far the largest group in Britain (about 170 species) and worldwide (more than 90% of all species). In the UK they belong to 15 genera, including Andrena, Anthophora, Osmia, Megachile, etc. The females of most of these bees, once they have mated, construct nests that they alone provision with pollen for their developing young. Nesting sites can be genus- or species-specific, and include soil, cavities in stone or wood, and snail shells. Some species are not strictly solitary at all and may produce colonies with varying levels of social structure, though without a queen or a strict caste system; we term them “primitively eusocial”. In fact sociality has evolved and been lost numerous times in the bees and in the rest of the Hymenoptera (Danforth 2002, Hughes et al. 2008, Danforth et al. 2019). It’s also been lost in some groups that have reverted back to a solitary lifestyle, and even within a single genus it can vary; for example in the carpenter bee genus Ceratina (Apidae: Xylocopinae) tropical species are more often social than temperate species (Groom & Rehan 2018).
The final group is termed the cuckoo bees and, like their avian namesake, they parasitise the nests of both social and solitary bees (though never, interestingly, honeybees). There are about 70 species in 7 genera, including the bumblebee subgenus, Psithyrus. Other genera include Melecta, Nomada and Sphecodes. In some cases the parasitic species are closely related evolutionarily to their hosts and may resemble them, for example some Psithyrus species. In other cases they may be only distantly related and in fact look more like wasps, e.g. Nomada species. Some genera of cuckoo bees are restricted to parasitising only a single genus of bees, others are parasites of a range of genera (Figure 2.4).
Although we often think of bees, overall, as being the most important pollinators, in fact species vary hugely in their importance. Pollinating ability depends upon factors such as abundance, hairiness, behaviour, body size, and visitation rate to flowers (Figure 2.1). Size is especially important for three reasons. First of all, larger animals can pick up more pollen on their bodies, all other things being equal. Secondly, in order to bridge the gap between picking up pollen and depositing it, flower visitors must be at least as large as the distance between anthers and stigma, unless they visit the stigma for other reasons. Finally, larger bee species tend to forage over longer distances on average (Greenleaf et al. 2007) thus increasing the movement of pollen between plants. However, most of the world’s bees are relatively small as we can see from the analysis of British bees in Figure 2.5. Many species have a maximum forewing length of only 4 or 5 mm, and the majority of species are smaller than honeybees. Remember also that these are maximum sizes measured from a sample; individual bees can vary a lot within populations and even (in the case of Bombus spp.) within nests (Goulson et al. 2002). So the assumption that all bees are good pollinators needs to be tempered by an acknowledgement that some are much better than others.
Figure 2.5: The sizes of British bees. Forewing length is a good measure of overall body size and the data are maximum lengths recorded for species, except for the social bumblebees and honeybee I have used maximum size of workers (queens are often much larger). The blue line indicates the honeybee (Apis mellifera). The biggest bee in this data set is the Violet Carpenter Bee (Xylocopa violacea) which, whilst not generally considered a native species (yet), has bred in Britain in the past. Data taken from Falk (2015).
Pollinators such as bees and butterflies are highly dependent on flowers to provide nectar as food; at the same time, those plants are reliant on the pollinators for reproduction. Over the past few decades, declines in both flower and pollinator diversity and abundance have prompted ecologists to wonder about the consequences of flower loss for pollinator communities and for plant pollination.
In a ground breaking new study, a team from institutions in the Czech Republic and the University of Northampton in the UK have published the results of experiments that seek to answer these questions. Led by PhD researcher Dr Paolo Biella, the team performed experiments in both countries that involved temporarily removing thousands of flower heads from grassland plant communities. They assessed how the pollinator assemblage responded to their removal, and how effectively the remaining flowers were pollinated. The team focused on generalist plant species that support the majority of pollinators within a community because these have traditionally been less well studied than highly specialised relationships.
The results are published today in the open access journal Scientific Reports and provide the first demonstration of the ways in which pollinators flexibly adjust their behaviour when faced with a sequential loss of resources. This flexibility is constrained by the type of flowers they visit, however: pollinators will tend to switch to flowers of a similar shape to the ones that have been lost. From the plant’s perspective, things are less clear: the patterns of pollination for the remaining species were idiosyncratic and not as predictable. Some plants received more pollination during the experiment than before, others less.
For the first time we are seeing the consequences of sudden loss of flowers for both the pollinators and the plants in a habitat. That the pollinators can respond flexibly to this loss is a welcome indication that these insects might be more resilient to sudden changes than we had thought. However, the erratic pollination of the flowers shows that there is a great deal of random chance within these ecological systems that is not easily predictable. In the same week that the UN’s Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) Global Assessment Report on Biodiversity and Ecosystem Services was published, our study reminds us that there is much that we do not currently understand about the consequences of sudden changes in the natural world.
