Impacts and control of alien Proteaceae invasion in the Western Cape Province, South Africa

Abstract

Research focused on ecological impacts and control of invasive alien species (IAS) is gaining attention worldwide. The eradication and control of invasive alien plants (IAP) is essential for the restoration of native plant communities. Understanding ecological impacts and potential invasive risks of IAP is important for their effective management, particularly for prioritisation. Most studies concerning impacts on vegetation structure and plant-pollinator interactions have measured few ecological metrics, resulting in a superficial understanding of plant species invasion. Additionally, most studies related to the control of IAP have focused on major invaders which have demonstrated severe impacts, with less focus on emerging invaders. This study assessed ecological impacts, invasive risks and chemical control options for alien Hakea drupacea and Banksia species in the Western Cape Province, South Africa. Multiple ecological metrics data on vegetation, soil and plant-pollinator parameters were measured and compared between invaded and uninvaded sites. The invasion risk of fourteen Banksia species which have been introduced to South Africa was evaluated by conducting a weed risk assessment (WRA). The herbicide efficacy of resprouting Banksia integrifolia and Banksia serrata was determined by rating plants response to different treatments, with percentage, height and resprout vigour as measures. Results revealed significant negative impacts of alien H. drupacea and Banksia speciosa invasion on native plant species richness and diversity and on the abundance of native pollinators. The study demonstrated that 79% of Banksia species have a high risk of invading the Fynbos Biome. Chemical control with triclopyr+picloram mix provided effective means of controlling resprouting Banksia species. The high invasive risk of Banksia species and competitive effects of invasive alien B. speciosa and H. drupacea with native plant species for biotic and abiotic resources represents a major threat to biodiversity conservation in the Fynbos Biome. The removal of both naturalised and invasive alien H. drupacea and Banksia populations is recommended in order to conserve native plant communities in the Fynbos Biome.