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Plant invasions on small Mediterranean islands 1127

tion of Acacia species in the study area should be & Motti 2013). The genus Oxalis includes perennial,
monitored closely, as this genus may be one example seldom annual, herbs that spread prevalently in
of those taxa whose expansion may be promoted by secondary disturbed habitats, such as settlements,
climate changes, particularly by a rise in the mini- wasteland, road sides, agricultural land and ruderal
mum winter temperatures, which are considered to areas. The most prominent invasive species of this
have previously limited their invasive spread in the genus in the study area is Oxalis pes-caprae (Bermuda
Mediterranean Basin (Di Castri 1989). buttercup), which is the fourth most common species
More generally, there is increasing evidence on the islands we investigated (32 islands, Table 1).
indicating that the role played by woody Fabaceae, This plant, which is native to the Cape region of South
and in particular by Mimosoideae, in the flora of Africa, forms large clonal colonies that reproduce
the study area has grown in recent decades. For through bulbs and is one of the main invaders in
instance, Paraserianthes lophantha (Tables 1 and 2), an southern Italy (Celesti-Grapow et al. 2010b) and the
ornamental tree from southern Australia, has shown a Mediterranean region (Vilà et al. 2006a; Lambdon
rapid demographic increase in these last years on the et al. 2008c; Traveset et al. 2008; Arianoutsou et al.
island of Vulcano (31 in Figure 1, Aeolian Islands, 2010) as well as in numerous countries throughout
Sicily) where its dense populations have spread on the the world (Weber 2003). The widespread presence of
volcanic soil in both human-made and less disturbed this species is also due to the fact that it is looked upon
environments (Domina & Spallino 2007). These favourably by the local populations of some islands
findings confirm the observations made by Pasta et al. in the Mediterranean Basin, which use the plant in
(2012), which point to a recent dramatic increase in various ways, e.g. as a means of preventing superficial
the number of successful invasion events in Sicily soil erosion in winter and on account of its aesthetic
by non-native Fabaceae such as Albizia julibrissin properties (Bardsley & Edwards-Jones 2007).
Durazz., Leucaena leucocephala subsp. glabrata (Rose) As regards Opuntia, the entire Cactaceae family,
S. Zárate, Parkinsonia aculeata L. and Senegalia visco a group that contains some of the most widespread
(Lorentz ex Griseb.) Seigler & Ebinger, in addition and damaging invasive plants in the world (Novoa
to the aforementioned acacias and Paraserianthes et al. 2014), was found to have spread throughout
lophantha (Raimondo & Domina 2007). Indeed, the study area, and has become one of the most
Fabaceae is the second largest family in the study numerous families in the flora analysed (Figure 2).
area (Figure 2) and all 17 recorded species are woody. A large number of succulent species belonging to this
The spread of emergent introduced woody Fabaceae family are cultivated extensively on the islands, mainly
has also been shown to affect the landscape in other as ornamentals, some of which were recently reported
Mediterranean sites in Italy (Stinca et al. 2015a). to have escaped and established. For instance,
Besides Acacia, the genera represented most Austrocylindropuntia subulata, which was recorded as
among the neophyte flora of the study area are a species that grows spontaneously outside cultivation
Amaranthus, Opuntia, Oxalis and Solanum (Figure 3). for the first time in Italy only in 2001 (Guiggi 2008;
The genera Amaranthus and Solanum, which include Nicolella 2013), now occurs on 16 islands. This makes
some of the most frequent species on all the islands it not only the most frequent naturalized species in
(Table 1) and of those species that have recently the investigated islands (Table 1), but also the species
spread to other islands (Table 2), contain some of that has expanded most since the previous survey, it
the most common weeds in agricultural and urban being found on an additional 10 islands (Table 2).
habitats in Italy (Ricotta et al. 2008; Celesti-Grapow The most frequent species on all the islands is
et al. 2010a; Iamonico 2015) as well as in Europe also a succulent plant, i.e. Agave americana (35 is-
(Lambdon et al. 2008c). lands, Table 1), a popular ornamental species found
The successful growth of the genera Oxalis and throughout the Mediterranean that spreads vegeta-
Opuntia, which have expanded remarkably in the study tively by abundant suckering from the root crown
area as regards the number of species, the number of (Weber 2003). On several of the islands investigated,
islands on which they occur and the overall level of this species has formed very abundant populations,
establishment (Tables 1–3), appears to be more typical such as those found in the La Maddalena archipela-
of the islands’ flora. This increase may, once again, be go on the north coast of Sardinia, particularly on the
due to the greater knowledge of introduced flora, since islands of La Maddalena and Caprera (17 and 18
recent advances in research on these groups in Italy in Figure 1) and on the island of Elba, where it has
(Guiggi 2008, 2010; Stinca et al. 2008; Piovan et al. become particularly invasive in rocky coastal habi-
2015) may have resulted in the identification of a larger tats, often in association with the almost ubiquitous
number of taxa. It is, however, undoubtedly also due (Table 1) prickly pear, i.e. Opuntia ficus-indica (Laz-
to the spread, reported by several sources in the study zaro et al. 2016). New species of the Agavaceae fam-
area, of several species of these genera (Domina et al. ily have recently established in a higher number of
2012; Ferretti et al. 2013; Lazzaro et al. 2013; Stinca islands (e.g. Agave sisalana Perrine ex Engelm., + 1