First records of Opuntia monacantha (Willd.) Haw. and Opuntia tomentosa Salm-Dyck (Cactaceae) from Algeria

We report here the first occurrences of two alien cacti species from Algeria. During field surveys from 2016 to 2022 in the Skikda region (north-eastern Algeria), ten populations of Opuntia monacantha (Willd.) Haw. were recorded. This species is reported here as naturalized alien species for the first time for Algeria and the second time for mainland North Africa. The same surveys also yielded three localities of Opuntia tomentosa Salm-Dyck, reported here as casual species for the first time for Algeria and mainland North Africa. Both species colonize various habitats (coastal dune, maquis, cliffs, roofs, gutters, tree trunks) and they propagate by stem fragments. However, O. tomentosa also reproduces from seeds, and birds seem to play a role in the dispersal of this species. We discuss the potential of both species for further spread in Algeria, and argue that increased botanical surveys may lead to the recording of further alien cacti species.