The most globally significant groups of pollinators are well known and have been studied for a long time: bees and wasps, flies, butterflies and moths, birds, bats and beetles are all familiar to those of us with an interest in pollination ecology. However, every few years a new type of pollinator or a novel pollination system is described from nature or from the fossil record, or we add further examples of previously neglected pollinator groups such as cockroaches.
This begs the question: how much is there still to discover? How close are we to describing the full diversity of animals that act as pollen vectors? Can looking at the past help us to predict what we might find in the future? That’s the topic of a Perspective article that I was invited to write for the special issue of the Journal of Applied Entomology on the theme of The Neglected Pollinators that I mentioned last month. It’s a subject that I’ve thought about a lot over the last few decades and it was great to get an opportunity to air some ideas and speculation.
The article is open access and you can download a copy by following the link in this reference:
Although huge progress has been made over the past 200 years in identifying the diversity of pollinators of angiosperms and other plants, new discoveries continue to be made each year, especially in tropical areas and in the fossil record. In this perspective article I address the following questions: Just how diverse are the pollinators and what are the phylogenetic limits to that diversity? Which other groups of animals, not currently known to regularly engage with flowers, might be found to be pollinators in the future? Can we predict, from the fossil record and from discoveries in under-researched parts of the world, which animal groups might turn out in the future to contain pollinators? I also discuss why adding to our knowledge of plant–pollinator interactions is important, but also stress that an incomplete knowledge may not be a bad thing if it means that remote, inaccessible and relatively pristine parts of the world remain that way.
Just after I arrived in Northampton in 1995, I set about looking for suitable local sites for conducting pollination ecology field work for myself and students. The campus on which we were situated at the time was adjacent to an urban park – Bradlaugh* Fields – parts of which were designated as local nature reserves. In the intervening years, data from that area have made their way into a wide range of published studies, including:
I still have data collected during that time that have never been published, but good data are hard won and they may see the light of day at some point. Case in point is that we’ve just published a paper based on data from Bradlaugh Fields, the first of which were collected in 2001!
In this paper we’ve tested how effective hoverflies, butterflies and bumblebees are at pollinating the flowers of a common generalist grassland plant, colloquially called Field Scabious (Knautia arvensis). The expectation was that bumblebees, being generally larger, hairier and more flower-focused than the other groups, would be the most effective at transferring pollen to stigmas. To our surprise, they were not: hoverflies and butterflies performed just as well! In fact we argue that butterflies may be MORE important as pollinators of this plant because they fly further distances between individual plants, rather than hopping between the inflorescences of the same plants, as bumblebees tend to do.
Crucially, the importance of these different groups of pollinators varies enormously as the relative abundance of the insects visiting the flowers differs between seasons. In some years butterflies dominate as pollinators, in other years bumblebees or hoverflies. This is driven, we think, both by fluctuations in the populations of these insects and by the availability of other, more preferred flowers that may bloom at the same time.
The paper is part of a special issue of the Journal of Applied Entomology devoted to The Neglected Pollinators. It’s open access and you can download a copy by following the link in this reference:
Plant-pollinator interactions exist along a continuum from complete specialisation to highly generalised, that may vary in time and space. A long-held assumption is that large bees are usually the most effective pollinators of generalist plants. We tested this by studying the relative importance of different groups of pollinators of Knautia arvensis (L.) Coult. (Caprifoliaceae: Dipsacoideae). This plant is suitable for such a study because it attracts a diversity of flower visitors, belonging to different functional groups. We asked whether all functional groups of pollinators are equally effective, or if one group is most effective, which has been documented in other species with apparently generalised pollination systems. We studied two subpopulations of K. arvensis, one at low and one at high density in Northampton, UK. To assess pollinator importance we exposed unvisited inflorescences to single visits by different groups of pollinators (butterflies, bumblebees, hoverflies and others) and assessed the proportion of pollinated stigmas. We then multiplied the effectiveness of each pollinator group with their proportional visitation frequency in five different years. For each group we also compared time spent on flowers and flight distance between visits. The relative importance of each pollinator group varied between years, as did their flight distances between flower visits. Butterflies were the best pollinators on a per visit basis (in terms of the proportion of stigmas pollinated) and flew further after visiting an inflorescence. Different measures and proxies of pollinator effectiveness varied between taxa, subpopulations, and years, and no one group of pollinators was consistently more effective than the others. Our results demonstrate the adaptive value of generalised pollination strategies when variation in relative abundance of different types of pollinators is considered. Such strategies may have buffered the ability of plants to reproduce during past periods of environmental change and may do so in the future.
During the 2020 lockdown caused by the COVID-19 pandemic, I coordinated an international network of pollination ecologists who used standardised methods to collect data in their gardens. I blogged about it at the time – see here and here for instance – and also put up a post when the data paper from that work was published.
Several research groups are now working with that huge data set and interrogating it for answers to a wide range of questions. The first group to actually publish a paper from the data is a largely Chinese set of researchers from the Key Laboratory of Plant Resources, Conservation and Sustainable Utilization, at the South China Botanical Garden in Guangzhou, assisted by Kit Prendergast and myself.
In this paper we’ve considered how robust these plant-pollinator networks are to simulated extinctions of species, and how this is affected by the elevation, latitude, and plant species diversity of the network.
Here’s the full reference with a link to the study:
If you can’t access it and need a PDF, please send me a request via my Contact page.
Here’s the abstract:
Plant-pollinator interactions play a vital role in the maintenance of biodiversity and ecosystem function. Geographical variation in environmental factors can influence the diversity of pollinators and thus, affect the structure of pollination networks. Given the current global climate change, understanding the variation of pollination network structure along environmental gradients is vital to predict how global change will affect the ecological interaction processes. Here, we used a global plant-pollinator interaction data collection by the same sampling method at the same period to explore the effects of elevation, latitude, and plant richness on the structure and robustness of pollination networks. We analyzed a total of 87 networks of plant-pollinator interactions on 47 sites from 14 countries. We conducted a piecewise structural equation model to examine the direct and indirect effects of elevation, latitude, and plant richness on the network robustness and analyzed the function of network structure in elucidating the relationship between robustness and these gradients. We found that plant richness had both positive effects on robustness under random and specialist-first scenarios. Elevation, latitude, and plant richness affected network connectance and modularity, and ultimately affected network robustness which were mediated by nestedness under specialist-first and random scenarios, and by connectance under the generalist-first scenario. This study reveals the indirect effects of elevation, latitude, and plant richness on pollination network robustness were mediated by nestedness or connectance depended on the order of species extinctions, implying that communities with different pollination network structures can resist different extinction scenarios.
Researchers at the University of East Anglia (UEA) are calling for volunteers to contribute to their investigation of the springtime pollen diets of bumblebees in the UK countryside.
Spring is a critical season for colony-establishing bumblebees, but little is known about what types of pollen they collect at this time. In particular, trees, shrubs, and woodland flowers may make significant contributions to bumblebee pollen diets.
Volunteers are needed from across the UK to collect pollen samples from live bumblebees in April and May this year. These will then be analysed by the researchers to determine their taxonomic composition.
Volunteers can collect as many or as few samples as they like – every contribution will help! For an information pack detailing what is involved, and to receive a registration form, please email Guthrie Allen (BIO – Postgraduate Researcher): G.Allen [at] uea.ac.uk.
