by Dave Goulson
Bee discrimination among flowers goes beyond discerning which species are most rewarding. Bees are also capable of learning which particular patches of flowers are best. I have spent many happy hours watching the bees, butterflies and moths visiting the lavender bushes along the front of the house at Chez Nauche. After a little while it is possible to start recognising individual bees. For example, you might spot a particularly brightly coloured shrill carder bee. The largest lavender bush, just to the right of the front door, always seems to be the most popular with insects, and that is where this bee is foraging. After ten minutes or so, when her honey stomach is filled, she flies back to her nest somewhere in the meadow, laden with nearly her own body weight of food. Half an hour later she returns, unladen, and this repeats itself through the day. You can almost set your watch by her return. Of course you might suspect that I am imagining it – there could be more than one brightly coloured shrill carder of similar size – but I know that I am not, because I have tried marking the bees with a little blob of correction fluid. This sometimes upsets them and then they zoom off never to return, but usually it doesn’t, and the same individual can then be seen coming back at regular intervals, day after day, visiting exactly the same bush.
This in itself wouldn’t be too remarkable. The bee has found a good patch of food, remembered its location and is simply flying backwards and forwards from her nest, a trip that takes a predictable length of time. What is perhaps more impressive is that she may be visiting a number of different flower patches on each trip. She can remember the locations of a dozen or more patches and navigate in a straight line from one to another. This behaviour is known as trap-lining, derived from fur-trapping, where the trapper will tour his traps along a fixed route every day. It has been studied extensively by the Canadian bumblebee expert James D. Thomson at the University of Toronto. The bee does not visit the patches in the order that she discovered them, but instead plots novel routes linking the nearest patches together, effectively minimising her flight time. When she can carry no more, she does not need to retrace her steps, but can manage to fly in a direct line back to her nest. It seems that most bumblebee species do this, and also honeybees, hummingbirds and those long-lived Heliconius butterflies. Trap-lines can remain stable for many days, or they can shift; if a patch becomes unrewarding, the pollinator skips it and may try out new patches and incorporate them into her route if they are worthwhile.
To return to nectarless flowers, it is clear from all of this that bees are well equipped to swiftly detect and avoid a species of plant that provides no nectar, such as many orchids, or to avoid individual plants (flower patches) that are less rewarding. If nectarless flowers are scarce, they may be visited occasionally by bees that are exploring new patches. Bugloss is generally highly rewarding and is visited by many bees, so you might imagine that a single nectarless plant among many rewarding ones would still receive bee visits. The nectarless plant would effectively be parasitising its neighbours, benefiting from the nectar they produce while contributing none of its own. You might predict that cheating of this sort might spread, for the cheats ought to be at an advantage. As unrewarding plants become more common, bees would be likely to switch away to feed on other plant species entirely, but if so this cost would be borne equally by all of the bugloss plants, and not just the cheats, so the cheating strategy would not be penalised. However, it would seem this does not generally happen – the cheating, nectarless strategy rarely takes over. The explanation may be that nectarless plants, although they may receive some exploratory visits, will never receive the regular, clockwork visits of a trap-lining bee, and hence will not be pollinated as effectively as their rewarding neighbours. In a sense, the intelligence of bees acts to police cheating strategies among the plants they pollinate.
As you will have gathered, getting pollinated is no simple business, with plants competing for the attention and fidelity of pollinators whilst attempting to minimise the costs of doing so. Equally, gathering pollen and nectar efficiently, when it is hidden in varying and unpredictable amounts amongst patchily distributed flowers of numerous colours and shapes, is far from simple. There is doubtless much more to discover about the ecology and behaviour of pollinators and flowers. You could spend a lifetime studying the insects and flowers at Chez Nauche and still have barely begun to scratch the surface. At present we have only a poor idea of how important particular linkages between flowers and pollinators are. What happens if a pollinator species goes extinct? Do others simply take up the slack, exploiting the unoccupied niche, or do some plants suffer from its absence? How many different species of pollinator does it take to support a healthy wild-flower meadow? These questions are pertinent because, as we shall see in chapter thirteen, we are slowly losing our pollinators.
