Invasive alien plants at Capraia Island (Italy): distribution and threats to Natura 2000 habitats

Island ecosystems face significant threats from biological invasions due to their unique biomes. In over a quarter of these ecosystems, the number of alien plants surpasses the total count of native ones, leading to notable impacts. This concern is particularly pronounced in the Mediterranean Basin, a globally important hotspot for plant diversity. In line with this, our study aimed to investigate the current distribution of six main alien plant species (Chasmanthe floribunda, Nicotiana glauca, Opuntia ficus-indica, Opuntia stricta, Senecio angulatus, and Zantedeschia aethiopica) on Capraia Island in the Tuscan Archipelago, Italy. We identified invaded vegetation types and N2000 habitats (sensu 92/43/EEC “Habitats” Directive), created a detailed map of the current distribution of these six target species, and analyzed the relationship between their distribution and environmental factors. The six target species were found to be more widespread in areas of the island with higher levels of anthropogenic disturbance, with only O. stricta also spreading into natural habitats. Overall, the invasion covers an area of 71 hectares (3.6% of the island’s surface), of which over 70 hectares are invaded by O. stricta. The habitats most impacted by alien plant invasion include 5330 “Thermo-Mediterranean and pre-desert scrub”, 5320 “Low formations of Euphorbia close to cliffs”, 1240 “Vegetated sea cliffs of the Mediterranean coasts with endemic Limonium spp.”, and 6220* “Pseudo-steppe with grasses and annuals of the Thero-Brachypodietea”, all primarily invaded by O. stricta. Environmental factors most correlated with alien plant distribution include the distance from infrastructure, altitude, and slope aspect.