The European shore-crab Carcinus maenas has been present in South Africa since 1983. Despite this species' international reputation as a biological invader, its distribution in this region has only been considered by three outdated 'snapshot surveys.' The present study is the most comprehensive to date, providing an update on the species' range and the first temporal assessment of its abundance and demographics. Along South Africa's Cape Peninsula and surrounding areas, C. maenas was absent from 12 intertidal sites surveyed, except for Sea Point, and no crabs were found during subtidal surveys along the open coastline. Subtidal harbour populations were recorded in the Cape Town harbours of Table Bay and Hout Bay (previously estimated as comprising approximately 164 200 and 6 500 individuals, respectively). Table Bay was surveyed monthly for one year, using baited traps, crab condos and postlarvae settlement collectors, to assess size distributions and reproductive seasonality of the crab. Reproductive females were recorded throughout most of the year. These results suggest that the harbour populations could be targeted by control programmes, but provide no strong evidence to support the initiation of management action during a particular season. The lack of detection of postlarval settlement, even among well-established populations, suggests this will not be a useful monitoring tool for detecting incursions.
Biological invasions severely impact on marine ecosystems around the world, but to date management is rare and has not previously been attempted in Africa. This study documents a trial management programme aimed at informing a national management strategy for the invasive European shore crab, Carcinus maenas, in South Africa. The approach involved testing control methods used elsewhere (baited traps, crab condos, diver collections and sediment dredging) and adapting these to the local context. Following these trials, baited traps were deployed over the course of the year, and the catch per unit effort (CPUE) tracked. A total of 36,244 crabs were collected during the management period, six times more than a pre-management population estimate. The population was not extirpated and CPUE increased once trapping ceased. The cost of attempting nationwide eradication is prohibitive, particularly given the lack of current impacts by this crab in this region and the possibility of reintroduction. We highlight key administrative challenges encountered, and the importance of such pilot trials in setting long-term goals when attempting alien species management interventions.
Background: The effectiveness of invasive alien species management in South Africa, and elsewhere, can be mproved by ensuring there are strong links and feedbacks between science and management. The CAPE Invasive Alien Animals Working Group (CAPE IAAWG) was established in 2008 to enhance cooperation among stakeholders such as implementing agencies and researchers, and thereby improve the management of invasive animals in the Greater Cape Floristic Region.Objectives: In this article we highlight where and how the working group has advanced our understanding of research and the implementation of management objectives and consider the working group’s successes and failures.Methods: We analyse the attendance of meetings by different stakeholders and the frequency of discussion topics on meeting agendas throughout the sequence of meetings from 2008 to 2019. We document insights based on published accounts or the experiences of the authors from eight different management projects.Results: Meetings are attended by stakeholders from NGOs, universities, and local, provincial and national government agencies as well as private individuals. Topics of discussion ranged from details of specific alien animal invasions (e.g. the House Crow in Cape Town), to considering the risks posed by broad groups (e.g. earthworms), to specific management techniques (e.g. guidelines for trapping invasive alien birds). Through the eight projects described here the CAPE IAAWG has: (i) contributed to capacity building through funding and advising on post-graduate research projects; (ii) provided ad hoc support to staff of agencies that implement invasive alien animal control; (iii) acted as a focal point for a community of practice that is supportive of decision making and policy development; and (iv) played a vital role in linking research, management and policy in a manner accessible to a broader range of stakeholders. The projects undertaken by the group reveal several lessons for managing invasive animals: (i) the importance of logistics and contract efficiency, (ii) the need for effective stakeholder engagement by the project team, (iii) the need to effectively address conflicts between role players, and (iv) the importance of including ethical and animal rights considerations in the decision making processes.Conclusion: The CAPE IAAWG has been a valuable forum to improve management effectiveness and support implementation decisions. Due to its small cost and time footprint, the working group has remained viable and retained a core of committed members, ensuring ongoing institutional buy-in. The working group will remain successful so long as the group is supported by its members and their organisations.
Knowledge of the introduction history and spread dynamics of invasive species can provide important insights for management (Ens et al. in Environ Rev (in press), 2022), however such information is often unavailable for accidental introductions. Here we infer how the European shore crab, Carcinus maenas, and its congener, the Mediterranean shore crab, C. aestuarii, were introduced to and spread within South Africa. We do this using nuclear microsatellite data and Bayesian assignment tests and Approximate Bayesian Computation (ABC) modelling that included samples from the native and other invasive ranges of these two species. We also compared the genetic diversity and structure of one of the South African populations during and after intensive management, with that of another, unmanaged, population. South African populations had higher genetic diversity than invasive Carcinus populations from elsewhere in the world. Moreover, the ABC analyses suggest that South African populations originated from an admixture event between individuals of C. maenas from a population in the native range and an invasive population from Canada. We also identified instances of hybridisation between Carcinus maenas and C. aestuarii in South Africa. South African populations showed no genetic structure, suggesting either extensive migration between them or that populations arose from the same initial introduction. Management of Carcinus did not affect genetic diversity or structure, and we suspect that the management duration was insufficient to target a full generation of crabs. Together these results suggest multiple introductions and/or high propagule pressure to South Africa, crab (larval or adult) movement between existing populations, and some hybridisation. For eradication from South Africa to be achieved, management would need to concurrently target all known invasive populations and clearly establish that new introductions could be prevented.
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