CHAPTER ELEVEN
Robbing Rattle
26 May 2012. Run: 39 mins 48 secs. People: 6 Lycra-clad old men rocketed past me on their bicycles, nearly making me jump out of my skin. Dogs: 5, including an unfortunate three-legged mongrel wandering the street in Épenède. Butterflies: 12 – spotted a male small blue butterfly perched on a grass stem right by my car on the drive, a tiny but aggressively territorial species with smoky-blue wings, the first I have seen at Chez Nauche. This is my favourite time of year; the meadow is in full bloom, the hedge banks lush and bursting with life, without the tiredness that comes in the heat of summer. Several red-shanked carder bumblebee queens were foraging on white deadnettle on the ditch banks; also one shrill carder queen, emitting her distinctively piercing buzz.
All, all is theft, all is unceasing and rigorous competition in nature; the desire to make off with the substance of others is the foremost – the most legitimate – passion nature has bred into us and, without doubt, the most agreeable one.
Marquis de Sade
My first encounter with a truly ancient flower-rich grassland was in Oxfordshire, when I was an undergraduate at Oxford. In some ways it formed the inspiration for my attempts to create a similar meadow in France, for it was in Bernwood Meadows that I first came to appreciate the extraordinary diversity of life that grasslands can support, if left undisturbed for long enough.
When I went to university I was shy, immature and more than a bit homesick. I was intimidated by the brash confidence of some of the students from public schools, and I struggled to fit in and make friends. My first few terms were a pretty lonely time, and I occupied myself by exploring the local countryside on my old and dangerously decrepit Yamaha 250cc motorbike. There was one place in particular to which I repeatedly returned: Bernwood Forest and the adjacent Bernwood Meadows, about eleven kilometres to the east of Oxford, the site where a few years later I would end up doing much of the work for my PhD on meadow brown butterflies. At the time I didn’t really notice the meadow; I walked through it looking for butterflies and other insects, particularly the elusive and rare black hairstreak butterflies that live in the dense blackthorn hedges, but I didn’t pay too much attention to the plant life.
It was later, when I had moved ‘up the hill’ to do my PhD at Oxford Brookes University (or Polytechnic as it was at the time, situated on Headington Hill to the east of the city), that I came to appreciate the meadow for what it is: a tiny surviving fragment of one of the most flower-rich habitats in Britain. The ‘Poly’ ran an annual field course for biology students in July, rather unexcitingly based in Oxford, but with day trips out to a range of different habitats, one of which was to Bernwood. Just a few months after I had started my PhD I was asked along as a demonstrator on this trip, something that made me very nervous as I wasn’t sure that I had any great knowledge to impart. Luckily for me, the member of staff in charge was Dr Andrew Lack, an enormously knowledgeable, twinkly-eyed botanist with a boundless enthusiasm for natural history of all sorts. He walked us through the meadow, pointing out the long, regular ridges and furrows, evidence that the field had not been ploughed since it was cultivated in strips in medieval times. Given 500 years or so in peace, the mea
dow had had time to develop a wonderful richness of plant life, which in turn supported a huge diversity of insects.
We introduced the students to the many different bumblebee and butterfly species that lived there, and Andrew showed off to the students by catching the male, stingless bumblebees in his hands – a trick that I have since borrowed from him and have used many times to great effect. We spotted lovely bright-yellow crab spiders, perched on the edge of ox-eye daisies waiting for a meal to arrive, and we caught five or six grasshopper species in the long grass. Eventually Andrew set the students to work in small groups, identifying and counting the multitude of different plants within metal quadrats that they scattered on the ground.
