Tag Archives: Biodiversity

Are plant-pollinator interactions more specialised in the tropics? A new global analysis suggests that it’s complicated…

Some scientific questions have a habit of following you around for much of your career. Back in 2002, Louise Cranmer and I published a paper in the journal Oikos asking a deceptively simple question: are tropical plants more specialised in terms of their pollinators?

At the time there was a widespread assumption in ecology that interactions between species become increasingly specialised as you move from the poles towards the tropics. It’s an attractive idea which had some (quite vague) theoretical support. Tropical ecosystems contain more species, have experienced relatively stable climates over long periods of evolutionary time, and seem like just the sort of places where tightly specialised interactions ought to evolve.

The trouble was that the evidence wasn’t especially convincing, in part because it hadn’t actually been well studied. In preparing the 2002 paper, Louise and I found only a handful of empirical studies that had addressed the question, not just for plants and pollinators, but for ALL kinds of interactions.

The question of tropical specialisation has subsequently cropped up repeatedly in my research. In 2014, Angela Moles and I went considerably further and asked whether the broader claim that species interactions are stronger and more specialised in the tropics might actually be a “zombie idea” – one of those scientific notions that persists despite accumulating evidence that the reality is considerably more complicated. That guest post on the Dynamic Ecology blog was then written up as an invited paper for Biotropica see: Tropical Zombies: Moles & Ollerton (2016) is now published.

Twenty-four years after that first Oikos paper, we can now address the question with a dataset that Louise and I could scarcely have imagined in 2002. A huge international collaboration led by Sailee Sakhalkar and Robert Tropek has just published “Global patterns in plant–pollinator specialization” in Nature Ecology & Evolution.

I’m delighted to be one of the co-authors and I find the answer to the question that we posed 24 years ago to be fascinating.

More than 110,000 interactions

The scale of the new study is worth emphasising. It assembled 3,415 quantitative plant-pollinator networks from 162 studies, covering 110,571 pairwise interactions between 5,343 pollinator species and 6,126 plant species. The data span almost all of the world’s terrestrial biogeographic regions, from 43.6°S to 81°N and from sea level to more than 4,000 metres elevation.

That makes this the most comprehensive attempt yet to examine the geography of specialisation in plant-pollinator interactions. But simply throwing thousands of networks into a statistical analysis would potentially create as many problems as it solves. Different researchers study pollination networks in very different ways. Some attempt to sample whole communities, while others concentrate on particular groups such as bees, hummingbirds or particular plants.

So an important feature of this study was that we explicitly separated these different types of datasets and accounted for sampling completeness and methodological differences. We also looked separately at different functional groups of pollinators rather than assuming that bees, beetles, butterflies, moths, flies and birds should all behave in the same way. That turned out to be important in ways that we could not predict.

So, are interactions more specialised in the tropics?

The short answer is no, not in any simple sense. There is geographical structure to plant-pollinator specialisation. But there is no straightforward increase in specialisation as you approach the Equator – nor, for that matter, a straightforward decrease. Instead, the patterns are distinctly nonlinear.

Network-level specialisation and pollinator specialisation tended to peak at relatively low northern latitudes, at around 20°–30°N, close to the tropical-subtropical boundary. I find this particularly interesting as I think the subtropics have been a bit neglected as a focus of ecological research: no one ever refers to themselves as a “subtropical ecologist” and there is no Journal of Subtropical Ecology. Yet the subtropics are not simply a transitional zone between the tropical and temperate worlds. They encompass an extraordinary range of ecosystems, including deserts, Mediterranean-type vegetation, subtropical forests, and some major centres of biodiversity and endemism. This includes the distinctive habitats in Yunnan that I’m exploring during my visits to China. Perhaps we need to start thinking of the subtropics as an important biogeographical region in their own right, rather than as the ecological space between two better-studied zones?

Plant specialisation showed a different and weak hemispherically asymmetric pattern. And once individual pollinator groups were examined separately, the picture became more complicated still. Hoverflies, for example, came closest to the traditional expectation of increasing specialisation towards the Equator. Birds showed something closer to the opposite pattern, while moths did something much more complicated.

In other words, there isn’t a latitudinal specialisation gradient. There are several overlapping geographical patterns whose shapes depend upon what organisms you study and what measure of specialisation you are considering.

