As species richness increases, habitat filtering should constrain the expansion of total niche volume, while limiting similarity should restrict the degree of niche overlap between species. How the patterns of niche occupancy vary from species-poor to species-rich communities is a fundamental question in ecology that has a central bearing on the processes that drive patterns of biodiversity. The phytosociological and statistical analysis has led to the description of three new associations two of which are classified in the Oleo-Ceratonion alliance (Ruto chalepensis-Oleetum sylvestris and Chamaeropo humilis-Oleetum sylvestris), the third in the Erico-Quercion ilicis alliance (Calicotomo infestae-Oleetum sylvestris). The fieldwork is supported by 120 phytosociological surveys carried out in various areas of Sicily, which were statistically analyzed and compared with other published data from surveys of other communities that have been described for the Tyrrhenian (Sardinia and Corsica) and Balearic Islands up until now. Furthermore, the residual expressions of the Oleaster forests are limited, and often exist as regenerated woodland made possible by the abandonment of agricultural land they are often small forest nuclei вЂ“ high maquis, woods and micro-woods вЂ“ with a more or less discontinuous distribution within the agricultural landscape. In Sicily, Oleaster formations show considerable climacic potentiality in the bioclimactic belts between the infra- and the thermomediterranean with single edapho-climacic penetrations that are also in the mesomediterranean however, these were largely destroyed by man in order to make room for crops. sylvestris of Sicily and of the smaller minor islands. This paper presents a phytosociological study on the forest vegetation of Olea europaea var. The habitat-based approach proved to be suitable for evaluating the habitat specificity and frequency of alien species at a regional scale, improving the capacity for risk assessment in different ecological contexts. Some habitats are invaded by few (or single) species, which attain a high cover, transforming the whole ecosystem. It must be noted, however, that the number of alien species occurring in a given habitat does not relate to the severity of the impact of invasion in that habitat. Most of the habitats were colonized by very few alien species or completely lacked them, except for sandy coasts, naturally-disturbed riverbeds, and synanthropic habitats. The investigated habitats differed strongly in their level of invasion by alien species, ranging from 0 to 15.6% of aliens of all species recorded. Our analysis shows that annual and woody species are the most represented life forms in the alien flora of Sicily and that alien species tend to have a longer flowering period than the native species. The fidelity of alien species to individual habitats was calculated using the phi coefficient.

Representation of different life forms and phenological patterns was compared between alien and native species. The invasiveness of each species was expressed in terms of its absolute and percentage frequency. Vegetation plots were classified based on the EUNIS classification of European habitats.

We used a large dataset of plant species presences/absences in vegetation plots to analyze the invasion patterns across habitats considering biogeography, life form and phenology of alien plants. Here we compare the levels of vascular plant invasion across habitats and plant communities of Sicily. Levels of plant invasions in different habitat types were assessed in several regional studies, but few of them were from the Mediterranean.

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