Temporal variation and efficiency of the pollinator community of the rare Dictamnus albus

The abundance and composition of pollinator communities may vary in space and time due to several natural factors, with consequences on the efficiency of pollination and on plant fitness; on the other hand, flowering phenology and abundance may act on the preferences and behaviour of an insect community. We present the results of a 4-year study performed in a natural population of Dictamnus albus within the LIFE+ Project “Plant-Pollinator Integrated CONservation approach: a demonstrative proposal” (LIFE09/NAT/IT000212, http://www.pp-icon.eu). We observed insect behaviour during flower visits, recording the collected rewards, the contacts with receptive stigmas, the movements along the raceme and among plants, and also noting flower abundance and phenological stage. The pollinating ability of each visitor taxon was estimated as an index of pollination performance based on the frequency of visits, fidelity to the dittany (i.e., the percentage of Dictamnus pollen on the total pollen load) and pollination efficiency (i.e., the percentage of contacts with receptive stigmas). Our results indicate a slight variation in the composition of visitor guilds among years, and a considerable ecological specialization of the plant-pollinator system: within the spectrum of visitors, only medium-big sized bees (Habropoda tarsata, Xylocopa spp., Bombus spp.) play a significant role as D. albus pollinators. Megachilids (Megachile, Osmia) show an intermediate efficiency, partly due to their smaller body size; however their high frequency of visit contributes to their role as pollinators. On the contrary, small halictid bees only act as pollen or nectar thieves. These results were helpful for the implementation of concrete conservation measures addressed to pollinator safeguard and habitat restoration, such as the setting of specific nesting sites for solitary bees or bumblebee colonies, as well as the selection of nectariferous flowering species for habitat management. Fruit and seed set of control and pollenaugmented flowers highlighted a variation in pollen/pollinator limitation among years, which may be due to Allee effect (i.e., quantitative limitation) or to high inbreeding between close relatives (i.e., qualitative limitation).