Current and future threats to plant biodiversity on the Cape Peninsula, South Africa

The biodiversity of the Cape Peninsula (49 127 ha in extent) has been considerably affected by various factors since European settlement in 1652. Urbanization and agriculture have transformed 37% of the original area of natural vegetation. Lowland vegetation types have been worst affected, with almost half of the transformation occurring in one of the 15 recognized vegetation types. Vegetation at high altitudes has been little affected by urbanization and agriculture, but alien trees and shrubs are now threatening biodiversity in these areas. Of the area not affected by urbanization and agriculture 10.7% is currently under dense stands (>25% canopy cover) of alien plants and another 32.9% is lightly invaded. Dense stands of Acacia cyclops, the most widespread invader, cover 2510 ha, 76% of the total area under dense alien stands. This paper assesses the impacts of these factors on aspects of the plant biodiversity of the area, namely, the distribution of major vegetation types and of endemic, rare and threatened plant taxa and of taxa in the Proteaceae (a prominent element in almost all communities, taken as an indicator of overall plant biodiversity). Possible future impacts on biodiversity are assessed by considering the effects of several scenarios involving increased urbanization and changes to alien plant control strategies and further spread over the next 50-100 years. The worst-case scenario for urbanization sees the area under natural vegetation reduced to 12 163 ha (39.3% of its extent in 1994, or 24.8% of its original extent). Under this scenario almost a quarter of the 161 endemic, rare and threatened ('special') taxa are confined totally to urban areas; 57.4% of the known localities of these taxa, and 40.1% of the remaining localities of Proteaceae taxa are transformed. Dense alien stands currently affect 29.8% of the localities of special taxa known from herbarium records and 8.4% of these taxa currently occur only in areas at present affected by aliens. Clearing all dense stands would result in 62.9% of special taxa having less than half of their known localities affected (49.1% at present). Under this scenario, 92% of Proteaceae taxa have more than 75% of their localities unaffected by aliens. If clearing is confined to specific areas (the Cape Peninsula Protected Natural Environment or all publicly-owned land) and the aliens spread further outside these areas, the area of natural vegetation remaining shrinks (to 82.4% of the current extent if control is confined to public land). The further losses in biodiversity associated with these scenarios are described. If control programmes collapse and all potentially invadable land is occupied by dense alien stands, only 407 ha of natural vegetation would remain (1.5% of the current extent). The probability of the various scenarios materializing is discussed. To minimize further losses in biodiversity it is essential that: (1) a major initiative is launched immediately to clear (firstly) the 10 184 ha of lightly invaded vegetation and then the 3313 ha of densely invaded vegetation; (2) no urban development be permitted within the boundaries of the Cape Peninsula Protected Natural Environment; (3) a systematic programme of prescribed burning (linked to the alien control programme) is initiated; and (4) contingency measures are implemented to improve the status of seriously threatened taxa, habitats and vegetation types.