That conclusion feels satisfying given the arguments Angela Moles and I made ten years ago. We suggested that attempts to find a universal latitudinal gradient could obscure the fact that different groups of organisms have different evolutionary histories, physiologies and ecologies. That is very close to what emerges from this much larger analysis.

Perhaps latitude was never really the interesting variable

One of the most intriguing results is what happens when we stop asking about latitude itself. We compared latitude with a suite of possible explanatory variables, including temperature, precipitation, environmental productivity, and plant and pollinator diversity. Climate generally did a better job of explaining specialisation than latitude, biodiversity or productivity. Again, however, there wasn’t a single climatic rule.

At the whole-network level, specialisation generally declined as mean annual temperature increased, which was not expected at all and runs counter to the argument that the tropics are always more specialised. Precipitation produced strongly nonlinear relationships, and plants and different pollinator groups responded differently. Birds, insects and even different groups of insects could show contrasting responses to the same climatic gradient.

There are some plausible biological explanations for these patterns. Warmer conditions may extend flowering periods and pollinator activity, creating more opportunities for species to encounter alternative partners and therefore encouraging generalisation. Colder or more seasonal environments might compress flowering and pollinator activity into shorter periods, producing tighter temporal matching between species. Rainfall adds another layer of complexity because it can influence flowering but also directly constrain pollinator activity.

These are hypotheses rather than demonstrated mechanisms, however. As we stress in the paper, these are correlational global patterns and climate covaries strongly with latitude. The results shouldn’t therefore be interpreted as demonstrating simple causal effects of temperature or rainfall.

Specialisation depends on how you look at it

There’s another connection here to research I’ve discussed previously on the blog. In work with Danish and Brazilian colleagues, for example, we showed that the apparent specialisation of plants and pollinators depends partly on the spatial scale at which interactions are examined – see Local and regional specialization in plant–pollinator networks. More recently I’ve written about why scale matters when analysing plant-pollinator networks.

And earlier this year, our global analysis of plant-pollinator interactions in gardens found that the factors explaining species richness were not necessarily those explaining interaction specialisation: Global drivers of plant-pollinator interaction specialization in gardens.

There’s also a nice connection to another study that I wrote about here in 2022. With Pablo Gorostiague and Pablo Ortega-Baes, I looked specifically at whether cactus pollination systems become more specialised towards the tropics. Our answer was yes and no: tropical cacti were visited by somewhat fewer pollinator species, but there was no latitudinal pattern in the number of functional pollinator groups that they used. As I wrote at the time, the answer depended on what we meant by “specialised”: Are cactus pollination systems more specialised in the tropics? A new study suggests yes…and no!

All of these studies point towards a similar conclusion: “specialisation” is not a single, fixed property of an ecological community. What we detect depends upon biological scale, geographical scale, the organisms being considered, and how the interactions have been sampled. That doesn’t make specialisation meaningless. Quite the opposite, it means we need to be precise about what sort of specialisation we’re talking about and at what scale.

The new global study puts that result into a much broader context. Different groups really can display different geographical patterns of specialisation, so studies focused on a particular lineage aren’t necessarily contradicting one another when they produce different answers. They may simply be measuring different pieces of a much larger ecological puzzle.

Why does any of this matter?

Specialisation is not just an esoteric property of ecological networks. It affects how plants and pollinators partition resources, how species coexist, how communities are assembled, and potentially how ecological systems respond when species disappear.

That becomes especially important in a changing climate. Our results suggest that changing temperature and rainfall regimes could restructure plant-pollinator networks, but they are unlikely to do so uniformly. Different pollinator groups and different parts of the world may respond in very different ways. Highly specialised interactions may also be particularly vulnerable to changes in phenology or the loss of interaction partners. Predicting the consequences therefore requires us to move beyond simple statements about “tropical” and “temperate” ecosystems.

For me, though, there is also a broader lesson here about how science progresses. In 2002, Louise Cranmer and I questioned whether the available evidence really supported the assumption that tropical plant-pollinator interactions were more specialised. Fourteen years later, Angela Moles and I argued that the wider claim about stronger and more specialised tropical interactions might be a zombie idea. Now, with more than 110,000 plant-pollinator interactions available for analysis, the answer is much clearer. Latitude matters; climate matters; the identity of the organisms matters; and ecology, as usual, refuses to obey a simple rule.

