DECEIT POLLINATION OF ORCHIS-SPITZEII (ORCHIDACEAE) ON THE ISLAND OF GOTLAND IN THE BALTIC - A SUBOPTIMAL SYSTEM

Interactions between anthophilous insects and Orchis spitzelii were studied on the island of Gotland, in the Baltic Sea of southern Sweden. The plant is nectar ‐ less and thus deceives its nectar‐seeking pollinators. No specific nectar‐plant model exists for mimetic resemblance, but the orchid produces an exuberant floral display. Pollinators comprise mainly bumble‐bee queens (Bombus spp.) that transport 90% of the pollinaria. The remaining pollinators are cuckoo bumble‐bee females (Psithyrus spp.). Pollinating bees make exploratory visits primarily to only a few of the lowermost flowers in inflorescences. Many‐flowered individuals are more attractive and have a higher fitness in terms of fruit set. Bees quickly learn to avoid the non‐rewarding orchid. Arctostaphylos uva‐ursi (Ericacae) and Rubus saxatilis (Rosaceae) are the most important subsidiary nectar plants for the bees during the flowering of O. spitzelii. Phenological, morphological and functional data indicate that the anth‐ecological adaptation to the main pollen vectors is suboptimal. Moreover, experiments indicate that O. spitzelii is an obligate outcrosser and lacks an effective isolating mechanism against the abundant and concurrently blooming Orchis mascula. It is therefore proposed that O. spitzelii has spread to Gotland in recent time. Copyright © 1990, Wiley Blackwell. All rights reserved