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-flowering rewarding species, either negatively through competition for pollinators, or positively by means of a 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 (yellow and blue flowers) rewarding, co-flowering species, Viola aethnensis, were explored in a natural stand in southern Italy. To evaluate the interactions between these two species, plots of all possible arrays of presence/absence of the four colour morphs were arranged in the field and fruit production of the orchid morphs assessed. Natural aggregations of both colour morphs of the orchid had the highest reproductive fitness for each colour morph. Patterns in fitness variation detected in treated plots provided direct and indirect evidence that D. sambucina may benefit from the co-occurrence of V. aethnensis through floral mimicry and/or magnet species effect. Since the fitness of each orchid morph was strongly increased by the presence of a viola morph of similar colour, a double mimetic effect occurs between the two species, which to our knowledge has not been previously reported. Moreover, the co-occurrence of an orchid morph with a non-matching viola resulted in competition for pollinators, whereas in the absence of the rewarding plant the fitness was balanced due to positive interactions between the two orchid colour morphs. These findings suggest that D. sambucina, like many other European deceptive orchids, possesses a set of display traits suitable for exploiting potential facilitative interactions with a number of rewarding species. In addition, the unequal morph frequencies occurring in natural populations could result from unbalanced events in floral mimicry.