Questions: 1. Which habitats have the highest degree of inva- sion? 2. Do native species-rich communities have also a high degree of invasion? 3. Do the patterns of association between native and alien species richness vary between habitats. Location: Catalonia region (NE Spain).
Methods: We conducted a large regional analysis of 15 655 phytosociological relevés to detect differences in the degree of invasion between European Nature Information System (EUNIS) habitats representative of temperate and Mediter- ranean European areas.
Results: Alien species were present in less than 17 % of the relevés and represented less than 2% of the total number of species per habitat. The EUNIS habitats with the highest alien species richness were arable land and gardens followed by anthropogenic forb-rich habitats, riverine and lakeshore scrubs, southern riparian galleries and thickets and trampled areas. In contrast, the following habitats had never any alien species: surface running waters, raised and blanket bogs, valley mires, poor fens and transition mires, base-rich fens, alpine and sub-alpine grasslands, sub-alpine moist or wet tall-herb and fern habitats, alpine and sub-alpine scrub habitats and spiny Mediterranean heaths. There was a unimodal relationship between the mean native and mean alien species richness per EUNIS habitat with a high number of aliens in habitats with intermediate number of native species and a low number of aliens at both extremes of the native species gradient. Within EUNIS habitats, the relationship was positive, negative or non-signiﬁcant depending on the habitat type without any clear pattern related to the number of native species. Alien species richness was not related to plot size, neither between habitats nor within habitats.
Conclusions: The analysis emphasised that the habitats with a higher degree of invasion were the most disturbed ones and that in general habitats rich in native species did not harbour less invaders than habitats poor in native species.
Abbreviations: EUNIS = European Nature Information Sys- tem; UTM = Universal Transverse Mercator.
Introduction Biological invasions are threatening the conservation of native species and habitats worldwide. However, not all native species are threatened to the same degree by invaders and not all habitats are equally invaded (Lons- dale 1999). Habitat differences in the degree of invasion depend on alien species traits compared to native species, environmental and biotic characteristics of the recipient habitat and the propagule pressure with which alien spe- cies are entering into the recipient habitat (Rejmánek et al.2005).
Several studies have compared differences in the diversity of alien and native species within habitats at the landscape scale (Levine et al.2003) and in general have found that habitats with a high number of native species also harbour a high number of alien species (Stohlgren et al.1999; Stohlgren & Chong 2002; Brown & Peet
2003). This positive relationship can be explained by the similarity of both groups of species in the abundance of propagules entering a community (Levine 2000) or by both groups of species occurring in resource rich and moderately disturbed sites (Davis et al.2000). Most of the patterns have been observed in surveys conducted after a prioriveriﬁcation of highly invaded habitats, e.g. riparian habitats (DeFerrari & Naiman 1994; Planty-Tabacchi et al.1996; Stohlgren & Chong 2002). Consequently, they are probably biased towards immigration driven systems characterized by processes leading to resource release and entrance of new species through intermediate intensity disturbances (Brown & Peet 2003). Moreover, most surveys, even if they have been conducted at a large scale, are performed within habitat types (e.g. Gilbert & Lechowicz 2005), not verifying if there are differences in the degree of invasion between habitats (but see Stohlgren et al.1999). It is possible that in some habitats, especially those with low diversity, alien and native species respond differently to environmental and disturbance parameters (McIntyre & Lavorel 1994).
