Sex in the city: Reproductive success of Digitalis purpurea in a gradient from urban to rural sites

Urbanization of natural habitats is considered to have large impacts on local plant populations and their associated pollinator faunas, which are expected to cause changes in both plant-pollinator interactions and increased pollen limitation. This study investigated the effects of increasing intensity of urban land use on pollinator diversity, flower visitation rates and reproductive success in Digitalis purpurea. To this end, we set up experimental populations of D. purpurea in three urban-rural gradients and contrasted these against two large, natural populations. In the experimental populations we quantified the number of flower visits per 20 min and applied supplemental hand-pollinations to determine the impact of pollen limitation on fruit-level seed set. We found no impact of urbanization intensity on pollinator diversity at all locations studied, and also the number of flower visits did not differ significantly between urban and rural populations. Reproductive success appeared, however, pollen limited in all sites, even in the large and optimal reference populations. This led us to conclude that the pollen limitation was caused by a bet-hedging strategy of the species rather than by insufficient pollination. (C) 2012 Elsevier B.V. All rights reserved.