Naturalized and invasive alien plants in the Kruger National Park, South Africa

Abstract

Kruger National Park (KNP), South Africa, covers an area of 20,000 km2 and is one of the world’s most widely known protected areas. Like many protected areas, KNP is facing an increasing problem with the introduction and spread of alien plants. However, species prioritization using poor baseline data remains a key challenge for managers globally. Publications, expert opinion, and field observations indicate that 407 alien plant taxa have been recorded in the KNP; this list also included hybrids, ornamentals, records that could only be identified to the genus level, and extralimitals (i.e., species native elsewhere in South Africa; 22 species in total); in addition, two species are considered eradicated, and three potentially eradicated. Such extensive lists of poorly defined species’ statuses accumulated over long periods of time poses challenges to current decision-making processes. This is especially important for the management of naturalized (maintaining self-sustaining populations) and invasive species (subset of naturalized species that have spreading populations), because management needs to identify and target high-priority species and vulnerable sites. Here, we provide an up-to-date inventory of alien plant species that occur in natural areas in KNP (i.e., beyond tourist camps and other infrastructure) and thus represent a potential threat to the native species diversity in the park. We identified 146 such alien taxa, of which 30 are casuals, 58 are naturalized, 21 have become invasive, and for 37 species, the status remains to be determined. Twelve of the invasive species in KNP are globally widespread, occurring in more than 100 regions, and five (i.e., Pontederia crassipes, Lantana camara, Opuntia stricta, Chromolaena odorata and Mimosa pigra) are listed among 100 of the world’s worst invasive alien species. The alien flora in KNP comprises 41 families. Solanaceae (45.5%) and Asteraceae (26.1%) are over-represented among invasive species compared to non-invasive species. The alien flora of KNP mostly originates from North America and South America, and largely consists of perennials and herbaceous species. We found no significant results regarding the effect of origin and life span on invasion status. Despite a steady increase in the numbers of alien and naturalized plants since the 1980s, species we classified as invasive generally represent earlier introductions and have not increased substantially since the beginning of this century. Our paper highlights that carefully revised naturalized and invasive species lists reflecting the current situation in protected areas present a strong knowledge base for effective management strategies. In addition to addressing data gaps related to the distribution of alien species on a global scale, knowledge from large protected areas such as KNP contributes to understanding invasions in landscapes with varying and unique habitat types.