Perspectives on and the status of sterile cultivars of invasive alien plants in South Africa

Abstract

Biological invasions are a major global threat causing detrimental effects on biodiversity, ecosystems, and economies. Nationally, various types of regulations have been developed to combat biological invasions. Such regulations can create conflicts between regulators and stakeholders, particularly within the horticultural industry, when trade restrictions on economically important species lead to financial losses. To address this, risk analyses are conducted on alien taxa which comprehensively assesses the risks, impacts, benefits, and management options of a specific taxon. However, most invasion risk analyses and subsequent regulations are applied at the species level, without considering infra- or interspecific entities (e.g., cultivars or hybrids). To balance economic needs and environmental risks for horticulture, a potential solution is the use of non-invasive forms of regulated invasive alien plant species. If invasive alien plants are bred to have severely reduced reproductive output (e.g., as sterile cultivars), then there may be insufficient reproduction for rapid spread from sites of cultivation, thereby reducing the invasion risk to an acceptable level. However, it remains challenging to demonstrate if sterility reduces invasion risk to an acceptable level, how any resulting regulations can be implemented and monitored in practice, and whether this approach is acceptable to key stakeholders. To address these challenges, I first evaluated various regulatory options for cultivars (and hybrids) of invasive alien plant species, and reviewed South Africa’s policy responses to the issue. From this evaluation I argue that South Africa should move from a Negotiated Guilt approach (exemptions based on negotiations without scientific evidence) to a Guilty until Proven Innocent approach. This approach allows for the exemptions of cultivars (or hybrids) based on scientific evidence of sterility and balances the needs of relevant stakeholders by providing regulatory exemptions for desirable taxa. In support of this I developed a pro forma (template) outlining the requirements for sterility assessments to guide future exemption evaluations. Secondly, to assess the efficacy of current regulatory exemptions, I analysed photographic distribution records of species in South Africa which have regulatory exemptions for sterile cultivars. In most cases, I could not readily distinguish cultivars from wild type forms. Nonetheless, results showed that most of the study species predominantly occur in cultivation. Only three species, which tend to spread vegetatively rather than by seed, are primarily found outside cultivation. Third, to determine if sterile cultivars of invasive alien plant species are acceptable to stakeholders in South Africa, I conducted a national online survey of members from the South African horticultural industry and environmentalists. Overall, respondents viewed species associated with sterile cultivars exemptions as relatively unimportant for horticultural trade, and general attitudes towards sterile cultivars were mostly neutral. This suggests that there might be a degree of hesitancy towards the use of sterile cultivars, particularly from more experienced industry members. As such, if sterile cultivars are to be widely acceptable to stakeholders, the benefits and risks would need to be clear. Fourth, I tested sterility in three cultivars of the ornamental plant Duranta erecta that are considered sterile (Sapphire Showers, Sheena’s Gold, and Goldmine) and compared results to the wild type (invasive form). Using the developed pro forma, I assessed: 1) long-term flower and fruit production; 2) pollen viability; 3) seed germination and viability; and 4) potential for vegetative reproduction. The Goldmine cultivar did not produce any flowers or seeds, consistent with it being sterile. The other cultivars exhibited 2-4 orders of magnitude lower reproductive output than the wild type, suggesting a substantially lower risk of invasion, consistent with the regulatory policy in South Africa. The findings presented here are relevant to any country facing challenges with invasive ornamental plants and cultivar regulation. The overall findings of this thesis highlight the considerable potential of sterile cultivars as a tool for reducing the spread of invasive alien ornamental plants. Conceptually and practically, this work represents significant progress in understanding how this strategy can be responsibly and effectively implemented (including the resources required). Through a multidisciplinary approach, this research provides a comprehensive assessment of the feasibility, risks and benefits associated with sterile cultivar use. While challenges remain, this thesis lays a strong foundation for a science-based implementation of the sterile cultivar approach as a strategy to address biological invasions while reducing conflicts between stakeholders.