The ecological dynamics of the invasive alien Common Myna Acridotheres tristis, L (Aves: Sturnidae) in South Africa

Urbanisation and invasive alien species constitute two of the most severe threats to biodiversity conservation in the 21st century. It is particularly concerning that these two threats often occur sympatrically and work synergistically in that continued alteration and deterioration of pristine ecosystems by the human urban sprawl changes intact biological communities’ species richness, composition and guild structuring, thereby allowing a numerically small ensemble of ecological generalists to prosper at the expense of richer and more diverse indigenous communities. An excellent South African example of such a prosperous generalist is the invasive alien Common Myna Acridotheres tristis. Centred on Pretoria, Gauteng, this study used point count surveys in combination with land-cover classification to conduct a community-level analysis of avian richness and diversity across a gradient of urbanisation. In particular, I identify and quantify bird communities associated with three levels of urbanisation (‘Urban’, ‘Suburban’ and ‘Semi-Natural’) and investigate land-cover variables that determine the distribution, numerical abundance and population density of the Common Myna in relation to sympatric indigenous species. Generally, species richness and ‘endemicity’ increased with decreasing urbanisation levels, whereas the number of individual birds showed the opposite trend. Common Mynas reach their highest densities in ‘Urban’ areas (3.25 birds/ha), compared to ‘Suburban’ (2.43 birds/ha) and ‘Semi-Natural’ (0.59 birds/ha). Mynas prefer areas dominated by urban greenery e.g. sports fields, parks and lawns, showing a strong positive correlation with this land-cover variable. In contrast, correlations for ‘Urban’ and ‘Suburban’-dominated areas were nearly neutral, while selection for ‘Semi-Natural’ sites was moderately negative. Despite claims that mynas compete with, and potentially displace indigenous species, evidence for this is lacking. The results of this study suggest that mynas are strongly attracted to areas severely altered by urbanisation where they potentially compete with a relatively species-poor avian community, many of which are also alien.