The rehabilitation value of a small culturally significant island based on the arthropod natural capital

Small islands are particularly vulnerable to degradation by invasive species and often lack source populations to replace those lost during localized extinctions. Robben Island, a significant South African cultural heritage site, has a long history of anthropogenic impact. Introduced alien mammals and trees have resulted in numerous localized plant extinctions with their impact on arthropod biodiversity remaining unclear. We used Robben Island's arthropod fauna as focal group to investigate the rehabilitation potential of this transformed island. Ground surface-living, foliage-inhabiting and flower-visiting arthropods were collected using five sampling techniques. Arthropod diversity was highest in the two 'natural' habitats compared to alien Eucalyptus plantations. Arthropod compositional diversity differed significantly between alien plantations and natural habitats. Nevertheless, a remarkable 17 % of all species were confined to plantations. However, when we corrected for the effect of rarity, only 15 species (4 %) were unique to alien plantations and all those identified to species level were either alien in origin or were widespread generalists only normally associated with transformed areas. This emphasizes the need to identify species that drive similarity indices to fully comprehend the conservation value of alien vegetation. Arthropods found in the remaining natural vegetation showed remarkable tolerance of long-term grazing pressures from introduced alien mammals. Only eight Orthoptera species were sampled, yet one was a rediscovery of a rare species and another a range extension. All indigenous species showed strong preferences for natural habitats. Thus, Robben Island could still hold conservation value if the natural habitats are rehabilitated and the Eucalyptus plantations removed.