Plant-pollinator meta-network of the Kashmir Himalaya: a new study just published

The hidden potential value of woodland trees for wild bee assemblages – lead author Guthrie Allen introduces his recently published study

Note: This is a guest blog post by Guthrie Allen who is a PhD candidate at the University of East Anglia.

Though often overlooked, woodlands have great potential to support wild bees in farmed landscapes. Both the understory and canopy can provide large quantities of early-season nectar. The pollen of woodland-tree taxa such as Acer can form a large proportion of the early-season diets of bumblebees and solitary bees. Even the pollen from wind-pollinated trees such as Quercus can form a significant part of the diets of several common solitary bee species. Woodlands are also believed to provide ample nesting opportunities. These factors may explain why several studies have found positive correlations between bee abundance and/or diversity and woodland cover at the landscape scale. Despite this, a large-scale European study has shown that interior woodland understories support very few bees compared to exposed woodland edges.

Sampling the canopy – at heights of up to 20 metres – is not easy, but unlike the understory, this habitat remains sun-exposed after canopy closure and could be favourable for bees. In Europe, however, our knowledge of bee activity in the woodland canopy is very limited. Furthermore, whilst canopy-tree taxa have been identified as suitable food sources for many bees, we have little evidence that these trees are used for forage when located within woodlands. To explore the role of the canopy, we trapped bees in late spring in the understory and canopy – at the exposed edges and in the interiors – of 15 woodland sites across an English agricultural landscape.

A significant proportion of bee abundance was found in the canopy, represented by 23 of the 29 total sampled species. Interior canopy activity was much higher when nectar-producing Sycamore (Acer pseudoplatanus) trees were nearby. Communities differed between the canopy and understory: of the seven most common species sampled, three were more active in the understory, while the opposite was true of one species – Bombus lapidarius. Interestingly, we found the sex ratio of the most abundant species – B. pratorum – to be female-skewed in the canopy. For four of these species, and contrary to expectations, we found no evidence that understory activity was any higher at woodland edges compared to woodland interiors.

Further research is needed to understand community differences between the canopy and understory. Nonetheless, our results suggest that both these habitats have a significant role to play in supporting farmland bees. We demonstrate that a diverse bee community has the potential to exploit canopy resources, with nectar-producing trees in woodland interiors likely to provide forage for many bees. And we show the unexpected potential of interior woodland understories to support bee abundance.

Here’s the full reference with a link to the open access paper:

Allen, G. & Davies, R.G. (2022) Canopy sampling reveals hidden potential value of woodland trees for wild bee assemblages. Insect Conservation and Diversity. Available from: https://doi.org/10.1111/icad.12606

Here’s the abstract:

Woodlands can play an important role in supporting bee abundance and diversity in agricultural landscapes. However, in temperate-region studies, the canopy is rarely sampled, and our understanding of its contribution is limited. To explore this, we sampled bees in late spring with blue vane traps in the understory (n = 30) and crowns of mature Quercus robur (n = 35) at the exposed southern edges and in the interiors (ca. 25–75 m from woodland edges) of 15 woodland sites across an English agricultural landscape. A significant proportion of bee abundance and diversity was found in the canopy: canopy-trap catches were estimated to be a third as large as understory-trap catches, and 23 of the 29 sampled species were present in the canopy. Of the seven most common species sampled, four were equally abundant in woodland edge and interior traps; three were more abundant in understory traps, and a single species—Bombus lapidarius—was more abundant in canopy traps. The sex ratio of the most abundant species, B. pratorum, was female-skewed in the canopy. Additionally, the presence of nearby Acer pseudoplatanus trees in flower greatly increased canopy-trap catches in woodland interiors. These results suggest that both the woodland canopy and understory have a significant role to play in supporting farmland bee communities; they indicate the importance of nectar-producing trees in woodland interiors, opening avenues for canopy-based management; and they demonstrate that a diverse bee community has the potential to exploit canopy floral resources.

Books as gifts this Christmas – here’s some ideas for adults and children

2 Replies

Although we are still in the early part of November, it’s never too soon to be thinking about buying Christmas gifts for friends and family. And what better gift than a book? Here’s a few suggestions for some that I’ve read this year that I think will make fine presents.

Who doesn’t love a good story, and Stephen King is a master storyteller. I’ve really enjoyed his more recent novels and Billy Summers stands out for me. It’s part thriller, part romance, and as always there’s a little touch of the uncanny. Very different, but just as much a page turner, is The Ministry for the Future by Kim Stanley Robinson. Although Robinson is known as a science fiction writer, this novel is set in the near future when climate change breakdown is forcing governments and businesses into making radical changes. As much as anything this is a road map for how we can get ourselves out of the current climate crisis.

