Dispersal limitation at the expanding range margin of an evergreen tree in urban habitats?

Dispersal limitations contribute to shaping plant distribution patterns and thus are significant for
biodiversity conservation and urban ecology. In fleshy-fruited plants, for example, any preference of
frugivorous birds affects dispersal capacities of certain fruit species. We conducted a removal experiment
with fruits of Ilex aquifolium, a species that is currently expanding its range margin in northern
Europe in response to climate change. The species is also a popular ornamental tree and naturalization
has been observed in many parts of its range. Fruits of native I. aquifolium and of three cultivars were
offered to birds at the expanding range margin in urban habitats in eastern Denmark. The four fruit types
were removed at different rates and red fruits were preferred over a yellow cultivar. Small fruit diameter
was positively related to fruit removal, and removal was faster under tree canopies compared with open
habitats. The preference for red cultivars compared with native I. aquifolium may contribute to naturalization
and potential invasion of garden escapes. Preferential foraging under closed canopies indicates
trees and shrubs as recruitment foci for fleshy-fruited plants in urban landscapes. The results should be
included in urban forestry and planting of potentially invasive ornamental species.
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