Why bother with nectar?

Abstract

Orchids comprise one of the most spectacular groups of flowering plants but they are rather a mean bunch: a large number of species go to some length to attract pollinators but produce no nectar or other reward for their visit. This is a highly unusual strategy amongst flowering plants. So how do such ‘rewardless’ species manage to propagate themselves while surrounding plants must reward their pollinators for breeding success?Ann Smithson and her colleague, Luc Gigord at the University of Exeter have been considering this question and more: not only were they interested in how rewardless orchids were pollinated, they considered whether there might be some distinct advantage in such a strategy (Proc R Soc Lond Ser B 2001, 268:1435–1441). So they carried out experiments on a rewardless orchid species found on sandy soils around the coasts of southern Europe, Barlia robertianaBarlia robertiana. They worked on three populations occurring in northern Spain. This orchid appears to be pollinated exclusively by members of a single bumble bee species (Bombus lucorum) without mimicry or other cues used by some other rewardless species.Fig. 1No rewards: recent studies have shown that although the southern European orchid, Barlia robertiana, provides no nectar, it can disperses more pollen than plants experimentally given a sugar reward for pollinators. (Photograph: Ann Smithson.)View Large Image | View Hi-Res Image | Download PowerPoint SlideSmithson and her colleague decided to add sucrose solution to some of the orchid flowers to compare the action of bees facing a rewardless flower and those where a sugar reward was added. The orchids produce pollen in clumps called pollinia which are deposited on the body of pollinating insects and are then carried by the insects to other flowers which can then be pollinated.Their results were quite striking. The bees were quite taken with the sugar rewards and spent much more time visiting individual flowers on these plants than on the rewardless plants. But the increased attention was accompanied by a markedly different outcome: up to eight times fewer pollinia were removed from plants harboring a sugar reward than from those naturally rewardless. The team found no difference in pollen deposition on subsequent flowers from bees visiting the two flower types.The decreased removal of pollinia from flowers containing a reward suggested that the rewardless individuals had the fitness advantage of greater pollen dispersion and seed paternity than those flowers with an added reward.The team then took a closer look at what might be going on. They found that bees facing a rewardless flower were more vigorous in probing the flower to seek a potential reward and were therefore more likely to detach the pollinia from the flower. Bees presented with a reward quickly moved onto further flowers to seek further rewards with much less disturbance and hence fewer pollinia detached.While rewardlessness may therefore have its benefits, it's a precarious strategy as bees soon learn to avoid such flowers. In northern Spain Smithson found the orchid growing among rosemary plants, Rosemarinus officinalis. “The rosemary starts flowering early in the year and the orchid flowers appear shortly afterwards,” says Smithson. “A new batch of naive bees emerge at this time and feast on the nectar provided by the rosemary,” she says. But the day's first production fizzles out during mid-morning and it's then that the bees, knowing no better, turn their attention to the orchid flowers. It's a very narrow window for the orchids, but by their success, it's one that appears in this case to work.