Talking of which, Michael Mann’s The New Climate War should be read by everyone interested in understanding how big corporations have colluded with the media and governments to trivialise and misrepresent the problems that we currently face. I reviewed this and a book about the insect biodiversity crisis, Silent Earth by Dave Goulson, in this blog post.

If you are looking for a book to help young children understand the importance of bees and other pollinators then I can highly recommend Can We Really Help the Bees? by Katie Daynes and Róisín Hahessy, for which I acted as science advisor.

For the ornithologically minded you might want to consider the lavishly illustrated Hummingbirds: A Celebration of Nature’s Jewels by Glenn Bartley and Andy Swash. Those who love writing and art with an environmental focus would appreciate an annual subscription to Dark Mountain, which gives you two beautifully produced volumes every year. A couple of my essays appeared in earlier volumes and the quality of the contributions never fails to impress me.

There’s lots of great natural history authors and books to choose from on Pelagic Publishing’s list, including of course my own Pollinators & Pollination: Nature and Society! I especially enjoyed Ian Carter’s recent memoir Human, Nature: A Naturalist’s Thoughts on Wildlife and Wild Places.

For the budding or experienced cook, Niki Webster’s Rebel Recipes serves up some amazing vegan dishes that Karin and I have really enjoyed trying. And speaking of my wife, of course I have to include her Essential Companion to Talking Therapy as the perfect gift for anyone considering or currently undergoing counselling or therapy, or who is thinking about becoming a practitioner.

Finally, if you’re looking for particular book ideas or just like browsing through lists of what others recommend, there’s lots of inspiration over at the independent Shepherd site.

I hope that you’ve found this useful. Feel free to comment with suggestions for other books that you’ve enjoyed.

Food system resilience: concepts, issues and challenges – a new, open access review just published

When organisms lose their friends: a new review of the “Missed Mutualist Hypothesis” just published

4 Replies

All organisms – be they plants, animals, fungi, or whatever – interact with other species throughout their lives, in relationships that include predation, parasitism, commensalism, and the many and varied forms of mutualism. But when species are transported to a different part of the world, as has happened often during the Anthropocene, these interactions typically break down because usually only one of the participants moves. This loss of ecological relationships can play a role in whether or not a species becomes established in its new home, and has been mostly explored in the “Enemy Release Hypothesis” (ERH) which predicts that, by leaving behind predators or parasites or herbivores, a species becomes more ecologically successful and ultimately invasive in its novel range.

Less well studied, though potentially just as important, is the “Missed Mutualist Hypothesis” (MMH) which in a sense is the twin of the ERH. As well as leaving behind “enemies”, introduced species leave behind “friends” such as pollinators, seed dispersers, mycorrhizal fungi, defensive partners, and other mutually beneficial associates. Negative effects arising from the loss of these relationships could potentially balance the positive impacts arising from the ERH.

In a new quantitative review just published, we review what’s known about the MMH (currently much less than the ERH) and suggest some fruitful lines of enquiry. The study is led by Angela Moles, my collaborator at the University of New South Wales where I spent time as a Visiting Research Fellow in 2019/20 (see my blog posts about that visit starting here). The paper has had a long gestation and gone through several iterations and revisions since we started writing it in late 2019, not least caused by the covid pandemic, but I think that it’s all the better for it.

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

Moles, A.T., Dalrymple, R.L., Raghu, S., Bonser, S.P. & Ollerton, J. (2022) Advancing the missed mutualist hypothesis, the under-appreciated twin of the enemy release hypothesis. Biology Letters 18: 20220220.

Here’s the abstract:

Introduced species often benefit from escaping their enemies when they are transported to a new range, an idea commonly expressed as the enemy release hypothesis. However, species might shed mutualists as well as enemies when they colonize a new range. Loss of mutualists might reduce the success of introduced populations, or even cause failure to establish. We provide the first quantitative synthesis testing this natural but often overlooked parallel of the enemy release hypothesis, which is known as the missed mutualist hypothesis.

Meta-analysis showed that plants interact with 1.9 times more mutualist species, and have 2.3 times more interactions with mutualists per unit time in their native range than in their introduced range. Species may mitigate the negative effects of missed mutualists. For instance, selection arising from missed mutualists could cause introduced species to evolve either to facilitate interactions with a new
suite of species or to exist without mutualisms. Just as enemy release can allow introduced populations to redirect energy from defence to growth, potentially evolving increased competitive ability, species that shift to strategies without mutualists may be able to reallocate energy from mutualism toward increased competitive ability or seed production. The missed mutualist hypothesis advances understanding of the selective forces and filters that act on plant species in the early stages of introduction and establishment and thus could inform the management of introduced species.

Are cactus pollination systems more specialised in the tropics? A new study suggests yes…and no!

