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The vascular flora of the satellite islands of Sicily

Geo-pedological variability
Another important consequence of the rather complex geological history
of Sicily is the diversity of rock outcrops, which in turn involve a high variety
of soil types. On the circum-Sicilian islands, the most common geological
substrata are: 1) sandy or compact limestones/dolomites (which cover most
of Lampedusa, Lampione, Egadi islands, and Stagnone islands and also all the
minor W, NW, and SE islets); 2) base-rich or acid vulcanites (which cover Linosa,
Pantelleria, Ustica, the Aeolian Islands and islets, Scogli dei Ciclopi, and part of
Capo Passero islet); and 3) marls (which cover part of Lampedusa). Calcareous
islands share the same pedological pattern: uneven soil depth, scattered
distribution of soil and rock outcrops, and a high variety of soil assemblages.
The co-occurrence of the latter three factors seems to enhance species richness
(Pasta, 1997) by multiplying the niches available to root systems (Figure 2).
The low plant species richness recorded on some volcanic islands (e.g., Linosa,
Vulcano, and Stromboli) may be due to their rather recent emersion and/or
to the frequent disruptive events, so that they are not yet saturated in terms
of species richness, which is consistent with the basic principles of island
biogeography (MacArthur & Wilson, 1963).

Fig. 2. The pattern of correlation between α- (i.e., species-richness) and β-diversity (i.e.,
number of suitable ecosystems) appears to be rather regular and distinct for volcanic and
calcareous islands.

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