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. © 2007 The Linnean Society of London.