2 Replies

The question of whether interactions between different species are more specialised in tropical environments (as theory predicts) has intrigued me for a couple of decades. In fact it’s just occurred to me that August 2022 was the 20th anniversary of my paper in Oikos co-authored with Louise Cranmer entitled: Latitudinal trends in plant-pollinator interactions: are tropical plants more specialised? That paper was one of the first to seriously challenge an idea that was long-embedded in the scientific and (especially) popular literature, that tropical ecology was in a sense “special” and that the ways in which species parasitised, consumed, or engaged in mutualistic relationships in the tropics was different to what was happening in the subtropics and temperate zones.

Since then I’ve written about this subject in a number of publications, most recently in my book Pollinators & Pollination: Nature and Society and it’s inspired some other researchers to address the topic.

One of the real challenges with asking questions about how plant-pollinator relationships change over large geographical areas is obtaining good, robust data to analyse. It’s a challenge to convince science funding agencies to give money to spend many years travelling the world and collecting the kind of data that are needed. However we can gain some idea of the patterns, and potential processes, that drive the macroecology of plant-pollinator interactions by piecing together databases of interactions for particular taxa, gleaned from published and unpublished sources.

That’s what we have done for the family Cactaceae in a new study led by Pablo Gorostiague from the Universidad Nacional de Salta in Argentina. This collaboration started when Pablo visited Northampton back in 2018 and spent some time with my research group, including helping out with field work in Tenerife. Since then the usual issues (work, COVID, etc.) have delayed publication of our paper, but now it’s finally out. Amongst other results we find that, yes, tropical cacti are pollinated by fewer species on average (though it’s hugely variable – see the figure above) but that functional specialisation (i.e. the number of pollinator guilds that are used by species) is no different in the tropics compared to the extra-tropics (that’s the figure at the end of this post).

The full reference with a link to the paper is below; if anyone wants a PDF, please send me a message via the Contact page:

Gorostiague, P., Ollerton, J. and Ortega-Baes, P. (2022) Latitudinal gradients in biotic interactions: Are cacti pollination systems more specialized in the tropics? Plant Biology https://doi.org/10.1111/plb.13450

Here’s the abstract:

Biotic interactions are said to be more specialized in the tropics, and this was also proposed for the pollination systems of columnar cacti from North America. However, this has not yet been tested for a wider set of cactus species. Here, we use the available information about pollination in the Cactaceae to explore the geographic patterns of this mutualistic interaction, and test if there is a latitudinal gradient in its degree of specialization.

We performed a bibliographic search of all publications on the pollination of cacti species and summarized the information to build a database. We used generalized linear models to evaluate if the degree of specialization in cacti pollination systems is affected by latitude, using two different measures: the number of pollinator guilds (functional specialization) and the number of pollinator species (ecological specialization).

Our database contained information about the pollination of 148 species. The most frequent pollinator guilds were bees, birds, moths and bats. There was no apparent effect of latitude on the number of guilds that pollinate a cactus species. However, latitude had a small but significant effect on the number of pollinator species that service a given cactus species.

Bees are found as pollinators of most cactus species, along a wide latitudinal gradient. Bat and bird pollination is more common in the tropics than in the extra-tropics. The available information suggests that cacti pollination systems are slightly more ecologically specialized in the tropics, but it does not support any trend with regard to functional specialization.

Scale matters when analysing plant-pollinator networks: a new research paper out today

3 Replies

It’s long been recognised that the scale at which we study the natural world – over long or short time periods, or across small areas or whole regions – affects the conclusions that we draw about ecological patterns and processes. This is certainly true of plant-pollinator interactions. For example, a widely distributed plant can have very different pollinators at the extremes of its range, and pollinators like bees may vary their focus on nectar and pollen sources from year to year.

The analysis of these interactions as networks of actors has become increasingly popular in the last couple of decades. However there is no consensus about how frequent sampling should be, or the geographical scale over which networks should be studied. In fact all scales (from regional “meta-networks” down to single-season, single-site, single taxon observations) are relevant, depending on the questions being asked or the hypotheses posed.

But it’s important that we acknowledge that conclusions drawn at one scale may not apply at other scales.

That’s the take home message from a paper published this week which is the latest output from the PhD work of Australian bee expert Kit Prendergast. We have collaborated on several papers based on her data and this is actually my 100th peer-reviewed publication: a proud milestone for me and one which I’m glad to share with a wonderful early career researcher like Kit!

