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clonal populations located in the Hyblaean district and only occurring along rivulets and sea-
sonal streams. Although the genus Zelkova belongs to the mesothermic deciduous tree flora
which gradually disappeared in the whole Europe during Pliocene and Pleistocene, the Sicili-
an species currently grows on basaltic lavas dating back to c. 3 to 1.7 Ma, so it may have colo-
nized the area afterwards, perhaps exploiting the cooler phases during glaciation events.
A regional phytogeographic partitioning of Sicily based on the distribution of differential
taxa (narrow endemics and exclusive species) has been published by BRULLO et al. (1995),
who were able to identify as many as 15 districts. Table 1 gives an idea of the floristic origi-
nality of the districts represented by the small archipelagos around Sicily (Fig. 1). Besides
local endemics, also broad ranging taxa are listed which, in the Sicilian context, only occur in
a single microinsular district. These latter are grouped as “Exclusive non endemic taxa”

2. Climate variability and heterogeneity
The Mediterranean climate is not only depending on the latitude, but also on the cyclonic
circulation of the oceanic air masses, whose position is ranging year by year. Annual climatic
variations and major climatic changes may locally lead to the severe reduction and splitting
of plant populations. Several noticeable disjunctions in the distribution ranges of plant species
growing in the Sicilian dry grasslands can be attributed to these climatic changes: for exam-
ple, the disjoint populations of Heteropogon contortus, Artemisia alba, Sesleria nitida subsp.
sicula, Koeleria splendens, Helictotrichon convolutum, have probably reached the island
during the dry interglacial periods of the Pleistocene. Similarly, during the coldest phases of
Pleistocene, many taxa reached the mountains of northern Sicily, exploited their most suitable
microhabitats as refugial areas, and eventually begun to differentiate on site. This is the case
of Allium ursinum, Aquilegia vulgaris subsp. sicula, Betula aetnensis, Circaea lutetiana, Epi-
lobium angustifolium, Fagus sylvatica, Lonicera xylosteum, Populus tremula, Ribes uva-cri-
spa, Sanicula europaea, and many other species typical to the Sicilian beechwoods (BRULLO
et al. 2012). So, the Pleistocene glaciations affected the Sicilian flora in at least two ways: (I)
they triggered floristic richness and heterogeneity in the mountain stands (but not on the top
of the highest ones, where periglacialism occurred repeatedly), whilst (II) sea level lowering
promoted not only the migration of Balkan species such as Carpinus orientalis, Cercis sili-
quastrum, Fritillaria messanensis, Nectaroscordum siculum, Ostrya carpinifolia, Paliurus
spina-christi, (PEZZETTA 2011) through S Italy thanks to the regression of the Adriatic Sea
but also some floristic exchanges between Sicily and northern Africa through the Sicilian
Strait (LAMBECK et al. 2014), like in the case of Pinus pinaster subsp. escarena in Pantelleria
and Calendula incana subsp. maritima along the western shores of the main island (even if,
for this species, an human introduction to Sicily cannot be excluded, see PASTA et al. 2017).
Millennial climatic trends are associated, as well, to remarkable year-to-year fluctuations
of rainfalls and temperatures, that are affecting Mediterranean climates more than any other
climatic type worldwide (DONLEY et al. 1979, HOFRICHTER et al. 2001). These fluctuation
could explain the general evolutionary tendency of many Mediterranean plants towards the-
rophytism (MOSSA et al. 2004), rewardless species-specific pollination strategies (DAFNI
1987), and high investment in seed productivity (WELLS 1969). Obligate seeders have greater
numbers of sexually produced generations, resulting in greater genetic recombination, which
in turn contributes to more rapid speciation. This could explain why the Mediterranean region
has such an high diversity of plant and insect species (MÉDAIL & QUÉZEL 1997). We know
that insects are quite precise in their flower-visiting habits and in the Mediterranean dry grass-
lands several examples of co-evolution between flowers and insects are known, especially

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