Abstract
Bees and flowering plants are two closely interacting groups of organisms. Habitat loss and fragmentation associated with urbanisation are major threats to both partners. Yet how and why bee and floral richness and diversity co-vary within the urban landscape remain unclear. Here, we sampled bees and flowering plants in urban green spaces to investigate how bee and flowering plant species richness, their phylogenetic diversity and pollination-relevant functional trait diversity influence each other in response to urban fragmentation. As expected, bee abundance and richness were positively related to flowering plant richness, with bee body size (but not bee richness and diversity) increasing with nectar-holder depth of flowering plants. Causal modelling indicated that bottom-up effects dictated patterns of bee-flower relationships, with urban fragmentation diminishing flowering plants richness and thereby indirectly reducing bee species richness and abundance. The close relationship between bees and flowering plants highlights the risks of their parallel declines in response to land-use change within the urban landscape.
Highlights
Bees and flowering plants are two closely interacting groups of organisms
We found urban fragmentation to be negatively related to flowering plant species richness, functional and phylogenetic diversity
Our causal modeling revealed an indirect negative effect of urban fragmentation on bee diversity that was mediated via its negative effects on flowering plant richness
Summary
Bees and flowering plants are two closely interacting groups of organisms. Habitat loss and fragmentation associated with urbanisation are major threats to both partners. The close relationship between bees and flowering plants highlights the risks of their parallel declines in response to land-use change within the urban landscape. Urbanisation leads to greater habitat fragmentation, degradation and loss, increased pollution and more impervious surfaces compared to non-urban habitats[3]. Such extreme land-use alteration can directly affect species persistence, alter competition and predation dynamics[4,5] and influence species evolutionary change, including that of wild bees and native flowering p lants[6,7,8]. If functional distances of the focal traits in the interacting partners are proportional to the time since species diverged, another metric of community diversity, phylogenetic diversity, might be expected to be correlated between the two interacting partners
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