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 origina l 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 b een little affected by urbanization and agriculture, but alien trees a nd 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 p lant biodiversity of the area, namely, the distribution of major veget ation types and of endemic, rare and threatened plant taxa and of taxa in the Proteaceae (a prominent element in almost all communities, tak en as an indicator of overall plant biodiversity).Possible future impa cts 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 vege tation 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 al ien stands currently affect 29.8% of the localities of special taxa kn own from herbarium records and 8.4% of these taxa currently occur only in areas at present affected by aliens. Clearing all dense stands wou ld result in 62.9% of special taxa having less than half of their know n localities affected (49.1% at present). Under this scenario, 92% of Proteaceae taxa have more than 75% of their localities unaffected by a liens. If clearing is confined to specific areas (the Cape Peninsula P rotected Natural Environment or all publicly-owned land) and the alien s spread further outside these areas, the area of natural vegetation r emaining shrinks (to 82.4% of the current extent if control is confine d 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). T he probability of the various scenarios materializing is discussed. To minimize further losses in biodiversity it is essential that: (1) a m ajor initiative is launched immediately to clear (firstly) the 10 184 ha of lightly invaded vegetation and then the 3313 ha of densely invad ed vegetation; (2) no urban development be permitted within the bounda ries of the Cape Peninsula Protected Natural Environment; (3) a system atic programme of prescribed burning (linked to the alien control prog ramme) is initiated; and (4) contingency measures are implemented to i mprove the status of seriously threatened taxa, habitats and vegetatio n types.