Pollen dispersal in an insect-pollinated wet meadow herb along an urban river

Urban sprawl has led over wide areas to alteration and fragmentation of so many ecosystem types that it becomes essential to consider biodiversity conservation in urban planning. As the urbanisation is usually irreversible, the urban context has to be taken into account when defining appropriate management measures to sustain viable populations. Connectivity through pollen flow appears to be a key-factor for the long-term persistence of populations of insect-pollinated plant species. We investigated pollen dispersal using fluorescent dye as pollen analogue in Lychnis flos-cuculi, an insect-pollinated wetland herb, along the Woluwe River, in Brussels urban zone (Belgium). Dye dispersal showed an exponential decay distribution, with particles mainly deposited on neighbouring individuals and populations. The mean distance of dye transfer was 133.6 m, with a maximal distance of 523.7 m. Dye deposition was positively related to the number of potential recipient plants when distance to dye source was held constant. This may be associated with a higher flowering plant density across the flying distance that increases attractiveness and visitation rate of pollinators, and/or with secondary dye transfer. When connected, small and large populations only differed in response to the number of potential recipients. The small isolated population received little immigrant pollen flow, but remained a dye donor. Our study demonstrates that small populations of L. flos-cuculi along an urban river may still exchange pollen thanks to pollinator movements, and might therefore be regarded in management planning as potential connecting components between populations.