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Non-native plant species in Mediterranean islands 2561

Mediterranean (Fig. 1). The small islands were national scale, by a team of experts from every
defined, according to Hess (1990), as islands that are administrative region in Italy, who used a common
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smaller than 10,000 km in size and have a resident approach and methodology and placed particular
population of less than 500,000 inhabitants. Although attention on islands, coasts and main urban areas
Sicily and Sardinia were consequently excluded on (Celesti-Grapow et al. 2009). While conducting this
account of their size, their non-native flora had, in any work, the information in the original database was
case, previously been examined in an analysis that reviewed and partially updated.
dealt with the determinants of the number of non- For the purposes of our analysis, information on
native species in the Italian administrative regions residence time was used to separate archaeophytes, i.e.
(Celesti-Grapow et al. 2010). those species introduced into Italy before 1500 AD,
The majority of the islands surveyed are smaller from neophytes. In view of the differences in ecolog-
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than 60 km and are situated relatively close to the ical requirements, habitat preferences and invasion
mainland, lying within 50 km of the coastline dynamics that have often emerged between ancient
(Table 1). Despite differences in their lithological and recent introductions (Pys ˇek et al. 2005), in this
substratum, these islands share a number of common study we chose to consider neophytes alone, which are
traits; for instance, many of them display marked hereafter referred to as non-native or introduced
topographic variation (e.g. the highest peak on the species. Indeed, the response of archaeophytes to
islands of Elba is over 1,000 metres) and prevalently environmental factors is often more similar to that of
rocky shores. Although all the islands belong to the native species than to that of neophytes (Celesti-
Mediterranean climatic region, slight variations in Grapow et al. 2004, 2010), which has in turn resulted
temperature and annual rainfall exist along the north– in archaeophytes generally being excluded from
south gradient. management programs. We assessed the status of
The history of human presence on these islands each non-native species on each island independently,
varies considerably. While some were colonized by dividing taxa into casuals and established (natural-
small groups of people in the Neolithic period, many ized), according to Pys ˇek et al. (2004). The group of
have witnessed repeated invasions throughout history, established species includes several invasive species,
which often resulted in the islands’ resources being which is a term used here to indicate naturalized plant
exploited, through agriculture or mining, long before species with a high rate of spread, with no reference to
the first permanent settlements (Grove and Rackam their socio-economic or ecological impact (Pys ˇek
2001). Indeed, many of the islands were so intensely et al. 2004).
cultivated and modified in the past that wooded areas
now cover a far smaller proportion of the island area Islands’ characteristics
than they once did, or are represented by non-native
plant species plantations (e.g. Acacia sp.pl., Ailanthus For each island, we collected 14 explanatory variables
altissima (Mill.) Swingle, Eucalyptus sp.pl., Robinia belonging to three different groups: four geographical
pseudoacacia L.). Some of the islands, such as variables (latitude, longitude, distance from the main-
Gorgona, Pianosa, Santo Stefano and Asinara, have land and distance from the nearest island), five
been used as prisons. Currently, all the islands are environmental variables (area, perimeter, maximum
subjected, even if in varying degrees of intensity, to elevation, percentage of volcanic lithology and percent-
the impact of tourism. age of sedimentary lithology) and five human-related
variables (human population density, percentage of
Species database island area covered by artificial surfaces, percentage of
island area covered by agricultural surfaces, percentage
Information on the occurrence and invasive status of of island area subjected to environmental protection
the non-native plant species on the 37 islands was measures and tourist pressure). Climatic variables and
extracted from the national non-native flora database, other human-related variables known to influence non-
which was built in the years 2005–2008 as part of the native species richness (e.g. the density of roads or the
project ‘‘A survey of the non-native flora of Italy’’. The GDP) were not included in the analyses because data
database was first compiled, on both the regional and were not available for a sufficient number of islands.

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