Abstract
Urbanization is a widespread and rapidly growing threat to biodiversity, therefore we need a predictive understanding of its effects on species and ecosystem processes. In this paper we study the impact of urbanization on a guild of nectar-feeding birds in a biodiversity hotspot at the Cape of Africa. The guild of four bird species provides important ecosystem services by pollinating 320 plant species in the Cape Floral Region. Functional diversity within the guild is related to differences in bill length. The long-billed Malachite Sunbird (Nectarinia famosa) plays an irreplaceable role as the exclusive pollinator of plant species with long nectar tubes. We analyzed the composition of the guild in suburban gardens of Cape Town along a gradient of increasing distance from the nearest natural habitat. Urbanization reduces the functional diversity of the nectarivore guild. Malachite Sunbirds did not penetrate more than 1 km into the city, whereas only the short-billed Southern Double-collared Sunbirds (Cinnyris chalybea) occurred throughout the urbanization gradient. The lack of data precludes conclusions regarding the detailed responses of Orange-breasted Sunbirds (Anthobaphes violacea) and Sugarbirds (Promerops cafer), however their absence across the entire gradient is suggestive of high sensitivity. The functional diversity of this guild of pollinators can potentially be restored, but the pros and cons of this conservation action need to be considered.
Highlights
A rapidly increasing portion of the Earth’s surface is urbanized
Malachite Sunbirds did not penetrate more than 1 km into the city, whereas only the short-billed Southern Double-collared Sunbirds (Cinnyris chalybea) occurred throughout the urbanization gradient
Cape Sugarbirds are closely associated with the Proteaceae; Malachite Sunbirds are the exclusive pollinators of plant species with long tubular flowers mostly in the Iridaceae and Amaryllidaceae; Orange-breasted Sunbirds specialize on the Ericaceae; and Southern Double-collared Sunbirds are generalists (Skead 1967, Geerts and Pauw 2009)
Summary
A rapidly increasing portion of the Earth’s surface is urbanized. a vast new habitat is being created and small islands of the original habitat are isolated in it (Radeloff et al 2005, McKinney 2006, Grimm et al 2008). Biologists often focus on the small islands of natural vegetation and ask how species diversity varies with fragment size and distance from the nearest large “mainland” of natural habitat (e.g., Bolger et al 1997, Pauw 2007). In this paper we focus instead on the urban habitat and ask how birds respond to it. The study is located in the hyper-diverse fynbos vegetation of the Cape Floral Region of South Africa. The largest urban area is the City of Cape Town. Typical of cities in the Developing World, Cape Town is expanding rapidly and engulfing many small conservation areas and the large Table Mountain National Park (Pauw and Johnson 1999, SinclairSmith 2009). Our study group is the small guild of specialized nectarivorous birds that occur in the region. We are interested in their response to urbanization because of the important ecological role that they play as pollinators of about 4% of the flora (320 plant species; Rebelo 1987)
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