One of the team’s recommendations is that pollination-generalist plant species should be given much more attention in conservation assessments than has previously been the case. These plants are at the core of plant-pollinator communities and without them the rarer and more specialised species could not exist.
Species extinctions undermine ecosystem functioning, with the loss of a small subset of functionally important species having a disproportionate impact. However, little is known about the effects of species loss on plant-pollinator interactions. We addressed this issue in a field experiment by removing the plant species with the highest visitation frequency, then measuring the impact of plant removal on flower visitation, pollinator effectiveness and insect foraging in several sites. Our results show that total visitation decreased exponentially after removing 1-4 most visited plants, suggesting that these plants could benefit co-occurring ones by maintaining high flower visitor abundances. Although we found large variation among plant species, the redistribution of the pollinator guild affected mostly the other plants with high visitor richness. Also, the plant traits mediated the effect of removal on flower visitation; while visitation of plants which had smaller inflorescences and more sugar per flower increased after removal, flower visitors did not switch between flower shapes and visitation decreased mostly in plants visited by many morpho-species of flower visitors. Together, these results suggest that the potential adaptive foraging was constrained by flower traits. Moreover, pollinator effectiveness fluctuated but was not directly linked to changes of flower visitation. In conclusion, it seems that the loss of generalist plants alters plant-pollinator interactions by decreasing pollinator abundance with implications for pollination and insect foraging. Therefore, generalist plants have high conservation value because they sustain the complex pattern of plant-pollinator interactions.
I have to confess that I forgot completely about Valentine’s Day, it’s not a celebration that I generally pay much attention to, as expressions of love are something that everyone should be doing all the time, surely?
Anyway, this bastardised version of “Roses are red” is for my wife Karin:
Some bees are red
Others are blue
There’s twenty thousand species
Of every hue
Some flies are yellow
Some wasps are cerise
Many of them pollinate
Better than bees
Attached to the email were a couple of images showing a wasp visiting flowers of Cynanchum obovatum, an endemic species of Apocynaceae from northern and eastern Madagascar. Ulli had taken the photographs during field work there in preparation for the Flora of Madagascar project. Here’s the global distribution of the species according to GBIFrecords:
I was excited because Madagascar has a very rich diversity of Apocynaceae (between 500 and 1000 species). However we have flower visitor observations for only a small fraction of them, fewer than 20 species, and good evidence that the visitors are pollinators for only a couple of those.
I didn’t immediately recognise the family to which the wasp belonged: it didn’t look like either Vespidae or Pompilidae, two groups that are known pollinators of Apocynaceae. So I uploaded the shots to the Hymenopterists Forum on Facebook and within minutes had received an answer: it was a species of Scoliidae, commonly referred to as scoliid wasps. The distinctive wing corrugation found in this family is clearly visible on this image:
Scoliids are parasitoids of beetles and are some of the world’s largest wasps, but it’s not a very diverse family, with only about 560 described species, and only a single species in the UK (on the Channel Islands). Compare that with the Pompilidae and Vespidae, both of which contain c. 5,000 species worldwide.
Ulli tells me that when he saw the scoliid on C. obovatum “the wasp knew what to do with the flowers”, something I’ve experienced with vespid and pompilid wasp pollinated species in Africa: these wasps are really familiar with the flowers, they know how to work them to get a reward as they are regular and committed visitors. We believe that this is likely to be the legitimate pollinator of the plant, in which case it’s one of the few records for Scoliidae pollinating Apocynaceae, and the first for Madagascar. Other examples are mainly in South America, India and South Africa, and usually as one of a broad set of other wasps and/or bees visiting generalist flowers.
It’s interesting that this species of Cynanchum is one of the few in which the corona which covers the gynostegium (the fused sexual parts) is closed over:
That means it requires quite a strong, large insect to get inside and access the nectar. So the prediction is that the pollen masses (pollinaria) will be found on the mouthparts of these wasps. Intriguingly, a very closely related species C. repandum has no such closed corona, begging the question of whether it might be pollinated by a different type of insect:
For now this record will go into the Pollinators of Apocynaceae database as pollinator unproven, but i would be great if someone working in Madagascar could confirm the status of this pollination system.
My grateful thanks to Ulli for sharing his pictures and allowing me to tell the story of what may be a whole new Madagascan pollination system for our favourite family. Apocynaceae is full of surprises!