I knew some of the plants, particularly those that I had taught myself to identify because they were food plants for the butterflies I had studied and bred during my childhood, but there were many others at Bernwood with which I was not familiar: eyebright, tormentil, lady’s bedstraw and pignut, among many others. One of the more common plants in the meadow was one that I had not seen before: yellow rattle, properly known as Rhinanthus minor, which Andrew explained belonged to the near-unpronounceable plant family of the Scrophulariaceae. These are not particularly remarkable plants to look at. They grow to sixty centimetres or so tall, usually less, with a central spike carrying small, tubular yellow flowers. The delicate petals of each flower form a yellow tube with a purple tip and protrude from within a tough, papery green calyx (the fused sepals of the flower), which shelters and protects them. Most of the flowers had already gone over – the main flowering period is May and June – but a few were still fresh and we spotted a garden bumblebee visiting them, a very long-tongued bumblebee and one of the few British bees able to reach the nectar in rattle flowers. Once pollinated, the rattle petals wither and a seed pod swells inside the calyx, the flat, disc-like seeds hardening and becoming loose inside the pod. As we walked through the meadow the disturbance of our feet caused the stems to sway and the seeds to rattle pleasingly – the origin of the plant’s name.
I had no more cause to think about yellow rattle until some years later, when I was investigating the declines of many UK bumblebee species. It had become clear that most of the bee species that had declined severely were the longer-tongued species and those that tended to emerge late from hibernation; species such as the great yellow, shrill carder, ruderal and short-haired bumblebees. When I began studying bumblebees in earnest I had seen none of these species and was desperately excited at the prospect. It also became clear that little was known about these bees. Searching the scientific literature revealed hundreds of studies of buff-tailed bumblebees and other common species, but almost nothing had been written about the rarer bees. I was determined to put this right – after all, we could not hope to conserve these creatures if we did not know anything about them.
By the time my interests in bumblebee conservation really came to the fore I had moved from Oxford to Southampton University, and I travelled far and wide in the UK to track down these exotic-sounding creatures. I saw my first brown-banded carder at Dungeness in Kent; red-shanked carders and ruderal bumblebees on Salisbury Plain; shrill carders on the Somerset Levels; and great yellows on South Uist. I went abroad, to Russia, Poland, Switzerland and even New Zealand, to find healthy populations of the rare species. Together with my students, I amassed data on the food plants used by all the different species that we saw, both common and rare, eventually accruing tens of thousands of records. We wanted to know how the various bee species differed with regard to the flowers they preferred, and whether the rare bumblebees were dependent on particular, rare flowers. In all this work, some clear patterns emerged. The sites that supported lots of bumblebee species, and those where the rare bumblebees were to be found, all tended to be flower-rich grasslands. There was usually a lot of red clover, which long-tongued bees favour as a source of pollen, and often there was yellow rattle.
While the queens of our common bumblebee species emerge early in the spring, sometimes as early as February, the queens of our rare species tend to emerge much later. Shrill carders and great yellows are not seen much before June, having slept since the previous September. This may be in part because the flower-rich grasslands they inhabit actually have few flowers in spring, despite their name. Grassland flowers tend to blossom from late May onwards, so it makes little sense for the bees that live there to wake up any earlier; if they did, they would starve. Yellow rattle is one of the first grassland flowers to bloom, and its deep flowers provide vital nectar and pollen for the hungry queens coming out of hibernation. It thus makes sense that the places where these bumblebees thrive tend to have good strands of yellow rattle.
It turns out that there is much more to the relationship between bumblebees and rattle than this. Since I was taught plant identification by Andrew Lack all those years ago, plant taxonomists have moved yellow rattle from the Scrophulariaceae to the no-more-pronounceable Orobanchaceae.1 The common name of this family is the broomrapes, and therein lies the clue to their unusual biology. Some members of the broomrape family are parasites on other plants, particularly legumes such as beans and broom. They don’t bother producing green chlorophyll to photosynthesise, but instead send out long roots that latch on to the roots of other plants and drain their nutrients. The bulk of the plant is below ground, for it has no need of sunlight, but in spring each plant sends up a yellow, sickly-looking flower spike which, despite its anaemic appearance, is quite attractive to bees. Broomrapes are rare in Britain, but they are common in more southerly parts of Europe and can even be a pest of broad-bean crops in the south of Spain. Yellow rattle is a cousin of the broomrapes, and although it has green leaves and photosynthesises like a normal plant, it does share some of their parasitic habits. The roots of rattle latch on to those of nearby grasses, drawing out sugary sap. Patches of rattle within a meadow are often conspicuous, for the surrounding grasses are stunted and yellowed, weakened by the parasitism – reminiscent of vampire victims that are being slowly drained of life.
Scientists at the UK’s Centre for Ecology and Hydrology in Wallingford have explored whether rattle might not be a powerful tool in the conservationist’s armoury. The recent realisation that we have destroyed most of Europe’s flower-rich grasslands has led to efforts to restore damaged meadows, or even to create them from scratch, as I have been doing at Chez Nauche. However, the great enemy of such projects is high soil fertility. Beautiful, diverse grassland that took centuries to develop can be reduced to a green sward of grass with barely a flower in sight within just a year or two, by the single act of adding chemical fertiliser. Ancient grasslands tend to have low soil fertility, so grasses grow slowly and there is lots of room for other plants. In these conditions plants that are able to extract nitrogen from the air are at a huge advantage, and so legumes tend to thrive, for they have root nodules containing nitrogen-fixing bacteria. Hence flower-rich grasslands are rich with trefoil, vetches, meddicks, restharrows and clovers – all legumes. This is also good news for bees, since they love the pollen and nectar of legumes. Chuck on a sack of fertiliser and the grasses sprout up like crazy and smother everything else.
Once fertiliser has been added to a meadow, it is exceedingly difficult to remove it. The fertility will slowly decline if no more fertilisers are added and if a hay crop is removed every year, but this can take decades. By attacking grasses and reducing their ability to dominate the sward, yellow rattle offers the possibility of speeding up the process. To test this idea, Richard Pywell and his colleagues at the Centre for Ecology and Hydrology set up an experiment at Little Wittenham Nature Reserve in Oxfordshire, a cluster of pretty, steep-sided chalky hummocks just south of the River Thames. By coincidence, I used to go sledging there in winter when I was a PhD student at Oxford. Pywell sowed patches of species-poor, ‘improved’ grassland with yellow rattle seeds, after first scarifying the surface to provide some bare ground. The rattle established well, and quickly began to suppress the grasses. After
two years he sowed a mix of wild-flower seed into the experimental plots, and then monitored whether they established successfully. The results were quite clear: the wild-flower seed mix took much better in plots with lots of rattle, presumably because there was less competition from the grasses.
It seemed to me that my meadow provided an opportunity to follow this up, and to simultaneously boost its floral diversity. Rattle has many hemiparasitic relatives; there are other species of rattle, such as greater yellow rattle and narrow-leaved rattle, and also eyebright, red bartsia, meadow cow-wheat, and so on. These other species might also prove effective at suppressing grasses and boosting floral diversity; perhaps some might even be better than yellow rattle? So in September 2010 I returned to Chez Nauche with a group of volunteers, my PhD students Andreia Penado and Leanne Casey, and two staff from the Bumblebee Conservation Trust, Pippa Rayner and Tasha Rolph. At the time I had a very old Yardman sit-on mower, and I set about mowing 120 ten-by-ten-metre plots; the mowing was needed to get rid of the vegetation that had sprung back since the July hay cut, and to provide some bare ground for the seeds to germinate in. Unfortunately my Yardman expired after just a handful of plots, and I have never managed to coax it back into life since. Not having the funds to replace it with a new one, I rushed out and bought a normal lawnmower, and spent the next six days mowing neat squares into the field. The team of four girls followed behind me, sprinkling in seeds and performing a strange, collective shuffling dance to bed the seeds in (Pywell’s team used a tractor-mounted roller, a far more sensible approach).
It was not long before my farming neighbour, Monsieur Fontaneau, happened to drive along the track adjoining the meadow, with the larger of his two sons in the passenger seat. He stopped, and they stared for a while. They turned their engine off and wound down the window. I waved, and he waved back. They stayed for perhaps ten minutes, before they drove away.