A Buzz in the Meadow (19 page)

Producing no nectar is an interesting but risky strategy for an individual plant. It has the clear advantage that the plant saves the energy it would otherwise have put into nectar, but it relies on the presence of enough other plants of the same species that are producing nectar to convince pollinators that the plant species is worth bothering with; or else it requires there to be a plentiful supply of naïve pollinators. The success of the cheating strategy is also very much dependent on the intelligence of the pollinators. Bumblebee workers, for example, are pretty sharp operators and are not fooled easily or for long. They spend their life learning and relearning associations between the colours, shapes and scents of flowers, and rewards. When they first begin foraging they experiment with visiting different flowers, and quickly learn to search out selectively those that are most rewarding. They commonly specialise on a particular flower species, be it red clover, tufted vetch or viper's bugloss, and become very adept at extracting the rewards swiftly and efficiently – much as we become better at any task with practice. If you follow a bumblebee in a flowery meadow, you will quickly notice that she usually visits the same flower type over and over again, bypassing most other flowers, a behaviour that was familiar to Charles Darwin in 1876 and has become known as ‘floral constancy':

That insects should visit the flowers of the same species for as long as they can is of great significance to the plant, as it favours cross fertilisation of distinct individuals of the same species; but no one will suppose that insects act in this matter for the good of the plant. The cause probably lies in insects being thus enabled to work quicker; they have just learned how to stand in the best position on the flower, and how far and in what direction to insert their proboscides.

As Darwin observed, this behaviour is exactly what the plants want, for it delivers pollen swiftly and efficiently from one flower to the next, without the pollens from different species getting mixed up and deposited on the wrong stigma. The complexity of flowers can be seen as a mechanism to encourage this, a way to manipulate bees into committing themselves to the relationship; if working out how to find and extract the reward from a particular species is difficult, then once the skill is mastered it makes sense to utilise it to the full, not least because competition with other pollinators is likely to be less on such taxing flowers.

All this said, floral constancy only makes sense if the flower that the bee has chosen to specialise on is providing a good reward. Bumblebees constantly reassess their strategy, and occasionally try other flowers that they pass, presumably to check they aren't missing out on something better. I have my favourite cheeses, but every now and then I try something different that I've never tasted before, in case it turns out to be fantastic. Often it is a disappointment – I recently experimented with some Saint Paulin and found it to be tasteless and rubbery, so I'll not try it again. Similarly, a bumblebee visiting viper's bugloss might quickly check out a geranium as it passes. If the geranium turns out to have sweeter or more copious nectar than the bugloss, the bee will try a few more; and if the reward is consistent, she will quickly abandon bugloss altogether. If not, she will tend to stick with the bugloss, but will occasionally experiment with new flowers that she happens to encounter. If the bee encounters a run of empty flowers of her favoured plant, she starts spending more time checking out the alternatives. She does the same if a flower becomes hard to find, such as might happen when the favoured flower is beginning to go over.

Bee foraging behaviour can be seen as a trade-off between the benefit of sticking with one flower type that the bee has become skilled in handling and trying to minimise the risk of missing out on something much better. In this way, bees keep tabs on the options available to them, and will swiftly abandon one flower in favour of another if it provides a better reward.

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 meadow 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.