Psidium cattleyanum (Myrtaceae) invasions in South Africa: Status and prognosis

Abstract

• Detailed current and potential distribution of Psidium cattleyanum in South Africa is presented. • The species is self-sustaining with evidence of spread. • The population in the Western Cape is the first substantial invasive population reported from a temperate winter-rainfall region globally. • Recommendations for regulating P. cattleyanum in South Africa are provided. Psidium cattleyanum (Myrtaceae) is a widespread invasive species in several countries, particularly on oceanic islands. The species was first recorded in South Africa in 1948 and has since established self-sustaining populations. We present the first comprehensive evaluation of the invasiveness of P. cattleyanum in South Africa by: 1) mapping the current distribution of the species; 2) estimating its potential range using species distribution models in MaxEnt; 3) analysing the risk posed to South Africa using the risk analysis for alien taxa framework; and 4) developing recommendations for regulation. Psidium cattleyanum was recorded at 52 sites in four provinces across South Africa, with the population in the Western Cape being the first substantial invasive population reported from a temperate winter-rainfall region globally. Invasive populations were assessed in detail at four sites: Newlands Forest (Western Cape; n=2193 plants; covering ∼12 ha), Eshowe (KwaZulu-Natal; n=1561, ∼7.6 ha), Southbroom (KwaZulu-Natal; n = 449, ∼2.9 ha), and George (Western Cape; n=214, ∼2.4 ha). At all four sites the species is self-sustaining and there is evidence of spread. In South Africa, the east coast is climatically suitable for the species to expand its range. Although damaging invasions of P. cattleyanum have only been recorded on several islands to date, we find no reason to suggest that climatically suitable continental regions (including parts of South Africa) will not suffer harmful impacts if invasions progress unmanaged. As such, we support the current regulation of P. cattleyanum in South Africa, whereby the species must be controlled as part of a national management plan (category 1b) noting that while it is advisable to ban cultivation, the forcible removal of plants from people's gardens should perhaps only be prioritised if such plantings clearly pose a high risk (i.e., in climatically suitable regions near to riparian areas or natural areas).