My particular thanks to Sailee and Rob for the monumental efforts they made to pull this work together, and cudos to all of the co-authors who helped to make the study possible.

Join me for “Pollinator Conservation: Threats and Opportunities” in Bedfordshire on Tuesday 15th September

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Bedfordshire Natural History Society (BNHS) has invited me to give a talk on the theme of “Pollinator Conservation: Threats and Opportunities” at Maulden Village Hall (MK45 2DP) on Tuesday 15th September. It starts at 8pm and non-members are encouraged to attend.

More details are available at the BNHS web site.

I’ll have signed copies of my books available to purchase, so if you’re in the area, please do come along.

Bird pollination finally confirmed in Britain!

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In my book Birds & Flowers, I included a chapter called “The curious case of Europe”. The point of that chapter was simple enough: compared with much of the rest of the world, Europe appears to be oddly deficient in bird pollination. There are no hummingbirds, no sunbirds, no honeyeaters, and very few native plants that obviously look as though they have evolved with birds as their main pollinators. And it Britain, it appears that bird pollination is totally absent.

But “appears” is doing a lot of work there.

For decades, European bird–flower interactions have tended to be treated as marginal curiosities: Blue Tits taking nectar from willow catkins, warblers dusted with pollen, finches messing about in blossom. Interesting natural history, certainly, but not necessarily pollination. The assumption has usually been that insects do the serious work, and birds are at best incidental visitors.

A new paper in Journal of Ecology called “Generalist passerine birds perform a functional role as pollinators in temperate Europe“, by Sandra Anderson, George Perry and Rose Thorogood challenges that assumption in a very useful way. Working at Wicken Fen in Cambridgeshire, they found that pollen transport by passerine birds was widespread. Most of the birds they sampled carried pollen, and several species — including Blue Tit, Blackcap, Chiffchaff, Wren, Redpoll and Bullfinch — regularly carried meaningful loads.

More importantly, they tested whether this mattered to the plants. By excluding birds from flowers while allowing insects access, they showed that fruit-set was reduced in several early-flowering woody plants, including Blackthorn (Prunus spinosa), Hawthorn (Crataegus monogyna) and Buckthorn (Rhamnus cathartica). In other words, the birds were not just getting dusty faces. They were contributing to plant reproduction.

I should say that I was one of the reviewers of this paper, so I have followed its development with particular interest. What I like about it is that it does not try to claim that Europe secretly has a hidden flora of classic “bird flowers”. These are not red tubular blossoms adapted to hummingbirds or sunbirds. They are familiar, open, pale, spring-flowering shrubs and trees. Nor are the birds specialised nectar-feeders. They are generalist passerines making use of seasonal resources.

That is precisely why the paper is interesting. It shifts the question from “does this look like bird pollination?” to “does bird visitation actually function as pollination?” That distinction matters. Pollination syndromes can be useful, but they can also blind us to interactions that do not fit the textbook categories.

The seasonal context is also important. These interactions peak early in spring, when willows, blackthorn and other woody plants are flowering, temperatures are still cool, insects may be unreliable, and birds are preparing to breed or arriving from migration. Under those conditions, even occasional bird visits could be valuable to plants needing pollen moved between individuals.

For me, this paper strengthens the argument I made in “The curious case of Europe”: Europe is not devoid of bird–flower interactions; rather, we have been looking for the wrong kind of bird pollination. Instead of obvious “ornithophilous” specialisation, we may have overlooked a more diffuse, opportunistic, generalist system involving common birds and common spring-flowering woody plants.

That may not be as spectacular as a hummingbird hovering at a tropical flower, but ecologically it is just as revealing.

The photo above is from Birds & Flowers and shows pollen on the face of a Eurasian Blue Tit in the early spring. Bird mist-netted under licence in Northamptonshire, UK. (© Lynne Barnett)

Help us uncover the hidden lives of Europe’s ground-nesting bees

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When most people think about pollinators, they picture bees visiting flowers. But for many species, a large part of life happens elsewhere: in the soil. Many pollinators do not just feed above ground, they also nest, shelter or develop below it

Discovering more about this aspect of pollinator life histories is the focus of ProPollSoil, a major new European research project exploring the links between soil health and pollinator health that kicked off last year. I’m excited to be part of it because the soil component of pollinator ecology and conservation is important but often overlooked.

