Plant-pollinator interactions within the urban environment

Urban ecosystems pose different environmental constraints on plant and animal communities than natural ecosystems, and this in turn might affect ecological interactions. We investigated plant pollinator interactions in urban vegetation communities in the context of local community structure and two landscape metrics describing the surrounding urban matrix. We recorded plant species diversity, floral density, pollinator species and floral visits at 89 flowering patches within the urban matrix, and used Trifolium pratense as a focal plant species. We correlated visits by all species, by bees, by bees when excluding the main visitor Bombus pascuorum, and by the main visitor alone T. pratense with the landscape metrics green area, representing the total extent of vegetated areas, and edge density of green areas, representing the degree of habitat fragmentation, at scales from 20 to 200 m. Extent of green area was positively correlated with visits by all species, by bees, and by Bombus pascuorum, but negatively with bees excluding the main visitor. The correlation increased with scale for all four variables, and the strongest correlations were found for B. pascuorum. The direction of correlation between edge density and the visitation variables was similar as for green area, but showed considerable variation at scales below 100 m radius. Visits by all bees and all visitors to T. pratense were negatively affected by flowering plant species diversity. There was a positive interaction between extent of green area and plant diversity on bee visitation, and a negative interaction between the extent of green area and heterospecific flowers for bees (excluding B. pascuorum). Increasing floral abundance and plant diversity reduced pollinator visitation to T. pro tense, suggesting competitive effects through both quantity and diversity of resources. Competition for flower visitors was mainly from Lotus corniculatus and Trifolium repens, both of which frequently co-occurred with T. pratense. We conclude that T. pratense competes for pollinators with other co-occurring plants, but that the nature of the surrounding urban matrix mediates interactions among plant species for pollinator services at different scales, particularly with respect to the proportion of green areas. (C) 2011 Elsevier GmbH. All rights reserved.