Conservation of the fynbos biome in the Cape Floral Region: the role of biological control in the management of invasive alien trees

摘要

Fynbos is a vegetation type in the Cape Floral Region (CFR), at the southern tip of Africa. Portions of the CFR are recognised as a ‘serial’ World Heritage site and acclaimed by UNESCO as the world’s ‘hottest hot spot’ for plant species richness and endemism. Habitat degradation and species losses through human intrusion in the CFR include transformations brought about by introduced invasive alien tree species. Since 1970, ten invasive tree species in the fynbos biome have been subjected to biological control, namely: six Acacia species and Paraserianthes lophantha (Mimosaceae), Hakea sericea (Proteaceae) and Leptospermum laevigatum (Myrtaceae), all from Australia, and Sesbania punicea (Fabaceae) from South America. A total of 19 species have been deployed as biological control agents, including nine weevil species (eight Curculionidae and one species in the family Brentidae: Apioninae), a seed-feeding moth species (Lepidoptera: Carposinidae), two species of bud-gallers (Hymenoptera: Pteromalidae), two species of flower-gallers (Diptera: Cecidomyiidae), and a gall-forming rust fungus (Uredinales: Pileolariaceae). Most of these agents primarily reduce seed production, directly or indirectly, but some also cause die-back and mortality of their host plants. The overall result, often in combination with mechanical clearing and herbicide applications, has been a substantial decline in the abundance and/or aggressiveness of most of the targeted host-plants. In this review, four representative but contrasting case studies are used to show that biological control is an indispensible supplement to other management practices for long-term conservation of the remnants of the fynbos biome.