Abstract
Background: Predicting which alien species may become invasive is important in prioritising scarce resources for management. Objectives: Sixteen naturalised ornamentals in Cape Town were assessed for invasion potential in relation to a mechanistic framework. The recently spreading species, Centranthus ruber (L.) DC., was studied in detail following management actions and vegetation fires. Method: The mechanistic framework was developed using nine features most likely to promote invasiveness. Species were assessed from their known characteristics, local usage and distribution records, including citizen science surveys. Surveys were conducted for C. ruber to assess its ability to survive and spread post-fire. Control efficacy for Centranthus ruber was assessed in plots at two sites. Results: Nine species with more than 25 naturalisation records had a median of seven features that promote invasion compared to five features in the less recorded group of seven species. Centranthus ruber was widespread in modified urban habitats and persisted in natural habitats following vegetation fires and is a high priority for control. Post-fire mechanical and chemical control of C. ruber significantly reduced its density and cover, but did not eliminate it. Conclusion: Naturalised ornamentals can move rapidly from latent to invasive phases; therefore, monitoring should start during the latent phase to detect sudden change. In firedriven ecosystems it is essential to have good pre-fire baseline data. More residents should be encouraged to become spotters through citizen science programmes and to report new naturalised ornamentals. It is important to act early in the invasion process and to allocate sufficient resources, if a newly invasive species is to be contained.
Highlights
Cities are hubs of human-mediated introductions and dissemination of alien species (Kowarik 2011)
From an environmental management perspective, it is important to know which naturalised ornamental species are potentially invasive, and what habitats to prioritise for control. This is important for cities located in global biodiversity hot-spots where significant endemic biodiversity is under threat, such as Cape Town, situated in the Cape Floristic Region (CFR), South Africa (Geerts et al 2017; Rebelo et al 2011)
Our analysis is confined to Cape Town, which is a coastal city of 2460 km2 in extent, located at the south-western corner of South Africa (Figure 1-A1)
Summary
Cities are hubs of human-mediated introductions and dissemination of alien species (Kowarik 2011). In Cape Town, ornamentals currently are the main source of new invasive alien species and have a higher overall chance of naturalising than accidental introductions (Gaertner et al 2016). They are promoted by the nursery trade or by citizenry informally swapping garden-adapted and attractive species and are established widely and exposed to a large range of local environments. From an environmental management perspective, it is important to know which naturalised ornamental species are potentially invasive, and what habitats to prioritise for control This is important for cities located in global biodiversity hot-spots where significant endemic biodiversity is under threat, such as Cape Town, situated in the Cape Floristic Region (CFR), South Africa (Geerts et al 2017; Rebelo et al 2011). Predicting which alien species may become invasive is important in prioritising scarce resources for management
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