Vegetation response to control of invasive alien European rabbits (Oryctolagus cuniculatus) on South African offshore islands: insights from Robben Island.

Abstract

The European rabbit (Oryctolagus cuniculus), a small mammal native to the Iberian Peninsula, exemplifies the catastrophic effects of introducing invasive alien species to new environments. Released on uninhabited islands as a food source for early world explorers, these rabbits have established populations on over 800 islands worldwide, posing significant threats to biodiversity and human livelihoods. European rabbits have been introduced to some of the more than 33 offshore islands that occur along the southern African coast, 24 of which are part of South African territory. While the impact of invasive European rabbits on global ecosystems has been extensively studied, research on their effects on southern African offshore islands is comparatively sparse. A handful of studies have assessed rabbit herbivory’s impact on the vegetation of some South African offshore islands, but the vegetation’s response to rabbit control remains largely unexplored. Given that European rabbits are classified as a Category 1b invasive species under South Africa's National Environment Management: Biodiversity Act 10 of 2004, understanding how offshore island vegetation might respond to rabbit control is crucial, especially considering the islands' long history of rabbit occupation and highly degraded ecosystems. This thesis examined an attempt to restore degraded vegetation on Robben Island, South Africa, through the control of invasive European rabbits. The aim was to understand the impact of rabbit control on the island's vegetation, particularly the potential effects of their removal. Changes in plant species cover were monitored at nine sites, including strand (n=3), eucalyptus (n=3), and inland dune (n=3), on the south-eastern part of Robben Island over four years (2010-2013). Vegetation monitoring followed an attempt to eradicate European rabbits from the island between 2008 and 2009. Robben Island, the largest and most environmentally degraded of South Africa's offshore islands, has a long history of rabbit occupation dating back to 1654. It is the only South African offshore island where there has been a deliberate and sustained effort to eradicate European rabbits. While South African offshore islands vary in geology, size, distance offshore, and climatic conditions, insights into vegetation dynamics on Robben Island following rabbit control could provide valuable understanding of potential vegetation responses on other invaded South African offshore islands. The thesis addressed these specific questions: Question 1: How did vegetation composition change following rabbit control on Robben Island and what ecological factors and vegetation processes underlay the observed changes? The thesis hypothesised that vegetation on Robben Island would revert to its original composition following rabbit control, with the re-establishment of late-successional perennial grasses, succulents, herbs, and an increase in woody shrubs. However, findings were contrary to this expectation in that instead of secondary succession towards the original composition, changes in plant species were solely driven by increases in early successional species, including the woody shrub Tetragonia fruticosa and grasses Ehrharta longiflora, Cynodon dactylon, and Ehrharta villosa. Secondly, there was no change in vegetation composition at eucalyptus sites due to subdued responses of Phyllobolus canaliculatus, Oxalis pes-caprae and T. fruticosa to rabbit control. These observations suggest a shift to an alternative stable degraded state rather than successional processes. Prolonged rabbit grazing has driven the island’s vegetation beyond a critical ecological threshold, eliminating competitive late successional species and preventing them from driving successional change post-rabbit control. This results in an arrested succession state dominated by herbivory-resilient, early successional species. The vegetation composition at eucalyptus sites remained unchanged most likely because of the inhibitory effects of the alien trees on understory vegetation dynamics. Question 2: How did vegetation species diversity change following rabbit control on Robben Island and what ecological factors and vegetation processes underlay the observed changes? The study hypothesised that rabbit control on Robben Island would boost species diversity by allowing suppressed palatable plant species to re-establish. It was further expected that residual non-rabbit herbivory would prevent these recovering species from dominating the vegetation and displacing less competitive species, leading to greater diversity. Findings showed that contrary to expectations, rabbit control led to a decrease in species diversity across strand and inland dune sites, while it remained unchanged at eucalyptus sites. The decline in diversity was due to the loss of seven plant species (Anagallis arvensis, Euphorbia peplus, Hypochaeris glabra, Wahlenbergia androsacea, Emex australis, Ornithoglossum viride and Trachyandra sp.), offset by the emergence of one species (Cynodon dactylon) at strand sites. In addition, species evenness declined across strand and inland dune sites following rabbit control. Before rabbit control, species cover at these sites was dominated by four species (Conicosia pugioniformis, Oncosiphon suffruticosum, E. peplus, and A. arvensis) and six species (O. viride, E. australis, Trachyandra sp., O. suffruticosum, C. pugioniformis, and Bromus diandrus) respectively, but after control, it was dominated by T. fruticosa and E. longiflora at strand and T. fruticosa and E. villosa at inland dune sites at each site. The changes in species diversity were attributed to the long-term effects of rabbit herbivory on the island’s vegetation. Prolonged rabbit herbivory depleted palatable plant species, leaving a community dominated by unpalatable and less palatable species. The release of vegetation from rabbit herbivory allowed the fewer, more competitive, palatable species to outcompete the more numerous, less competitive, unpalatable, and less palatable species. This led to a decrease in their abundance and a decline in species diversity. Eucalyptus sites showed no change in diversity likely due to the inhibitory effects of the alien Eucalyptus trees on understory vegetation dynamics. Question 3: How did vegetation grazing capacity change following rabbit control on Robben Island, and what ecological factors and vegetation processes underlay the observed changes? An increase in grazing capacity on Robben Island, following the control of invasive European rabbits, was anticipated based on the premise that the removal of these herbivores would alleviate the pressure on the island's vegetation, allowing for the recovery and re-establishment of palatable plant species previously suppressed by rabbit grazing. Results confirmed this as the control of rabbit herbivory led to an anticipated increase in grazing capacity across strand and inland dune sites on Robben Island. This increase was driven by the recovery and re-establishment of palatable forage species, such as T. fruticosa, E. villosa, E. longiflora, and Avena fatua. Being selective herbivores, rabbits preferentially browse and graze on these palatable plants. When rabbit herbivory was reduced, these palatable plants were able to recover, leading to an increase in grazing capacity. However, grazing capacity remained unchanged at eucalyptus sites following rabbit control, due to the subdued responses of T. fruticosa, E. longiflora, O. pes-caprae, and P. canaliculatus to rabbit control. The lack of change in grazing capacity at sites under Eucalyptus plantations was likely because of the inhibitory effects of the alien trees on understory vegetation dynamics. Question 4: What insights do the results provide into the potential response of vegetation on other South African offshore islands? The research posited that studying vegetation dynamics on Robben Island could offer valuable insights into how vegetation on other invaded offshore islands (Dassen, Jutten, Schaapen, Seal, and Vondeling Islands) might respond to rabbit control. Although the available data limited the precision of the predictions, the theoretical framework developed in the thesis provided a valuable starting point for understanding the potential consequences of rabbit control on these islands. Findings from the thesis revealed a promising outlook for vegetation recovery post-rabbit control on invaded islands. Across the islands, an initial resurgence of early successional, palatable species like T. fruticosa and Ehrharta spp. is anticipated. In the long term, recolonisation by late successional species and increased vegetation diversity are expected, particularly on islands closer to the mainland where dispersal is facilitated. This shift towards a more complex and resilient vegetation structure signifies a potential return to a functional island ecosystem. However, the specific trajectory and pace of these changes will depend on various island-specific ecological factors and necessitate ongoing monitoring and adaptive management. With more comprehensive data, this thesis's predictions could be further refined, leading to enhanced optimisation of restoration intervention effectiveness.