As part of ProPollSoil, we are asking the public to help us find and document ground-nesting bees through a citizen science initiative called EuroBeeSoil. If you spot a bee nest in soil, sand, a lawn, a path edge or a sunlit bank, you can photograph it and upload your record to iNaturalist.

It’s a simple idea, but potentially a very powerful one. These observations will help us understand where bees are nesting, what kinds of soils they use, and how we can better conserve them.

We often talk about pollinator-friendly flowers. But bees need more than food, they also need somewhere to live. For many species, that means the ground beneath our feet.

So please follow this link and consider how you might take part if you can. You do not need to be a bee expert. You just need to look down a little more often.

Join me for two webinars exploring the links between biodiversity, pollinators and the UN Sustainable Development Goals

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The diversity of life on Earth, and the interactions between the species that make up that diversity, are fundamentally important to the functioning of ecosystems and to human well-being. Yet these connections are often poorly appreciated, despite the fact that biodiversity supports everything from food production and clean water to climate resilience and human health.

At the end of March and the end of May I will be presenting two lunchtime webinars which explore this as part of the Biological Recording Company’s Skills for Ecology series. These talks will look at how biodiversity in general, and plant–pollinator interactions in particular, connect to the United Nations Sustainable Development Goals (UN SDGs), showing why the conservation of nature is central to a just, healthy, and sustainable future.

Here are the dates and the links for booking:

I look forward to seeing some of you there!

Do birds pollinate the iconic Golden Lotus? A new study suggests that they do!

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The Golden Lotus (Musella lasiocarpa) is one of China’s most iconic plants — a striking member of the banana family (Musaceae) that seems to bloom forever. Its brilliant yellow, lotus-like bracts have long made it a favourite of subtropical gardeners, though it also has utility as a food and fibre crop, and is associated with Chinese Buddhism. As you can see above it often features stylistically in Chinese temples, and in my visits to Yunnan we frequently encounter it during fieldwork on farms, planted to support terraced fields:

But despite its fame, one mystery has lingered for decades: what actually pollinates it?

Until now, Musella was thought to rely mainly on insects, particularly bees, for pollination. That assumption made it something of an outlier within the banana family, where most species are pollinated by birds or bats. But a new study, in which I was involved as part of an international team of predominantly Chinese and Brazilian researchers, has turned that view on its head.

By combining careful field observations with citizen science records, our team found that the Golden Lotus is regularly visited by an impressive diversity of birds — twelve species from five different families. As I documented in my recent book Birds & Flowers: An Intimate 50 Million Year Relationships, many of these visitors, such as bulbuls and sunbirds, are known nectar-feeders, and their behaviour at the flowers suggests that they are acting as effective pollinators. This discovery significantly expands what we know about the pollination ecology of the Golden Lotus, and places it firmly within the broader pattern of bird pollination that characterises much of the banana family.

Interestingly, the plant’s features — large, robust, vividly coloured bracts, abundant accessible nectar, and long-lived blooms — make perfect sense in this new light. These are traits that favour bird pollination rather than the short, concentrated visits typical of bees.

But the significance goes beyond one species. Bird pollination plays a vital, and often overlooked, role in China’s native flora, linking ecosystems from tropical rainforests to mountain valleys. Understanding these relationships is important not only for biodiversity conservation but also for horticulture — helping gardeners and landscape designers to create spaces that attract and sustain pollinators of all kinds.

The Golden Lotus has always been celebrated for its beauty and longevity. Now, we can add another layer to its story: a reminder that even the most familiar plants can still surprise us, and that nature’s partnerships are often more complex — and more colourful — than we imagine.

