The coexistence of closely related cogeneric species poses a conundrum: How do species persist as discrete taxa without fusing through hybridization or excluding each other through competition. Differentiation in flowering time and pollinator niche reduce species competition and constitute some of the pre-zygotic barriers recently found to be more important in reducing gene flow between related sympatric species.
The study system consists in five sympatric closely related perennial herbs of the genera Liatris (L. pauciflora, L. gracilis, L. tenuifolia, and L. laevigata) and Carphephorus (Carphephorus corymbosus), that coexist in a Sandhill community, Florida. They present small pink-purplish tubular corollas, offer pollen and nectar as a reward, and flower during fall each year. Hence, it is expected that these flowers will attract very similar pollinator guilds or even same species of pollinator. Accordingly it is not obvious how these species interact for pollination and how they avoid hybridization.
The goal of this study is to understand the importance of flowering time and pollinators in reducing negative interactions and in limiting pollen mediated gene flow between species. During Fall 2010 we studied their flowering phenologies (plots 5x5), we recorded insect visitation (plots 2x2), and described some aspects of floral morphology.
We found that these sympatric species of Liatris flowered sequentially, presenting some overlap between species. L. pauciflora starts flowering in late August, followed by L. gracilis, then L. tenuifolia, and lastly L. laevigata, with the flowering times of L. gracilis and L. tenuifolia presenting more overlap. The flowering period of C. corymbosus starts after L. pauciflora is done, and coincide with L. gracilis, L. tenuifolia and ends about halfway of L. laevigata flowering period. There is essentially no overlap between L. pauciflora and C. corymbosus. Likewise, these species are pollinated mostly by butterflies, while the rest \ are pollinated mostly by bees. In addition, the typical flowering stem of the study Liatris species is a raceme, that grows erect with the heads arranged in spiral fashion. Conversely, the flowering stem of L. pauciflora usually bends and the heads also bend facing up, in a manner capable of facilitating butterfly foraging, which coincides with the corymb type inflorescence that distinguishes C. corymbosus.
On the other hand, around 50% of the insect visits to L. gracilis were performed by a single Apidae species, one rarely found visiting the other Liatris species, and therefore suggesting that bee pollinated species present further floral and pollinator specialization.