Potential conflict extent between two invasive alien pests, Rhynchophorus ferrugineus and Paysandisia archon, and the native populations of the Mediterranean fan palm

The introduction of alien species is one of the main threat to native biodiversity worldwide. The spreading of Rhynchophorus ferrugineus and Paysandisia archon across the Mediterranean basin caused enormous damages to cultivated palms, representing a paradigmatic case of uncontrolled diffusion of invasive alien pests. The infestation of natural populations of Chamaerops humilis, which has a particular conservation relevance being the only native palm species in Western Europe, by the two pests can be a serious threat. Nevertheless, no clear evaluation of this risk is available. I provided a first estimation of the potential geographic extent of this threat, comparing the specific climatic niches of the three species and identifying high-conflict areas and potential refuges for C. humilis through a spatial explicit approach based on niche modeling. I evidenced that the climatic niche of the palm is similar to those of the two pests. This produces frequent co-occurrence of host and pests causing potential high levels of pressure on palm populations. More specifically, R. ferrugineus represents a very widespread threat to palms, with conflict areas in most of the coasts and islands included in the C. humilis distribution. On the contrary, P. archon represents a threat only in the northeastern part of the palm range, and large potential refuges occur in southwestern Iberia, North Africa, and several islands. However, the threat by P. archon can be locally stronger than by R. ferrugineus. Programs of assessment, monitoring, and containment in host-pest conflict areas and measures of protection, prevention, and monitoring in refuge areas could mitigate the threat to C. humilis, avoiding the impact that the same pests have on cultivated palms across Europe.