Here’s the reference with a link to the paper, which is open access:

Albuquerque-Lima, S., Ferreira, B. H. d. S., Rech, A. R., Ollerton, J., Lunau, K., Smagghe, G., Li, K.-Q., Oliveira, P. E., & Ren, Z.-X. (2025). Beyond Bees: Evidence of Bird Visitation and Putative Pollination in the Golden Lotus (Musella lasiocarpa)—One of the Six Buddhist Flowers—Through Field Surveys and Citizen Science. Plants, 14(20), 3157. https://doi.org/10.3390/plants14203157

Pollinators need more space and 10% habitat is not enough says a new study just published in Science

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Pollinators such as wild bees, butterflies, and hoverflies are in trouble worldwide. A major new study, published in Science and led by Gabriella Bishop and other scientists at Wageningen University & Research, shows that the oft-quoted figure of 10% semi-natural habitat in farmland landscapes is far too little to safeguard pollinators. Instead, the evidence points to a need for somewhere between 16% and 37% habitat cover, depending on the type of pollinator, if we are serious about halting declines. Suitable habitats include hedgerows, patches of woodland, species-rich grasslands, and flowering margins, and as a general rule, hoverflies need less of it whilst bumblebees and butterflies require more.

I was fortunate to play a part in this global assessment, contributing an unpublished dataset collected with my former PhD student, Sam Tarrant, who studied plant-pollinator interactions on restored landfill and established grassland sites. Seeing those data joined with dozens of other studies from around the world underlines something we have known for years: no single dataset, however carefully gathered, can give us the whole picture. To really understand what is happening to biodiversity—and to design conservation solutions that work—we need these kinds of global, mega-author syntheses that draw together evidence from many landscapes, taxa, and approaches.

The message from this analysis is stark but hopeful. More habitat means more pollinators, across all groups. Richer habitats with abundant flowers give an additional boost, but the overriding priority must be to increase the sheer area of natural habitat in farmed landscapes. Small-scale fixes like wildflower strips offer short-term benefits, but without enough space they can’t deliver recovery at scale. Long-term, secure habitat creation—on the order of decades, not seasons—is what pollinators, farmers, and ecosystems need.

Although the policy debate in Europe provided the backdrop for this study, the lessons (and the data) are global. Wherever agriculture dominates, the health of pollinator populations—and by extension our food security and biodiversity—depends on our willingness to give these insects the space and quality of habitat they require.

Looking ahead, we need to think bigger and work together. That means more international collaborations, more sharing of data, and more commitment to long-term solutions that transcend borders. The image at the start of this post is from my trip back to China in July this year. I deliberately chose it because, as you’ll see from the map below which is taken from the paper, there was no suitable data available for the study from that country. Or from Africa. Or Australasia. Or from most of tropical South America. That shows that as pollination ecologists we need to coordinate more in advance on these types of syntheses, and maximise the value of the kinds of data that we collect. The main take away from this study, however, is that if we want to reverse the declines in biodiversity, scientists, policymakers, businesses, farmers, and citizens all have a role to play. Pollinators remind us that nature is interconnected and global—our conservation efforts must be, too.

Here’s the full reference with a link to the study:

Bishop, G.A., Kleijn, D., Albrecht, M., Bartomeus, I., Isaacs, R., Kremen, C., Magrach, A., Ponisio, L.C., Potts, S.G., Scheper, J., Smith, H.G., Tscharntke, T., Albrecht, J., Badenhausser, I., Åström, J., Báldi, A., Basu, P., Berggren, N., Beyer, N., Blüthgen, R., Bommarco, B.J., Brosi, H., Cohen, L.J., Cole, K.R., Denning, M., Devoto, J., Ekroos, F., Fornoff, B.L., Foster, M.A.K., Gillespie, J.L., Gonzalez-Andujar, J.P., González-Varo, J.P., Goulson, D., Grass, I., Hass, A.L., Herrera, J.M., Holzschuh, A., Hopfenmüller, S., Izquierdo, J., Jauker, B., Kallioniemi, E.P., Kirsch, F., Klein, A.-M., Kovács-Hostyánszki, A., Krauss, J., Krimmer, E., Kunin, B., Laha, S.A.M., Lindström, Y., Mandelik, G., Marcacci, D.I., McCracken, M., Monasterolo, L.A., Morandin, J., Morrison, S., Mudri Stojnic, J., Ollerton, J., Persson, A.S., Phillips, B.B., Piko, J.I., Power, E.F., Quinlan, G.M., Rundlöf, M., Raderschall, C.A., Riggi, L.G.A., Roberts, S.P.M., Roth, T., Senapathi, D., Stanley, D.A., Steffan-Dewenter, I., Stout, J.C., Sutter, L., Tanis, M.F., Tarrant, S., van Kolfschoten, L., Vanbergen, A.J., Vilà, M., von Königslöw, V., Vujic, A., WallisDeVries, M.F., Wen, A., Westphal, C., Wickens, J.B., Wickens, V.J., Wilkinson, N.I., Wood, T.J., Fijen, T.P.M. (2025) Critical habitat thresholds for effective pollinator conservation in agricultural landscapes. Science 389: 1314-1319

