Biotic homogenization and alien bird species along an urban gradient in South Africa

Similar to the process seen in invasion biology, urbanized environments lead to biotic homogenization with a few species, often alien, dominating the urban habitat. We investigated avian communities across an urban gradient in Pretoria (South Africa). We defined three urbanization zones: urban, suburban and semi-natural, based on land-cover. We conducted point counts of all species at each of 13 sites in these zones over a 4-month period. We found significantly more species in the semi-natural zone than the urban zone but abundance was significantly lower in semi-natural than urban. The increase in abundance was mainly as a result of alien species with the three most abundant species in the urban zone being all alien. Semi-natural contained fewer alien species than urban and significantly fewer than suburban. The Common Myna Acridotheres tristis was the only alien species that was observed in all three urbanization zones and was the second most abundant species in the suburban and urban zones. Our study supports the process of biotic homogenization, where an increase in alien species occurs in the urban compared to semi-natural zone. Although the suburban zone made an important contribution towards native bird abundance in a larger city environment, this environment was also an important environment for alien species, especially for the Common Myna.