Double floral mimicry and the magnet species effect in dimorphic co-flowering species, the deceptive orchid Dactylorhiza sambucina and rewarding Viola aethnensis

Reproductive success of food-deceptive orchids may be affected by interactions with co-floweringrewarding species, either negatively through competition for pollinators, or positively by means ofa magnet species effect and floral mimicry. In this study, potential interactions between a dimorphic(yellow or purple flowers) non-rewarding orchid Dactylorhiza sambucina and a dimorphic (yellowand blue flowers) rewarding, co-flowering species, Viola aethnensis, were explored in a naturalstand in southern Italy. To evaluate the interactions between these two species, plots of all possiblearrays of presence/absence of the four colour morphs were arranged in the field and fruit productionof the orchid morphs assessed. Natural aggregations of both colour morphs of the orchid had thehighest reproductive fitness for each colour morph. Patterns in fitness variation detected in treatedplots provided direct and indirect evidence that D. sambucina may benefit from the co-occurrenceof V. aethnensis through floral mimicry and/or magnet species effect. Since the fitness of each orchidmorph was strongly increased by the presence of a viola morph of similar colour, a double mimeticeffect occurs between the two species, which to our knowledge has not been previouslyreported. Moreover, the co-occurrence of an orchid morph with a non-matching viola resulted incompetition for pollinators, whereas in the absence of the rewarding plant the fitness was balanceddue to positive interactions between the two orchid colour morphs. These findings suggest thatD. sambucina, like many other European deceptive orchids, possesses a set of display traits suitablefor exploiting potential facilitative interactions with a number of rewarding species. In addition, theunequal morph frequencies occurring in natural populations could result from unbalanced events infloral mimicry.