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1126 L. Celesti-Grapow et al.

species on each island does not allow us to assess the pycnantha have progressed to a subsequent stage of
relative contribution of these two possible sources the invasion process (Table 3). This general process
of the new species, we believe that new introduc- has been fairly fast. For example, on the southernmost
tions contribute far less to the increase in the num- islands in the study area, i.e. Linosa and Lampedusa
ber of non-native species than the recent escape of (36 and 37 in Figure 1, Pelagie islands, Sicily), the
species that were already present. Besides, owing to number of seedlings, saplings and young trees of A.
time lags between each stage of the invasion process cyclops, which was only recorded as established in
(Essl et al. 2011; Rouget et al. 2016), the progres- Italy in 2012, has increased significantly in recent
sion from the introduction of a new species to the years. This Australian species is considered to have
subsequent processes of spontaneous reproduction, been introduced on these islands by migratory birds
establishment and spread would most likely require from the nearby northern coasts of Africa (Pasta
a longer time span than the time lapse between our et al. 2012). The general expansion of acacias is also
surveys. clearly exemplified in the Tuscan archipelago, where
In addition to the increase in the total number A. melanoxylon and A. saligna were recently recorded
of species, a second finding indicating that the phe- for the first time, while A. dealbata and A. pycnantha
nomenon of plant invasion has progressed quickly in have spread to a larger number of islands and have
recent years is the high number of taxa (87 species) become particularly invasive, especially on the island
whose distribution has extended within the study of Elba (4 in Figure 1). Recent studies conducted
area. This means that more than half of the species on this island, the largest in the study area, have
that were recorded on at least one of the 37 islands shown that these species form extensive and dense
during the previous survey are now found on a larger monospecific populations that are expanding rapidly,
number of islands (Table 2). Further evidence lies in are reducing biodiversity and are causing detrimental
the increase in the overall level of establishment, as impacts on the invaded ecosystems (Lazzaro et al.
demonstrated by the 24 species that have progressed 2013, 2014b, 2015, 2016; Giuliani et al. 2016). Several
within the study area to a subsequent stage of the projects are under way to manage these invasions in
invasion process, such as 15 casual species that have the archipelago; the populations of A. pycnantha and
established or even become invasive (Table 3). A. saligna are being monitored on all the islands and
eradication projects (LIFE08 NAT/IT/000353) for
these two species have been launched on the island
of Pianosa (5 in Figure 1).
Most prominent plant taxa and major environmental
threats These results reflect a more general trend in
Mediterranean Italy, on both the mainland and the
One noteworthy example of the taxa that have dis- largest islands, where a number of Acacia species have
played an overall higher rate of escape, establishment been used for various purposes, e.g. as ornamentals,
and spread is the genus Acacia (Figure 3; Tables in forestry, for soil stabilization and as wind breakers.
1–3), a group of the Fabaceae family that includes Despite being described as species that seldom
some of the leading plant invaders in the world escaped cultivation in the country territory until
(Richardson & Rejmánek 2011; Rouget et al. 2016) the second half of the twentieth century (Viegi et al.
and some of the most invasive species in coastal habi- 1974; Viegi & Cela Renzoni 1981; Pignatti 1982),
tats in Europe (Lorenzo et al. 2010; Marchante et al. some Acacia species have more recently established
2015). Once they have established in the new terri- and spread spontaneously from their planting sites
tory, some of these fast-growing woody species form both by seed and by vegetative reproduction (Conti
dense, impenetrable thickets that compete for space, et al. 2005; Celesti-Grapow et al. 2010b). In southern
water and light with other plants, thereby displacing Italian regions, where the climate is Mediterranean,
the native vegetation and reducing invertebrate and a few of these species, particularly A. saligna, are
plant species richness (Wilson et al. 2011). regarded as successful and persistent invaders with
Six species of Acacia were recorded in the study a highly negative impact on the environment and on
area (without counting two close entites now included biodiversity, especially in coastal sand dunes (Bernardo
in the genus Vachellia, i.e. V. farnesiana (L.) Wight & et al. 2010; Medagli et al. 2010; Del Vecchio et al.
Arn. and V. karroo (Hayne) Banfi & Galasso). Two of 2013) and woods (Fascetti 2010). In Sardinia, invasive
these six species, i.e. A. cyclops A.Cunn. ex G.Don acacias are among the species that cause the most
and A. provincialis A.Camus, are recent additions to serious impacts on soil characteristics and biodiversity
the flora in the study area. Besides, the presence of all (Camarda et al. 2010; Meloni et al. 2015).
the previously listed species of this genus has recently Although the effect of climate change on the
extended to other islands, with A. saligna becoming spread of acacias in the Mediterranean is still debated
one the most frequent species on all the islands (Tables and the results of studies on this topic have been con-
1 and 2), while A. dealbata, A. melanoxylon and A. trasting (González-Muñoz et al. 2014), the distribu-