Floral biology and pollination ecology of the micro-endemic Stachytarpheta cassiae (S. Atkins) (Verbenaceae)

Knowledge on pollination biology is crucial to understand the basic aspects of sexual plant reproduction and the long-term viability of plant populations. Nowadays, the relevance of these studies on rare and micro-endemic species is ever more important, given the increasing anthropogenic pressures. Here, we characterize the floral biology and identify the pollinators of Stachytarpheta cassiae (S. Atkins), a micro-endemic species from the rupestrian grasslands of northern Minas Gerais, Brazil. Flowering of S. cassiae occurs throughout the year and flower anthesis is diurnal, with most of the flowers opening in the morning. Flowers offer nectar and pollen as rewards to a wide range of flower visitors. We observed 15 bee species, five hummingbird species, three butterfly species and one wasp species. All Euglossine bee species and hummingbird species effectively contacted the fertile whorls of the flowers and were classified as pollinators. Thus, S. cassiae appears to have a generalist pollination system. We also identified primary (Bumblebees and carpenter bees) and secondary (non-Euglossine bees and a wasp) nectar robbers and nectar thieves (butterflies). Overall, Euglossine bee and hummingbird pollinators showed significantly higher relative visitation frequency as compared to the rest of floral visitors (nectar robbers and thieves). Flowers of Stachytarpheta cassiae represent a key resource for many animal species throughout the year in these harsh rupestrian environments. Future studies should assess the nectar secretion patterns and the relative contribution of floral visitors to the reproductive success of the species.