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1120 L. Celesti-Grapow et al.

Owing to their considerable biomass and potential et al. 2015; Peruzzi et al. 2015). Some are also used
to deeply transform the biotic and abiotic nature of as major stop-over sites by migratory bird species
the environment over extensive areas, invasive plants (Spina & Volponi 2008); others are places of out-
may alter ecosystem structure and functions by in- standing cultural or natural importance that are
ducing changes in chemical, physical and structural listed among the UNESCO World Heritage Sites
ecosystem characteristics (Richardson et al. 2000). (WHC 2016).
They may, for instance, act as sand stabilizers, ero- A general survey of the non-native plants of
sion promoters and litter or salt accumulators, and Italian islands was carried out for the first time as
modify nutrient and water cycling or disturbance part of the national project on the non-native flora of
regimes (Pyšek & Richardson 2010). These modifi- Italy in the years 2006–2007 (Celesti-Grapow et al.
cations are often irreversible and may result in the 2009, 2010a, 2010b), followed by an overview of the
degradation of habitats, changes in community com- main patterns of plant invasion on small islands pro-
position and the decline of pre-existing populations vided by Pretto et al. (2012). These works have drawn
(Blackburn et al. 2014). attention to the threat posed to the conservation of
However, the relevance of this phenomenon de- the natural habitats of these islands by the spread of
pends on the invaded habitat. Indeed, the majority invasive plants and have highlighted some critical
of the introduced species establish either entirely or situations, particularly on small islands, which are
prevalently in sites that have been markedly trans- highly vulnerable to biodiversity loss owing to
formed by man, and that are therefore generally of their size (MacArthur & Wilson 1967). A growing
little conservation value, such as road sides and an- awareness of these environmental risks, which are
thropogenic habitats (Pyšek et al. 2010). By contrast, due above all to the marked environmental trans-
when introduced species establish in natural areas, formations taking place in the region following the
particularly in the vicinity of populations of high abandonment of agriculture and the expansion of
biogeographic and conservation value, they pose a tourism, has led to more research being conducted in
greater danger as their spread may threaten the pres- these areas. New projects have been launched lately
ervation of local, and even global, biodiversity. A case both on single islands and in the archipelagos lying
in point is the small islands in the Mediterranean off the Italian coasts, and extensive survey campaigns
Basin. have been carried out (e.g. Domina & Mazzola 2008;
Ever since the earliest studies by Elton (1958), Lazzaro et al. 2014a; Del Guacchio 2015; Camarda
islands, and particularly oceanic islands, have been et al. 2016; Stinca et al. 2016).
considered to be particularly vulnerable to the im- Following this recent progress, the need arose to
pacts of invasive species. Since then, a large body of gather a new comprehensive data-set to document
literature has repeatedly shown that biological inva- plant invasion patterns on these islands and to use
sions have become one of the main drivers of habi- as a fundamental scientific tool to draw up coordi-
tat modifications and a leading cause of biodiversity nated management strategies both on a national and
loss in island ecosystems worldwide (Lonsdale 1999; European scale (Brunel et al. 2013). To address
Reaser et al. 2007; Kueffer et al. 2010). Recent ad- this need, we compiled an up-to-date inventory of
vances in plant invasion research in Europe and the non-native vascular plant species that occur sponta-
Mediterranean Basin have shown that the spread of neously on the numerous islands that surround the
invasive species also poses a serious environmental Italian peninsula.
threat on Mediterranean islands because they are hot On the basis of the results of this work, the main
spots of biodiversity and are rich in rare habitats and aim of this paper is to provide an overview of the
endemic species (Vilà et al. 2006b; Lambdon et al. present-day introduced vascular flora on small Med-
2008a, 2008b). Indeed, the islands in the Mediter- iterranean islands based on a sample of 37 islands lo-
ranean Basin are considered to be among the eco- cated off the coast of Italy. By comparing the current
systems in which biodiversity is threatened most by data with those gathered during the previous sur-
drivers of global change, particularly by invasive spe- vey conducted in the same study area (Pretto et al.
cies (Sala et al. 2000; Genovesi & Carnevali 2011; 2012), we also aim to highlight the main changes
Brundu 2013). that have occurred in species diversity, distribution
Numerous islands and archipelagos surround and establishment status in recent years. Lastly, we
the Italian peninsula, including Sicily and Sardinia, aim to present a first general overview of the most
the two largest islands in the Mediterranean; many prominent plant taxa in the island’s introduced flora,
of these are important sites for the conservation focusing on those most responsible for these chang-
of biodiversity as they contain protected habitats es and those that pose the greatest environmental
and rare, endangered and endemic species (Fenu threats.