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2560 F. Pretto et al.

generalizations in invasion ecology, remains contro- (Duarte et al. 2005; Kallimanis et al. 2010; Kreft et al.
versial. A high proportion of non-native plant species 2008). By contrast, relatively few papers have verified
have been observed in the flora of temperate and whether the factors that influence native plant species
tropical oceanic islands (Pys ˇek and Richardson 2006), also affect the diversity and the composition of
though the long-term consequences of such invasions introduced plant species. Moody (2000) demonstrated
on native communities are not clear yet (Sax and that the number of introduced species in the Channel
Gaines 2008). Chytry ´ et al. (2008) suggested that Islands (California) is primarily influenced by island
invasion risk assessment should be based on habitat area, while McMaster (2005) highlighted the effect of
types rather than other factors such as insularity status, both area and population density on the number of
and Vila ` et al. (2010) found that plant communities in non-native species on 22 islands off the Atlantic coast
the Balearic Islands (Spain) display a lower level of of the United States and Canada. Long et al. (2009),
invasion than their continental counterpart. Although who studied the relationship between non-native and
there is so far little evidence of extinctions directly native diversity on 25 islands in Boston Harbor, found
caused by introduced plant species on islands (Sax and a differential effect of isolation which, while being
Gaines 2008), insular biota are considered to be detrimental to the richness of both groups, was
particularly vulnerable to the deleterious effects significantly weaker in the case of non-native species;
caused by the spread of non-native taxa (Reaser indeed, the proportion of non-native plant species in
et al. 2007). One of the consequences of the introduc- the total flora in their study increased in proportion to
tion and the stabilization of non-native species, the distance from the mainland. Lastly, Kueffer et al.
together with the potential loss of native endemics, (2010) observed a consistent increase in the number of
is the increasing similarity of species assemblages in invasive plant species in oceanic islands on which
space, a phenomenon known as biotic homogenization habitat diversity co-occurred with economic develop-
(La Sorte and McKinney 2007; Stohlgren et al. 2011) ment. In this paper, we deal with human determinants
which has been extensively investigated for oceanic of non-native plant species richness and composition
islands (Castro and Jaksic 2008; Castro et al. 2010). on Mediterranean islands.
When compared with other areas with similar Island size is closely related to the island’s suscep-
climatic conditions, the Mediterranean Basin is char- tibility to environmental degradation. Moreover the
acterized by a very high number (about 5,000) of flora of small islands may display distinctive features,
islands that differ in terms of environmental features hosting endemic, rare or endangered species that are
(e.g. size and geological origin), time elapsed since absent on larger islands nearby (Blondel et al. 2010).
colonization by humans and intensity of current The study of plant species that have been introduced
human activity. As a whole, these islands are recog- on small islands and of the mechanisms that underlie
nized not only as hotspots of biodiversity worldwide, their establishment might, therefore, make a major
but also as areas that support highly sensitive ecosys- contribution within a more general perspective of
tems (Me ´dail and Que ´zel 1999). In this regard, biodiversity conservation in the Mediterranean Basin.
biological invasions are considered to pose one of In the present study, we examined the non-native
the main threats to the conservation of biodiversity of flora of 37 small Italian islands to identify the main
Mediterranean ecosystems (Underwood et al. 2009). geographical, environmental and human-mediated
Moreover, climate projections for the Mediterranean factors that drive non-native plant species richness
Basin predict conditions of greater aridity and higher and composition on small Mediterranean islands.
temperatures, that can be emphasized by the current
intensity of human activities (Gritti et al. 2006). These
changes might alter the distribution, physiology and Methods
phenology of native plant species, with potentially
negative effects on their ability to compete with Study area
introduced taxa (Thuiller et al. 2008).
Several papers have focused on the effects of The 37 small islands are located along the Tyrrhenian
islands’ characteristics on the diversity and composi- coast of Italy and around the islands of Sicily and
tion of the native plant communities they support Sardinia, which are the two largest islands in the

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