Abstract
BackgroundBumble bees and other wild bees are important pollinators of wild flowers and several cultivated crop plants, and have declined in diversity and abundance during the last decades. The main cause of the decline is believed to be habitat destruction and fragmentation associated with urbanization and agricultural intensification. Urbanization is a process that involves dramatic and persistent changes of the landscape, increasing the amount of built-up areas while decreasing the amount of green areas. However, urban green areas can also provide suitable alternative habitats for wild bees.Methodology/Principal FindingsWe studied bumble bees in allotment gardens, i.e. intensively managed flower rich green areas, along a gradient of urbanization from the inner city of Stockholm towards more rural (periurban) areas. Keeping habitat quality similar along the urbanization gradient allowed us to separate the effect of landscape change (e.g. proportion impervious surface) from variation in habitat quality. Bumble bee diversity (after rarefaction to 25 individuals) decreased with increasing urbanization, from around eight species on sites in more rural areas to between five and six species in urban allotment gardens. Bumble bee abundance and species composition were most affected by qualities related to the management of the allotment areas, such as local flower abundance. The variability in bumble bee visits between allotment gardens was higher in an urban than in a periurban context, particularly among small and long-tongued bumble bee species.Conclusions/SignificanceOur results suggest that allotment gardens and other urban green areas can serve as important alternatives to natural habitats for many bumble bee species, but that the surrounding urban landscape influences how many species that will be present. The higher variability in abundance of certain species in the most urban areas may indicate a weaker reliability of the ecosystem service pollination in areas strongly influenced by human activity.
Highlights
Bumble bees and other wild bees play an integral ecological role as pollinators of a large number of wild flowers and cultivated crops [1,2,3,4]
Conclusions/Significance: Our results suggest that allotment gardens and other urban green areas can serve as important alternatives to natural habitats for many bumble bee species, but that the surrounding urban landscape influences how many species that will be present
We found that bumble bee diversity was negatively affected by urbanization as measured by increasing proportion of impervious surface within the surrounding landscape
Summary
Bumble bees and other wild bees play an integral ecological role as pollinators of a large number of wild flowers and cultivated crops [1,2,3,4]. The main causes of this decline are considered to be habitat destruction and fragmentation as consequences of human activity [5,6,7]. Still our understanding of the response of wild bees to habitat fragmentation is rather limited [8,9,10] Human activities, such as increasing urbanization and agricultural intensification, imply extensive modifications of the landscape and the environment and lead to destruction and fragmentation of natural habitats. Bumble bees and other wild bees are important pollinators of wild flowers and several cultivated crop plants, and have declined in diversity and abundance during the last decades. The main cause of the decline is believed to be habitat destruction and fragmentation associated with urbanization and agricultural intensification. Urban green areas can provide suitable alternative habitats for wild bees
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