This morning I spent a very pleasant couple of hours walking around the farm that’s at the heart of the Warner Edwards Gin Distillery, in Harrington just north of Northampton. We are setting up some collaborations around conservation and sustainability between the university and Warner Edwards. The first of these involves surveys of their farm by one of our final year undergraduates, Ellie West, to assess pollinator diversity and abundance, and opportunities for habitat enhancement on the farm.
One of the highlights of this morning’s visit was seeing this gorgeous hornet (Vespa crabro) taking nectar from common ivy (Hedera helix). I think that she’s a queen stocking up on energy prior to hibernating. But just look at how much pollen she’s carrying! There’s every chance that she’s a very effective pollinator of ivy, which is a key nectar resource at this time of year. It’s such an important plant in other ways too: ivy binds the landscape physically and ecologically, in ways few other native plants do. Pollination by insects such as hornets (and hundreds of other species) results in berries that are eaten by birds and mammals, whilst the branches and dense, evergreen canopy provides nesting sites for birds and shelter for over wintering insects.
Hornets and ivy: two of my favourite native British species.
Interactions between flowering plants and the animals that pollinate them are known to be responsible for part of the tremendous diversity of the angiosperms, currently thought to number at least 350,000 species. But the diversity of different types of pollination system (bird, bee, moth, fly, etc.) is unknown for most large, related groups of plants (what systematists term “clades”) such as families and subfamilies. In addition we know little about how these interactions with pollinators have evolved over time and in different parts of the world. Only a handful of groups of flowering plants have been studied with respect to questions such as:
How much do we currently know about the diversity of pollination systems in large clades?
How is that diversity partitioned between the smaller clades (e.g. subfamilies, tribes, genera) of a family, and what are the evolutionary transitions between the major groups of pollinators?
Do these pollination systems vary biogeographically across the clade’s range?
These sorts of questions have been addressed for the massive, globally distributed Apocynaceae(one of the top 10 or 11 largest angiosperm families with more than 5,300 species) in a study just published using a new database of pollinators of the family. What’s more, the work is open access and anyone can download a copy for free. Here’s the citation with a link to the paper:
In this study we have shown that (among other things):
The family is characterised by an enormous diversity of pollination systems involving almost all of the major pollen vectors and some that are nearly unique to the Apocynaceae.
Earlier diverging clades have a narrower range of pollination systems than those that evolved later.
Transitions from one type of pollination system to another are evolutionarily constrained, and rarely or never occur, whereas others have taken place much more often, e.g. between wasp and beetle pollination.
There is significant convergent evolution of pollination systems, especially fly and moth pollination, by geographically and phylogenetically distinct clades.
You’ll notice that there are 75 (!) authors on this paper. That’s because we’ve pulled together a huge amount of previously unpublished data and used some state of the art analyses to produce this work. It was a monumental effort, especially considering that my colleague Sigrid Liede-Schumann and I only decided to push ahead with this project about a year ago when we chatted at the International Botanical Congress that I posted about at the time. In truth however the origins of this paper go back over 20 years to 1997 when when Sigrid and I published a study of what was then known about pollination systems in the Asclepiadaceae (the asclepiads).
In that paper we said that the research “is intended to be ongoing…[we]…hope to re-review asclepiad pollination within the next decade”. At the time I didn’t think it would actually take more than 20 years! However over that period a lot has changed. For one thing the Asclepiadaceae no longer exists, broken up and subsumed within a much larger Apocynaceae. Also, I’ve done a lot of work in the field and in the herbarium on some of the smaller groups within the family, such as Ceropegia. Others, including many of my co-authors, have also been working on different groups in various parts of the world. Finally the level of sophistication of the analyses we are now able to do has increased beyond recognition compared to what we could achieve in the mid-1990s. All of this means that now is the right time to produce this study.
Having said all of that, this is still a work in progress. Our Pollinators of Apocynaceae Database contains a sample of just over 10% of the species in the family. So lots more data on plant-pollinator interactions needs to be collected before we say we fully understand how pollination systems have evolved in this most remarkable family. I’d be happy to talk with anyone who is interested in the family and being involved in future data collection.
The database will be freely available to anyone who wants to use it – lots more can be done with this information and, once again, I’m happy to chat with potential collaborators.
I was recently interviewed about the study, and about plant-pollinator interactions and the Apocynaceae more generally, for the In Defense of Plants podcast –here’s a link to that interview.
Finally, I’d like to express my sincerest thanks to my co-authors on this study – I really couldn’t have done it without you guys!