Here’s the abstract:

Biodiversity in human-dominated landscapes is declining, but evidence-based conservation targets to guide international policies for such landscapes are lacking. We present a framework for informing habitat conservation policies based on the enhancement of habitat quantity and quality and define thresholds of habitat quantity at which it becomes effective to also prioritize habitat quality. We applied this framework to insect pollinators, an important part 5 of agroecosystem biodiversity, by synthesizing 59 studies from 19 countries. Given low habitat quality, hoverflies had the lowest threshold at 6% semi-natural habitat cover, followed by solitary bees (16%), bumble bees (18%), and butterflies (37%). These figures represent minimum habitat thresholds in agricultural landscapes, but when habitat quantity is restricted, marked increases in quality are required to reach similar outcomes.

A new review gives us a deeper understanding of the evolution of plant-pollinator interactions

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If you’ve read my book Birds & Flowers: An Intimate 50 Million Year Relationship, you’ll know that I spend a few pages discussing the long-standing paradigm of how interactions between plants and their pollinators evolve and result in the formation of new plant species. This is referred to as the Stebbins (or Grant-Stebbins) Most Effective Pollinator Principle (MEPP). The MEPP is fairly straightforward and intuitive: flowers evolve their colour, shape, scent, rewards, and so forth as adaptations to the type of flower visitor that successfully moves the most pollen between flowers.

However, the MEPP is not the only Principle in town – there’s also Aigner’s Least Effective Pollinator Principle (LEPP) which is not so intuitive. In the LEPP, flowers can adapt to pollinators that are less successful at pollination, as long as those adaptations do no interfere with the pollination services provided by other flower visitors.

As I note in Birds & Flowers, we don’t know which of these Principles is more frequent in nature, because the LEPP has been much less intensively studied than the MEPP. That’s in part because it’s less well known, but also because the field work and experimental procedures required to test the LEPP are much more challenging.

Hopefully this is about to change with the publication of a brilliant critical review of the MEPP by pollination ecologists Kathleen Kay and Bruce Anderson published in the journal Annals of Botany, entitled: Beyond the Grant–Stebbins model: floral adaptive landscapes and plant speciation. The paper is open access – follow that link and you can download a copy.

Kathleen and Bruce discuss not just the MEPP v the LEPP, but also other ways in which flowers can evolve, framed around the idea of floral evolution as movement across an “adaptive landscape,” where plants are not shaped only by one pollinator but by the need to maximise overall reproductive success. This perspective allows us to explore how flowers evolve when influenced by multiple pollinators, how transitions between floral forms take place, and how speciation occurs through a combination of factors beyond pollination alone. It emphasises that pollinators are important drivers of floral change, but speciation is more likely when divergence happens across several aspects of a plant’s ecology, not just through its flowers.

It’s a great review and well worth your time reading in detail. Perhaps my favourite line in the paper comes from the abstract: “The Grant–Stebbins model, while inspiring decades of empirical studies, is a caricature of pollinator-driven speciation and explains only a limited range of adaptive outcomes.” This is something that many of us have been arguing for years: the natural world is extremely complex, so we should not expect these ecologically critical interactions between flowers and their pollinators to have simple origins or ecologies.

Join me for a “Birds & Flowers” talk in Cambridge on the 12th September!

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If you are in or around Cambridge next week, I’m giving a talk on Friday 12th September at the Cambridgeshire Bird Club about my recent book Birds & Flowers: An Intimate 50 Million Year Relationship.

The event takes place in the Wilkinson Room, St. John’s Church, Hills Road. Doors open at 7pm and the talk begins at 7.30pm. There’s a £2.00 charge for non-members – more details can be found by following this link.

