Page 5 - CELESTI-GRAPOW

Plant invasions on small Mediterranean islands 1123

naturalized species is Austrocylindropuntia subulata,
which occurs on 16 islands. The most frequent
casual species is Solanum lycopersicum, which occurs
on 14 islands. Sixty-six species (2 invasive, 32
naturalized and 32 casual) are currently restricted
to one island.
Of the 154 species which were also listed in
the previous data-set (Pretto et al. 2012), 87 were
detected on a higher number of islands. Those
showing the greatest increase are listed in Table 2
and include Austrocylindropuntia subulata (+ 10
islands), Erigeron sumatrensis and Opuntia amyclaea
(+ 9 islands), Solanum lycoperscum (+ 8 islands) and
Anredera cordifolia (+ 6 islands). In addition, 7 species
were recorded on a further 5 islands, 9 species on
4 islands, 11 species on 3 islands, 17 species on
2 islands and 38 species on 1 other island.
As many as 24 species, which are listed in Table 3,
Figure 3. The most represented genera in the non-native flora have progressed to a subsequent stage of the invasion
of the 37 islands analysed classified according to their level of
establishment. The numbers next to each bar refer to the total process. These include 15 previously casual species
number of species in the genus. that have established. Thirteen species (4 casual and
9 naturalized) have become invasive on at least one
island.
The genera represented most are Amaranthus
(10 species), Opuntia (7 species), Acacia, Oxalis
and Solanum (6 species). Ninety-five genera are Discussion
represented by only one species. The genera with the General features of the introduced flora and recent
highest number of invasive species are Erigeron (4), changes in species diversity, establishment and
Opuntia (4) and Acacia (3). distribution
The most widespread species, i.e. those that occur
on at least one third of the islands, are shown in Our results show that the plant invasion phenome-
Table 1. They include the invasive Agave americana, non on Mediterranean islands has intensified con-
Opuntia ficus-indica, Erigeron bonariensis, Oxalis pes- siderably in recent years. Indeed, we detected an
caprae and Carpobrotus acinaciformis, which occur on overall increase in non-native species diversity, level
more than 30 (80%) islands. The most widespread of establishment and spread within the study area.

Table 1. List of the most widespread non-native vascular plant species in the 37 investigated islands.
Taxon Family Est N
Agave americana L. Agavaceae I 35
Opuntia ficus-indica (L.) Mill. Cactaceae I 34
Erigeron bonariensis L. Asteraceae I 33
Oxalis pes-caprae L. Oxalidaceae I 32
Carpobrotus acinaciformis (L.) L.Bolus Aizoaceae I 31
Ailanthus altissima (Mill.) Swingle Simaroubaceae I 26
Mesembryanthemum cordifolium L.f. Aizoaceae I 26
Symphyotrichum squamatum (Spreng.) G.L.Nesom Asteraceae I 26
Amaranthus deflexus L. Amaranthaceae I 23
Mirabilis jalapa L. Nyctaginaceae I 23
Erigeron canadensis L. Asteraceae I 19
Amaranthus retroflexus L. Amaranthaceae I 18
Carpobrotus edulis (L.) N.E.Br. Aizoaceae I 18
Senecio angulatus L.f. Asteraceae I 17
Acacia saligna (Labill.) H.L.Wendl. Fabaceae I 16
Erigeron sumatrensis Retz. Asteraceae I 16
Nicotiana glauca Graham Solanaceae I 16
Austrocylindropuntia subulata (Muehlenpf.) Backeb. Cactaceae N 16
Robinia pseudoacacia L. Fabaceae I 16
Solanum linnaeanum Hepper & P.-M.L.Jaeger Solanaceae N 15
Tropaeolum majus L. Tropaeolaceae N 15
Opuntia amyclaea Ten. Cactaceae I 14
Solanum lycopersicum L. Solanaceae C 14
Notes: Est = establishment status in the study area, assigned on the basis of the highest stage in the invasion process documented in any
island. I = invasive, N = naturalized, C = casual (see text for details on the methods). N = number of islands where the species occurs.