Do ambush predators prefer rewarding or non-rewarding orchid inflorescences?

Previous research has shown that the presence of a reward on average doubles reproductive success in orchids by correspondingly increasing the frequency of pollinator visitations. We examined whether such reward-induced increases have a downstream effect on the behaviour of ambush predators concealed in orchid inflorescences, extending observations begun at Downe by Charles Darwin. Specifically, we studied three orchid-rich sites in southern England, in order to compare the occurrence of crab spiders on three coexisting species of terrestrial orchids: the nectariferous Gymnadenia conopsea versus the nectar-less Dactylorhiza fuchsii and Anacamptis pyramidalis. No significant difference was observed between rewarding and non-rewarding inflorescences at Risborough, whereas at Aston Clinton the nectar-less species supported significantly more crab spiders, albeit mainly in relatively short grass. Comparison of the two non-rewarding species present at Downe approximated a significant preference by the spiders for Anacamptis, which more closely resembles the rewarding Gymnadenia. The presence of a floral reward does not result in a higher frequency of crab spiders. We speculate that concealment quality of the inflorescence, the nature of the dominant pollinator(s), and/or ease of movement of spiders between inflorescences may be more important than frequency of visits by potential prey, particularly when no other form of concealment is readily available. (c) 2007 The Linnean Society of London.