In this study we take advantage of the habitat clas- siﬁcation of the European Nature Information System
(EUNIS) developed and managed by the European Topic Centre for Nature Protection and Biodiversity (ETC/NPB in Paris), the European Environment Agency (EEA) and the European Environmental Information Observation Network (EIONET). This habitat type classiﬁcation is a comprehensive, pan-European system that covers all types of habitats from terrestrial to aquatic and from natural to artiﬁcial; URL: http://eunis.eea.eu.int/index. jsp . Therefore, we used EUNIS classiﬁcation to compare the relationship between alien and native species richness within habitats and between habitats. Our main questions were: 1. Which habitats have the highest degree of inva- sion? 2. Do native species-rich communities have also a high degree of invasion? 3. Do the patterns of association between native and alien species richness vary between habitats? Our main hypothesis was that habitats and plots with a large number of native species also have a large number of alien species, especially for highly disturbed habitats. To the best of our knowledge this is one of the largest regional analyses of native-alien richness associa- tion between and within habitats. Furthermore, it adds to the knowledge of the degree of plant invasion in Spain (Sanz-Elorza et al. 2004).
Material and Methods
Study area Catalonia (ca. 32 000 km2) is situated at 40°30' N - 42°40' N and 0°15' E - 3°15' E. This region was chosen because of its contrasting topography, climate, dominant vegetation and land use; altitudes range from
0 to 3350 m a.s.l. It receives Mediterranean, Atlantic and Saharan inﬂuences. Catalonia forms a boundary between two phytographic regions – the Eurosiberian and the Mediterranean. Rainfall decreases and mean temperature increases southwards. A continental gradient can also be observed from the coast, with moist temperate climates, to inland, with contrasting dry conditions (Ninyerola et al.2000).
The landscape structure of Catalonia reﬂects the typical secular interaction between man and climate in western Europe and the Mediterranean region. Forest currently occupies 40% of the region (Burriel et al.
2001). Broad-leaved forests (evergreen Quercusspp. in Mediterranean areas, deciduous Quercusspp. and Fagussylvaticain sub-Mediterranean and Eurosiberian areas) have been mostly substituted by coniferous forests (Pinushalepensis and P. pinea in Mediterranean areas and P.nigraand P.sylvestrisin sub-Mediterranean and Eurosiberian areas). In recent decades, abandonment of marginal agricultural areas is leading to a progressive afforestation challenged by an increasing wildﬁre fre-
quency. In the favourable plains and plateaux for human settlement progressive crop intensiﬁcation and urbaniza- tion have occurred. The central coast of Catalonia is one of the most populated and industrialised areas along the northern Mediterranean coast (Anon. 1995).
Species database The high phytogeographic diversity of Catalonia results in a rich ﬂora with more than 3200 species (Bolòs et al.1993). Due to a long tradition in botany many ﬂoristic records have accumulated, available in both published work (more than 500 references from journals, books, dissertations and local atlases) and unpublished information (mainly Ph.D. and M.Sc. the- ses). The FLORACAT project (Font & Ninot 1995) has been devoted to the gathering, organisation and online exploitation of these ﬂoristic data, with the agreement of the Global Biodiversity Information Facility (GBIF) effort (Edwards et al.2000), URL: http://biodiver. bio. ub.es/biocat/homepage.html and http://www. gencat. net/mediamb/pn/e-bdbiodiversitat.html
Presently, FLORACAT accounts for ca. 1 200 000 ﬂoristic records and 17 000 phytosociological relevés organised following the 10 km UTM grid. From the total of FLORACAT relevés, we selected 15 655 relevés with phytosociological assignment. They were used to calculate the number of alien, native and total species, and the percentage of alien species per relevé. A species was considered an alien if it originated in another region outside Spain and when it was introduced accidentally or deliberately by man. Only neophytes (i.e. introduced after the 15th century) were considered. Each relevé was assigned to a ﬁrst or second hierarchical level of EUNIS habitat classiﬁcation (Table 1) through the phytosocio- logical alliance it belongs to. The correspondence among alliances and EUNIS classes was established by expert knowledge and it is summarized in Table 2. Publication
Fig. 1. Study area with main land cover types (Anon.1993).
Table1.F-values and Correlation coefﬁcient (r2) and of the multiple regression number of alien species = number of native species
+ plot area for EUNIS habitats represented in the FLORACAT relevés of Catalonia (NE Spain). P-values for the number of native