Here’s the reference with a link to a read-only version of the paper:

Prendergast, K.S. & Ollerton, J. (2022) Spatial and temporal scale of analysis alters conclusions about the effects of urbanisation on plant-pollinator networks. Arthropod-Plant Interactions https://doi.org/10.1007/s11252-022-01290-z

And here’s the abstract:

Bipartite networks of flowering plants and their visitors (potential pollinators) are increasingly being used in studies of the structure and function of these ecological interactions. Whilst they hold much promise in understanding the ecology of plant– pollinator networks and how this may be altered by environmental perturbations, like land-use change and invasive species, there is no consensus about the scale at which such networks should be constructed and analysed. Ecologists, however, have emphasised that many processes are scale dependent. Here, we compare network- and species-level properties of ecological networks analysed at the level of a site, pooling across sites within a given habitat for each month of surveys, and pooling across all sites and months to create a single network per habitat type. We additionally considered how these three scales of resolution influenced conclusions regarding differences between networks according to two contrasting habitat types (urban bushland remnants and residential gardens) and the influence of honey bee abundance on network properties. We found that most network properties varied markedly depending on the scale of analysis, as did the significance, or lack thereof, of habitat type and honey bee abundance on network properties. We caution against pooling across sites and months as this can create unrealistic links, invalidating conclusions on network structure. In conclusion, consideration of scale of analysis is also important when conducting and interpreting plant–pollinator networks.

Plant–bee interactions and resource utilisation in an urban landscape: a new study just published

1 Reply

Within the last decade there’s been a growing awareness of the importance of urban environments for supporting populations of pollinators, especially bees. Indeed, I devoted a whole chapter of my book Pollinators & Pollination: Nature and Society to the topic, though even then I was only able to scratch the surface of the research that’s been done. Since then there’s been some important studies published and this 2020 review by Kath Baldock provides a good starting point for the topic, whilst a recent pre-print by Pietro Maruyama and colleagues emphasises how little we know about pollinators in tropical cities.

One of the most detailed studies of urban solitary bees in a British town was conducted by Muzafar Sirohi when he was a PhD researcher in my department in Northampton. The first paper from that work, documenting the diversity and abundance of bees, came out in 2015, but since then commitments to other projects, plus Muzafar’s return to his university in Pakistan, have meant that we’ve struggled to find the time to publish more. Hopefully that’s changing and the second publication from Muzafar’s thesis is now out, with a third in progress.

This new paper uses a network approach to study the use of flowers by these bees; here’s the reference with a link to a read-only copy of the paper, followed by the abstract.

Sirohi, M.H., Jackson, J. & Ollerton, J. (2022) Plant–bee interactions and resource utilisation in an urban landscape. Urban Ecosystems https://doi.org/10.1007/s11252-022-01290-z

Abstract:

Biodiversity is declining through human activities and urbanisation is often seen as a particular concern. Urban settings, however, provide diverse microclimatic conditions for plants and pollinating insects, and therefore may be significant habitats for the conservation of solitary and primitively eusocial bees, a major group of pollinators. This study analysed the interactions between these bees and the plants on which they forage, using a network approach. We compared urban habitats (gardens, roadsides, and open vegetation) in a large British town with nearby nature reserves. One native plant Taraxacum officinale (dandelion) was a core generalist species visited in all habitat types. Other core plant species restricted to particular habitats include species of Geranium, Bellis, Crepis, and Ranunculus. Two generalist bee species, Anthophora plumipes and Osmia bicornis were the core visitor species within the networks. The networks were comparatively more nested in urban habitat types than nature areas, suggesting more frequent interactions between generalist and specialist species in urban areas. Network connectance, network level specialisation (H2’ index), and plant generality (network level) were not significantly different in urban and nature areas. However, visitor generality was found to be significantly higher in urban gardens than in nature areas. Careful management of common urban vegetation would be beneficial for supporting urban wild pollinators.

Using museum specimens to look at long-term stresses on pollinator populations: two new papers just published

Under every stone, an ecosystem: photosynthesis beneath rocks in the Kenyan savanna

1 Reply

We’re coming to the end of our time here in Kenya and we’ve amassed some amazing memories of the wildlife with great views of large mammals such as giraffe, zebra, elephant, hippos, and a variety of antelopes including the ubiquitous dik-diks. In addition, between us we’ve put together a bird list of about 130 species for the site. But as always it’s the smaller things that have fascinated me the most and some of the student project groups have worked with pollinators, ants, and other invertebrates.

One of those fascinations has been the life that exists beneath some of the rocks that are embedded in the red soils of this part of the savanna. Turn them over and you often find the green growths of what are probably cyanobacteria – so-called “blue-green algae” – which are true bacteria and not at all related to algae, even though they also photosynthesise.

The question arises, of course, of how an organism that requires sunlight to survive is able to grow under a stone? Our investigations have shown that they only live under quartz rocks, mainly those that are lighter in colour. Quartz is of course a crystal and it allows a small amount of sunlight to pass through, typically only one or two percent of the sunshine hitting the rocks. There’s also greater humidity under the stones so it’s a relatively more benign place to grow than the savanna, especially in the dry season.

There’s a great review of these microbial communities, and their ecological importance, by Chan et al. called Hypolithic microbial communities: between a rock and a hard place.

Here’s some photographs that I took of these hypolithic photosynthesisers: