Considerazioni sui popolamenti di uccelli terrestri delle Isole Mediterranee

Abstract

BIOGEOGRAPHIA — vol. XI - 1985 Principi e metodi delfanalisi biogeografica Considerazioni sui popolamenti di uccelli terrestri delle Isole Mediterraneei‘ BRUNO MASSA Istituto di Zoologia dell’Um'z2er5z'tz2, via Arc/9z'rafz' 18, 90123 Palermo (I) SUMMARY Comideratlons on the breeding land bird species of tlae Meditemmean Islands. The author first consid- ers some problems of the biogeographical studies about islands (exploration of islands, extinctions due to the human presence, and so on); then he explains the results obtained by the bibliographic analysis and personal explorations in the Mediterranean Islands (Tabs. I and II). The species of steppe are more abundant in the most eastern Islands (Crete and Cyprus), while the lowest values for the species of forest refer to Balearic Islands and to Crete, the less tirnbered islands (Tab. III). The avifaunistic poorness follows a typical insular state along ecological successional stages: after an increase of the number of species in the shrub stages, there is a decrease in the copse and in the old forest (Tab. IV). Then he considers some problems concerning the area-species ratio: the different slope, according to the size of the islands, the influence that physical conditions and isolation can exert on the number of the species. Insularity, meant as impoverishment of species, is sometimes a gradual phenomenon, due to the gradual impoverishment on the peninsulas: e.g.: Crete, that represents the top of the Balckan peninsula, and Sicily that concludes the Italian peninsula (Tab. V). The author considers also other parameters which influence the number of species, as space heterogeneity and the complexity of food chaines (this one is valid only for groups of numerous species: Tabs. VI and VII). Interspecific competition is indirectly examined, supposing that species belonging to the same genus have greater possibilities of overlapping resources. Genera with many species are frequent only in the isles with a high richness of habitat and then of species (Tab. IX). Wideness of niche realised by some species of genus Sylvia is also examined by the parameter habitat (Tab. VIII). Also the average weight of species follows an insular trend (Tab. X), but the most important problem to be examined during an insular study on terrestrial avifauna is the different dispersal power of each species. This has been calculated by the exam of the presence or absence of the species in the 37 examined islands, so that it is possible to obtain a classification of this potential power (Tabs. XI and XII, figs. 1, 2, 3). The species with the lowest dispersal power seem to be present only in the greatest islands. At last, the particular situation of Calabria is considered. It seems to be an island, as for the number of species (in total and along successional ecological stages), but the the number of species along the successional stages shows a continental trend, with a regular increase up to the old forest (Tab. XIII). 1. PREMESSA Le isole mediterranee sono un esempio atipico di isole. Certamente non isolate, in nessun caso eccessivamente distanti dalla terraferma, queste isole si trovano in un bacino circondato da continenti, da cui e possibile una conti- (*) Lavoro realizzato con un finanziamento del Ministero Pubblica Istruzione (60%, 1985, B. Massa). 261