Effects of urbanisation and management practices on pollinators in tropical Africa.

Abstract

Urban expansion is an increasing threat to biodiversity, especially in tropical Africa where biodiversity hot spots are being encroached upon by fast-growing cities. Threatened species include bees and other pollinators, which deliver important ecosystem services but are sensitive to land use changes. We investigated the impact of urbanisation and vegetation management practices on pollinator abundance, bee diversity, and bee functional traits. We sampled 126 locations in a stratified random design across an urbanisation gradient in two medium-sized cities in the West African Forests biodiversity hot spot, encompassing three management practices (farmed sites; amenity lands, i.e. green spaces managed for aesthetics; informal green spaces), and tested their effect with generalised linear models. Urbanisation did not affect bee abundances or diversity but had a negative impact on both wasp and beetle abundances. There was also a management-mediated impact of urbanisation on bee abundances, which decreased with urbanisation on farmed sites but not amenity land or informal green spaces. Management practices alone influenced bee abundances with farms harbouring fewer bees, and amenity lands fewer beetles. Bee genera occurrence and dominance patterns were influenced by both urbanisation and management, with some otherwise common genera rare in urban areas. Most functional traits were influenced by management, with fewer polylectic bees, cavity-nesting bees and long-tongued bees in farmed sites. Amenity lands hosted smaller bees and fewer savanna specialists. Some traits were influenced by urbanisation, with more long-tongued bees and cavity-nesting bees found in urban areas. Synthesis and applications. Pollinator responses to urbanisation are complex. In our research, we demonstrate how bee, lepidopteran, and non-fruit fly abundances have been maintained across an urbanisation gradient in tropical Africa, but not wasp and beetle abundances. Moreover, bee community composition and the distribution of traits shifted markedly. How green spaces were managed was also critical. We found that farmed sites hosted the lowest bee abundances and amenity lands the fewest beetles. Retaining informal green spaces and amenity lands in African cities, including protecting nesting sites for stingless bees, and limiting pesticide application would be important for conserving bees and the pollination service they provide to both crops and native vegetation.

Key words

• adverse effects
• animal behaviour
• biodiversity
• breeding places
• cities
• conservation
• disturbance
• disturbed land
• ecosystem services
• ecosystems
• effects
• endangered species
• forests
• grasslands
• human activity
• impact
• land use
• management
• models
• nesting
• pesticides
• pollination
• pollinators
• savannas
• traits
• tropics
• urban areas
• urbanization
• vegetation management