Effects of habitat fragmentation on the reproductive success of the nectar-producing orchid Gymnadenia conopsea and the nectarless Orchis mascula

Habitat fragmentation decreases plant population sizes and increases spatial isolation, which hampers the exchange of seeds and pollen between fragmented populations. This may result in decreased population viability. We compared the effects of population size and isolation on the reproductive success of two orchid species, Gymnadenia conopsea (nectar-producing) and Orchis mascula (nectarless) growing in highly fragmented calcareous grassland in southern Belgium. We expected that the nectar-producing species would be more susceptible to the negative reproductive consequences of habitat fragmentation compared to the nectarless species. Nectar production has been associated with increased geitonogamous pollination and, therefore, with lower seed viability. Our results show that seed viability increased with increasing population size in O. mascula, whereas it was always low in G. conopsea, even in large populations. In contrast, percentage fruit set was positively related to population size in the nectar-producing G. conopsea, but no such effect was observed in the nectarless O. mascula, where fruit set was low even in large populations. Population isolation was not related to reproductive success for either species. Our results suggest that even in large populations, where pollinators are expected to be more abundant, increased geitonogamous pollination reduces seed viability in the nectar-producing G. conopsea. In contrast, seed viability in O. mascula seems to benefit from increased pollinator availability in larger populations. For the latter species, however, fruit set remains low, even in large populations, compared to G. conopsea. This may be explained by the relatively low attractiveness of nectarless orchid species for pollinators. Our results indicate that small population size may negatively influence reproductive success in both nectarless and nectar-producing orchids by reducing seed viability and fruit set, respectively.