Well, we’re back in the UK now and have just about got over the jet lag. I’ve returned to teaching, admin, and meetings, and both Karin and I are trying to find time to finish our books. But the persistent backdrop to our stay in Australia – the bushfires and the role of climate change, and the ensuing tensions between scientific evidence and politics – is still fresh in our minds. It’s timely, then, to highlight two new papers that focus on extreme events, climate change and pollinators. The first is one of my own, led by Dr Hilary Erenler who carried out her PhD research in my group. It’s an invited mini-review in the journal Current Opinion in Insect Science entitled “Impact of extreme events on pollinator assemblages” (Erenler et al. 2020). The review is available as a pre-print on the journal’s website; we’ve not yet even seen the proofs, though the final version should not be too different. If you want a copy, just ask.
In this essay we focus on what we term SHOCKS: events that provide a Sudden, High-magnitude Opportunity for a Catastrophic ‘Kick’ to the environment that can negatively affect pollinator assemblages in many different ways. Such events can be natural, human-mediated or human-enhanced, and occur suddenly, at a high-magnitude and with possibly catastrophic outcomes for those pollinators. There are many examples of such SHOCKs, as we illustrate in the figure above which comes from the paper. However one of our main conclusions is just how little we understand about the outcomes of such events on pollinators. Ideally we need before, during and after event monitoring to assess how pollinators have been affected and may respond. But SHOCKs are, by their very nature, infrequent and unpredictable, and often we don’t have the baseline data with which to compare to post-event data. I know from conversations with Australian pollination ecologists that some have had their field sites burned and they are going to use this as an opportunity to assess how the fires have impacted pollinators. Field experiments such as the one by Biella et al. (2019) that I discussed last year, in which flowers were removed from a plant community, may also give us some insights into the response of plant-pollinator networks to sudden SHOCKs. But we need more research focus on this topic, especially consideration of how the impacts of SHOCKs can be reduced and mitigated.
One set of emerging human-enhanced SHOCKs highlighted by Erenler et al. (2020) is extreme weather events that are being exacerbated (in scale or frequency) by anthropogenic climate change. We cite several papers and reviews that have considered this, but there’s still few empirical studies that have actually looked at how weather SHOCKs might be impacting pollinators. It’s therefore timely that this week’s Science includes a very impressive study of how climate change has affected populations of bumblebees (Bombus spp.) in Europe and North America (Soroye et al. 2020).
The title of the paper rather gives away its findings: “Climate change contributes to widespread declines among bumble bees across continents“. This study shows that, for the 66 species of Bombus studied, there had been a decline in species diversity in 100 km x 100 km quadrats of, on average, 46% in North America and 17% in Europe. This loss of diversity has occurred in the period 2000–2014, relative to a baseline of 1901–1974. Using some sophisticated analyses they show that climate change has been the main driver of these losses, and has been more important than factors such as changes in land use, pesticides, etc. Which is not to discount those other contributors to pollinator loss: they can interact with climate change and are all part of the assault that we are imposing on the environment.
The most significant finding of the Soroye et al. (2020) study, and the reason why I’m discussing Erenler et al. (2020) in the same post, is that it’s extreme heat which seems to be the driving factor in determining Bombus declines. Bumblebees are large, hairy insects because they are adapted to cooler conditions: they are not, by and large, tropical insects, except in mountainous areas. Not surprisingly, then, it is the number of days of temperatures higher than those historically encountered by particular bee species that is the main driver of their loss from a region. In relation to the figure above, this is the result of human-enhanced SHOCKs, and for heat-sensitive species like bumblebees, they are occurring more often than we had imagined when we wrote our review. I fear that the coming years will see more examples of this as the effects of anthropogenic climate change continue to play out and our world experiences more extremes of weather events that are hotter, wetter, colder, drier, windier, and more combustible than we have previously known.
Biella P., Akter A., Ollerton J., Tarrant S., Janeček Š., Jersáková J. & Klecka J. (2019) Experimental loss of generalist plants reveals alterations in plant-pollinator interactions and a constrained flexibility of foraging. Scientific Reports 9: 1-13
Erenler, H.E., Gillman, M.P. & Ollerton, J. (2020) Impact of extreme events on pollinator assemblages. Current Opinion in Insect Science (in press)
Soroye, P., Newbold, T. & Kerr, J. (2020) Climate change contributes to widespread declines among bumble bees across continents. Science 367: 685-688 [see also the commentary by Bridle and van Rensburg pp. 626-627 of the same issue]