I’ll bring copies of both Birds & Flowers and Pollinators & Pollination: Nature and Society, if anyone wants to buy a signed book.

I hope to see you there!

Is Common Elder an under-appreciated habitat for bats? [updated]

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Regular readers of my blog may recall that I have ~~an obsession with~~ something of an interest in Common Elder (Sambucus nigra) that goes back to my childhood, as I recounted in an article for British Wildlife back in 2022. In that article I mentioned that the larger hollow trunks and branches of elder “can offer nesting opportunities for birds and small mammals”, but didn’t go into detail. The mammals I was thinking of at the time were small rodents, but following a long country walk with Karin recently I wondered whether bats might also make use of these hollows as roosts for breeding and/or hibernation.

I posed this as a question for the bat specialists in the British Ecologists Facebook Group and received several replies, with respondents mentioning that they had encountered Natterer’s Bat (Myotis nattereri), Common Pipistrelles (Pipistrellus pipistrellus), and Brown Long-eared Bat (Plecotus auritus), in hollow elder trees, during summer and autumn surveys.

A couple of people suggested that I check out Arbology’s Look-up Tool for the Bat Tree Habitat Key (BTHK) database which records trees that are used by bats, but to my surprise it returned the following message:

“There are no positive results which match your query.
This does not mean that bats won’t use the feature type in the species and habitat you have selected, but current data suggests that survey effort may be better focused on features which have a proven occupation”.

The BTHK relies on bat surveyors adding their observations, but clearly no one has submitted records of bats in elder, despite the fact that we know they occur. This concerns me for two reasons.

Firstly, of all of our smaller native woodland edge and hedgerow trees, elder is (in my experience) the one most likely to have significant cavities in their trunks and branches. The specialists in the Facebook Group introduced me to the phrase “if they fit, they sit”, meaning that almost any cavity might contain bats, even quite low to the ground: one respondent mentioned that a friend had found two Common Pipistrelles during an autumn survey, in a dead elder stem less than ten centimeters in diameter at about one metre above ground level.

Secondly, it’s not unusual for old elder trees to be cut right to the ground or even removed completely during work on hedgerows. It’s a neglected, even despised native British tree that, as I noted in that British Wildlife article, is:

“generally considered by naturalists, when it is considered at all, as rather boring, so commonplace that we hardly give it a second glance…[and by some as]…’barely a tree at all, more of a weed'”

Another respondent mentioned that tubular structures, such as elder branches, are less likely to be identified as active bat roosts in the absence of bats, as they provide limited shelter and often lack droppings, which tend to fall out, leaving minimal evidence. I’m sure that’s not the whole story, however, I think it’s more likely that small trees generally are overlooked when it comes to habitat for bats: the BTHK has a single entry for Common Hawthorn (Crataegus monogyna) and nothing for Blackthorn (Prunus spinosa), for instance.

In addition to bats, another respondent noted that Willow Tits (Poecile montanus), a species experiencing significant decline and now red-listed in the UK, often nest in elder trunks, where they excavate cavities in decaying wood. That’s yet another reason why we should pay more attention to this most interesting of trees!

My thanks to all of the British Ecologists who replied to my query. As always, feel free to comment or get in touch via my Contact page.

UPDATE: After I posted this on Bluesky, Richard Broughton, author of The Marsh Tit and the Willow Tit, pointed out that elder is also a significant nesting site for Marsh Tits (Poecile palustris), another red-listed species. To quote Richard’s comment:

“Elder is prob[ably] the very best cavity-bearing shrub, far better than hawthorn, hazel, blackthorn (very poor). Important nesting shrub for Marsh Tits & Willow Tits, but only if left to develop old trunks and cavities, not cut. Like Hazel, they self-coppice without management, with new growth from base….in Wytham Marsh Tit studies Elder was the main nest tree/shrub. Though it’s not common/available in woods everywhere. It develops *really* good hollow nest cavities for the small hole-nesting guild, and also very amenable for Willow Tits to excavate. Prob[ably] important in hedges, where holes rare.”

Richard kindly shared a scan from his book showing that for Willow Tits, elder ranks second (after willow and birch) and for Marsh Tits it ranks second after Ash.