The idea that members of the plant family Orchidaceae (the orchids) “typically have exclusive relationships with their pollinators“, such that each orchid has only one pollinator, is a persistent one. Recently I’ve encountered it on horticultural websites (follow that last link), in grant proposals, and on Wikipedia.
The problem is that it’s not true: it’s a myth that is perpetuated by people (often botanists or horticulturalists) who may know a lot about orchids but don’t know as much as they think they know about pollination ecology.
Orchids certainly have some fascinating and often quite intricate floral mechanisms to ensure pollination, but these have not necessarily evolved to attract and exploit just one species of pollinator. Even in the case of sexually deceptive orchids that fool their (male) pollinating insects into believing that they are mating with a female of the same species, it is sometimes the case that more than one insect species is involved. For example, in the well studied genus Ophrys “flowers are pollinated by a narrow taxonomic range of pollinators, from a single species to up to five closely related species“. As the authors of that last paper state, this is not the same as the mythological “extreme case of one orchid/one pollinator”.
Likewise different species of orchid bees may pollinate the same orchid flowers as they visit to collect scent compounds; for example in the Brazilian species Dichaea pendula, species from at least two different bee genera act as pollinators (Nunes et al. 2016).
The fact that “one orchid/one pollinator” is a myth is not new knowledge, it’s been widely discussed in the pollination ecology literature for decades. For example, in our 1996 paper “Generalization in Pollination Systems, and Why it Matters” we showed data from the late 19th/early 20th centuries that clearly indicated a range of specialization in European orchids (follow that link and look at Figure 3B). Even earlier than this, in his 1992 paper “Trends in the pollination ecology of the Orchidaceae: evolution and systematics” Raymond Tremblay showed that only about 62% of species for which he could find data had a single pollinator, and that this varied considerably between different subfamilies of Orchidaceae, with some subfamilies being more specialized than others.
More recently, in a chapter in the 2006 book I co-edited with Nick Waser entitled “Geographical Variation in Diversity and Speciﬁcity of Pollination Systems” Steve Johnson, Andrew Hingston and myself looked at data from southern African compared to North American and European orchids; here’s the figure from that assessment:
Orchids are more specialized in southern Africa compared to Europe and North America (as are a number of other plant groups including the asclepiads, which we’re comparing them with here). But even in southern Africa, only about 65% of the orchids studied have a single pollinator species. It’s worth pointing out, though, that many of the species included in this analysis, and in Raymond Tremblay’s paper, have been studied only at single sites and often in single years, meaning that we have no idea if there is any spatio-temporal variation in the pollinators a particular orchid species exploits.
Why does this myth persist? I think it’s for the same reason that myths are retold from generation to generation: they are great stories that fascinate the teller and the audience. Indeed, orchids are very special plants with some amazing floral and vegetative adaptations, fascinating relationships with fungi, and incredible diversity. But we don’t have to mythologise their relationships with their pollinators to try to make orchids more special than they already are.