Citizen science reveals incipient naturalisation of non-native palms (Arecaceae) in South Africa

Palms (Arecaceae) are among the most widely cultivated plant families globally, yet their invasion ecology remains poorly understood outside a few well-studied tropical systems. South Africa has cultivated non-native palms for over a hundred years, but no palm species are currently listed as invasive in national legislation. We used iNaturalist data, supplemented with herbarium and arboretum records, to compile the first comprehensive inventory of non-native palms in South Africa, model habitat suitability, identify areas of high naturalisation susceptibility, and propose monitoring priorities. In total, 145 non-native palm taxa were recorded in the country. From 4,220 quality-filtered iNaturalist records, 3,873 represented planted individuals and 347 were wild-growing records indicative of naturalisation. Naturalisation probability, estimated using empirical Bayes shrinkage with Wilson score confidence intervals, was highest for Livistona chinensis , followed by Caryota mitis and Phoenix dactylifera . Seventeen taxa showed evidence of naturalisation, substantially exceeding the four currently recognised for South Africa in the Global Naturalized Alien Flora database. An ensemble habitat suitability model evaluated using spatial block cross-validation (calibration TSS = 0.853, AUC = 0.972) identified the Human Footprint Index and minimum winter temperature as the dominant predictors. A composite naturalisation susceptibility assessment integrating habitat suitability, propagule pressure, and proximity to existing naturalising populations identified 72 high-priority quarter-degree cells concentrated in three urbanised regions. These findings demonstrate the value of citizen science platforms for detecting incipient naturalisation events and a spatially explicit framework for prioritising early detection before invasive palm populations become widespread and costly to control.