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 lack of late summer flowering resources for pollinators is a contentious issue however as plant conservation groups have in the past recommend that meadows and road verges are cut in late summer to maximise plant species richness. Mowing road verges once or twice a year certainly benefits plant diversity, as this recent review by Jakobsson et al. (2018) demonstrates. But there’s very little data available that assesses how timing of cutting can affect pollinators. The only study that I know of (and if I’ve missed any, please let me know) that has considered this is the PhD work of one of my former students, Dr Sam Tarrant who looked at pollinators and plants on restored landfill sites compared to nearby nature reserves. In a paper that we published in the journal Restoration Ecology in 2012 we showed that on restored landfill sites the abundance of pollinators in autumn surveys (conducted September-October) was just as high as for summer surveys. On nature reserves, which are routinely cut from mid-July onward, this was not the case.
Here’s the full citation of Nick’s study with a link to the publisher’s website, and a copy of the abstract is below. If anyone wants a PDF, drop me a line:
Balfour, N., Ollerton, J., Castellanos, M.C., Ratnieks, F.L.W. (2018) British phenological records indicate high diversity and extinction rates among late-summer-flying pollinators. Biological Conservation 222: 278-283
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.