Pollinators as isolation mechanisms: field observations and field experiments regarding specificity of pollinator attraction in the genus Ophrys (Orchidaceae und Insecta, Hymenoptera, Apoidea)

Abstract

It is demonstrated that the biological species concept of the genus Ophrys corresponds to the general concepts of Ernst Mayr. On the example of two closely related forms of the fusca group in Andalucia, it is shown how to handle the different criteria of the biospecies concept. One is the high specifity of attraction of a male bee followed by a complex pseudocopulation behaviour which enables the correct pollen transfer within the species. The high specifity is caused by a highly remarkable imitation of the sexual pheromones of the mimicked female of the pollinator. Sexual pheromones of animals act as intraspecific signals which result in attractions of only males of the same species. Exactly this specific behaviour is triggered by the Ophrys flower, too. Thus the pollinator male is attracted only to those flowers which have the correct mixture of the pheromone molecules. To further ensure success the pollinating males also select other characters of the Ophrys flower as labellum size, colour, labellum hair characters, phenology and possibly habitat selections etc. In the sexual reproduction behaviour of the male insect acts as pregamic isolation mechanisms for the given Ophrys like for their own female.

We were successful in confirming the high specifity of the pollination mechanisms in numerous field experiments, mainly choice experiments, this in contrast o the statements of Hennecke & Munzinger (2013). They had argued that there were many different pollinator species listed in the literature for a given Ophrys species. However, they did not critically assess the reliability of these citations. For this, they would have had to ctitically read the primary literature and not only the secondary quotations like in Delforge (2005, 2016) or others. Therefore, they rehashed many of the old mistakes concerning wrong identifications of the bees or even the Ophrys taxons which had long been corrected. Especially a mixture of old and actual names of the same species in different combinations is annoying and seems to demonstrate that the authors are not really fit in the nomenclature of Ophrys and their pollinators. This is demonstrated by some examples, which are very confusing without knowledge of the bees. For example, these authors did not realize that a 3–4 mm bee ( Andrena hesperia) could not serve as a pollinator of a 20 mm labellum of a large Ophrys omegaifera species.

However, there are some cases with more than one pollinator. Besides the main pollinator the others I call them “secondary pollinators” which in those cases we could check their pollination contribution in the population is very weak. In many cases, these secondary visitors are mainly pollinaria thieves because they are only attracted and try to copulate. However, they seem learn very quickly that Ophrys is not a true female and will never visit another flower of this kind. In some quantitative field experiments (with Eucera nigrescens/longicornis on Ophrys holosericea) we could confirm this hypothesis.

The conclusion of the two authors that Ophrys species are not species-specific regarding sexual attractivity does not correspond to our field experiments regarding olfactory compound investigations in biotests, and does not agree with the molecular data of population biology and with the genetic analyses.

The other proof the authors used in their argumentations are the supposed frequency of hybrids. But this is also a spurious argument. Hybrids are only frequent in literature and not in nature. In places frequently visited by orchid enthusiasts, the number of observed “hybrids” is conspicuously high, which may indicate “hand pollination”.

The aim of the two authors has been to reduce the many species within Ophrys because they cannot be discriminated. Consequently, they try to establish a simplified classification system without any biological background. A complete anthropomorphic typological systematic like the “Index of accepted plant names” is issued by Kew Gardens. However, this is in the 21 ^st century, 150 years after Darwin, a kind of middle age taxonomy. They use a quite typological morphospecies concept like in the book of Pederson & Faurhold (2007). This does not fit with nature. If you pay attention to the different criteria of the biospecies concept within the genus Ophrys you will recognise the many species which the genus makes so interesting.

A critical list of the known specific pollinators of the genus Ophrys is given as an appendix.