Natural history records of plant flowering and pollinator foraging, much of them collected by well informed amateurs, have huge scientific importance. One of the values of such records to ecology is that it allows us to document where these species occur in space and when they are active in time. This can be done at a range of spatial and temporal scales, but large-scale patterns (for example at a country level) are, I think, especially useful because they provide scientific evidence that can inform national conservation strategies.
During 2017 I collaborated with a young early career researcher at the University of Sussex, Dr Nick Balfour, on an analysis of the phenologies of British pollinators and insect pollinated plants. That study was recently published (see citation below) and I think that the results are fascinating.
Nick did most of the leg work on this, which involved assessing more than one million records that document the activity times of aculeate wasps, bees, butterflies and hoverflies held in the databases by three of the UK’s main insect recording organisations, the Bees, Wasps and Ants Recording Society (BWARS), the UK Butterfly Monitoring Scheme (UKBMS) and the Hoverfly Recording Scheme (HRS). Information on flowering times was taken from a standard British flora (Clapham et al. 1990 – Flora of the British Isles. Cambridge University Press).
As well as looking at annual flight periods and flowering trends for these organisms we also focused on pollinator and plant species that were endangered or extinct. Here are some headline results and thoughts on what the work shows:
About two-thirds (62%) of pollinator species peak in their flight times in the late summer (July and August), though there was some variation between the different groups – see the figure from the paper above). Particularly noticeable was the double peak of the bees, with the first peak denoting the activity of many early-emerging solitary bees, such as species of the genus Andrena, whilst the second peak is other solitary bees plus of course the bumblebees which by that time have built up their colonies.
A rather fixed phenological pattern with respect to different types of plants was also apparent, which I was not expecting at all: insect pollinated trees tend to flower first, followed by shrubs, then herbaceous species (again, refer to the figure above). This might be because larger plants such as trees and shrubs can store more resources from the previous year that will give them a head start in flowering the following year, but that idea needs testing.
Putting those first two points together, what it means is that trees tend to be pollinated by those earlier emerging bees and hoverflies, whereas the herbs are mainly pollinated by species that are active later.
When looking at the extinct and endangered pollinators, the large majority of them (83%) were species with a peak flight times in the late summer, a much larger proportion than would be expected given that 62% of all species are active at that time. However this was mainly influenced by extinct bee species and the same pattern was not observed in other groups.
The obvious explanation for that last point is that historical changes in land use have led to a dramatic reduction in late summer flowering herbaceous species and the subsequent loss of floral resources has been highly detrimental to those bees. But intriguingly no such pattern was apparent for the endangered pollinators and clearly there are complex reasons why pollinators should become rare or extinct, a point that I have discussed previously on the blog.
The long-term decline of wild and managed insect pollinators is a threat to both agricultural output and biodiversity, and has been linked to decreasing floral resources. Further insight into the temporal relationships of pollinators and their flowering partners is required to inform conservation efforts. Here we examined the
phenology of British: (i) pollinator activity; (ii) insect-pollinated plant flowering; and (iii) extinct and endangered pollinator and plant species. Over 1 million records were collated from the historical databases of three British insect monitoring organisations, a global biodiversity database and an authoritative text covering the national flora. Almost two-thirds (62%) of pollinator species have peak flight observations during late-summer
(July and August). This was the case across three of the groups studied: aculeate wasps (71% of species), bees (60%), and butterflies (72%), the exception being hoverflies (49%). When species geographical range (a proxy for abundance) was accounted for, a clear late-summer peak was clear across all groups. By contrast, there is marked temporal partitioning in the flowering of the major plant groups: insect-pollinated tree species blossoming predominantly during May (74%), shrubs in June (69%), and herbs in July (83%). There was a positive correlation between the number of pollinator species on the wing and the richness of both flowering insect pollinated herbs and trees/shrubs species, per calendar month. In addition, significantly greater extinctions occurred in late-summer-flying pollinator species than expected (83% of extinct species vs. 62% of all species). This trend was driven primarily by bee extinctions (80% vs. 60%) and was not apparent in other groups. We contend that this is principally due to declines in late-summer resource supplies, which are almost entirely provisioned by herbs, a consequence of historical land-use change. We hypothesize that the seasonality of interspecific competition and the blooming of trees and mass-flowering crops may have partially buffered spring flying pollinators from the impacts of historical change.
It’s been a couple of years since I posted my previous “virtual conferences” on Pollinators, Pollination and Flowers and Ecology and Climate Change, a lapse that has largely been due to lack of time (my default excuse for most things these days….). However Judith Trunschke at Uppsala University in Sweden has risen to the challenge of guest-curating her own virtual conference*. The theme here is how pollinators impose (or sometimes don’t impose) natural selection on flowers that results in the formation of new plant species: