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Regional assessment of plant invasions across different habitat types Vilà, Montserrat1*; Pino, Joan

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Fig. 2. Plot sizes (mean + SD) in relevés grouped according to main EUNIS habitats in Catalonia. See Table 1 for EUNIS type classification. Different letters above columns indicate significant differences among habitats according to pair-wise Bonferroni tests.

(mean ± SE, 20.87 ± 0.09 and 20.47 ± 0.22 respectively, t-test = 1.76, p = 0.08). Similarly, habitats which were never invaded do not harbour more native species than habitats where invasions occur (mean ± SE, 17.84 ± 1.7 and 17.71 ± 1.4 respectively, t-test = 0.052, p = 0.96). The same lack of association was found when plots or habitats without aliens were excluded from the analysis. The relationship between mean native and alien species richness per habitat type appears to be unimodal (hump-

In contrast, the number of native species per relevé was 20.80 ± 0.09 (mean ± SE) ranging from 1 to 102 species per plot. As expected, there were also significant differences in native species richness among habitats (χ2

= 5886.63, p < 0.0001). The habitats with the highest native richness were different from the ones with high- est alien richness: dry and mesic grasslands (E1 and E2, respectively), broad-leaved deciduous woodlands (G1), garrigues (F6), and coniferous forests (G3) (Fig. 3). The habitats with the lowest number of native species were inland surface water habitats (C1, C2 and C3), inland saline and brackish marshes and reedbeds (D6) and inland saline grass and herb dominated habitats (E6) (Fig. 3).

There was not a significant linear relationship be- tween mean native and mean alien species richness per habitat type even when we included plot area in the analysis (F = 0.339, p = 0.715). Plots without alien species were not more native species-rich than plots with alien species

Fig. 3. Native and alien species richness (mean + SD) for dif- ferent EUNIS habitats in Catalonia. See Table 1 for EUNIS type classification. Different letters above columns indicate significant differences among habitats according to pair-wise Bonferroni tests.

Fig. 4. Percentage of alien species per plot (mean + SD) for different EUNIS habitats in Catalonia. See Table 1 for EUNIS type classification. Different letters above columns indicate significant differences among habitats according to pair-wise Bonferroni tests.

shaped), with the highest number of alien species at intermediate values of native species richness and the lowest number of alien species at both extremes of low and high native species richness (Fig. 5).

For the 24 invaded EUNIS habitat types we found a positive association between native and alien species in seven types, a negative association in seven and a non- significant relationship in ten types (Table 1). There were non-significant differences in mean plot size between those habitats with positive, negative and non-significant alien-native relationships (Kruskall-Wallis, H = 3.16, p

= 0.206).

Fig. 5. Relationship between mean (± SE) number of native and mean (± SE) alien species for EUNIS habitats in Catalonia. See Table 1 for EUNIS type classification.

The mean number of alien species was low (represent- ing less than 2% per relevé) matching results for similar phytosociological database analysis (Rejmánek et al.

2005; Chytrý et al. 2005). For Catalonia, alien richness analysis conducted at the scale of UTM grid squares of floristic mapping has found larger percentages (Pino et al. 2005). Similarly, local field surveys conducted in several regions of Spain have found percentages of alien species higher than 10% probably because field surveys were deliberately biased towards highly invaded areas such as riverine (Sobrino et al. 2002) or coastal habitats (Campos et al. 2004). We are confident that this low value of alien representation in relevés is not due to under-representation of relevés with high number of aliens because in Catalonia there is a strong tradition of vegetation research in anthropogenic habitats (Masalles et al. 1997; Casasayas 1990). There is not a trend towards low sampling of relevés with few aliens. In fact, there is not a significant relationship between the number of relevés per habitat type and number of alien species per relevé (r2 = 0.002, p = 0.808). Furthermore, the analysed database is extensive enough (15 655 phytosociological relevés across a 32000-km2 area) to be certain that it is representative of all vegetation types of the region. As in other European phytosociological surveys, sampling was conducted in sites where there is a high probability of including presumed diagnostic species (Chytrý 2001).

The total lack of alien species in certain habitats such as in sub-alpine and alpine habitats mirror observation analysis in which there is a decrease of alien species richness with altitude and low temperatures (DeFerrari

& Naiman 1994; Pyšek et al. 2002; Pino et al. 2005). This negative correlation does not necessarily imply a

causal relationship between temperature and alien spe- cies richness, or with components of invasibility (i.e. community susceptibility to invasion) at high elevations but could also be related to lower propagule pressure in such habitats due to remoteness and low human activi- ties (Pyšek et al. 2002). In contrast, the habitats with the highest frequency and number of alien species are anthropogenic habitats such as agricultural, ruderal and trampled areas together with riparian habitats, all of them being frequently disturbed areas (DeFerrari & Naiman

1994; Planty-Tabacchi et al. 1996) with usually a high propagule pressure due to their closeness to urban areas and communication networks.

Contrary to expected, we did not find a positive association between native and alien species richness between EUNIS habitats. The relationship appears to be unimodal, indicating a high number of aliens in habitats with intermediate number of native species and low number of aliens at both extremes of the native species gradient. The relationship might be better viewed as an area below an upper boundary of an envelope filled with data points than a line of fitted values. This type of association mirrors observational patterns of plant species diversity-productivity relationships when data from different habitats along a productivity gradient and a broad range of variation in species richness are analysed (Mittelbach et al. 2001). In fact, our analysis encompasses the whole range of local variation in alien and native species richness across habitats within Catalonia.

Within EUNIS habitats, the native-alien species richness relationship was positive, negative or non-sig- nificant, independent of plot size. This result is consist- ent with studies conducted in other regions which have also found that the relationship between native and alien species depends on the vegetation type (Planty-Tabacchi et al. 1996; Stohlgren et al. 1999; Brown & Peet 2003; Cully et al. 2003); emphasising that hot spots of native plant diversity are not immune to alien species invasion (Stohlgren et al. 1999; MacDougall & Turkington 2005). Furthermore, correlation values, even if significant, were very low indicating that at the local scale alien species richness can not be predicted by native species richness (MacDougall et al. 2006). Habitats with low alien species richness could be independent of native species richness but might reflect environmental constraints as well as dispersal limitations (MacDougall & Turkington 2005). Observations are not tests of causality. The mechanisms underlying the relationship between native and alien species richness across and within habitats could only be elucidated with large-scale, long-term experimental manipulation of plant species richness and environmental limiting factors.

By analysing thousands of phytosociological relevés expanding a broad range of habitats, we found the per- centage of alien species to be low indicating that at the local scale, and when floristic surveys are not biased towards sampling the most invaded habitats, aliens are not very common, compared to values found at the re- gional scale, where the percentage of aliens comprise a large amount of rare aliens increasing the total diversity of the flora (Sax & Gaines 2003).

As previously stated, the habitats with a higher degree of invasion were the most disturbed (e.g. riparian) and anthropogenic (e.g. agricultural, trampled). Contrary to our expectations, these habitats were not the most native species rich. In fact, patterns of association between na- tive and alien species richness were highly idiosyncratic within habitats emphasising that native species richness is not a good indicator of the degree of invasion, and that alien species richness is probably more dependent on environmental and invasion event factors (e.g. propagule pressure, residence time) than on biotic factors.

Acknowledgements. We thank D. Sol for statistical advice, M. Sanz-Elorza, H. Bruelheide and an anonymous reviewer for comments on an early version of the paper. This study was partially financed by the European Commission VI Framework Programme project ALARM (Assessing Large scale environ- mental Risks for biodiversity with tested Methods-contract GOCE-CT-2003-506675) and The Ministerio de Ciencia y Tecnología project ʻDeterminantes biológicos del riesgo de invasiones vegetalesʼ (RINVE).

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Received 15 March 2006; Accepted 23 July 2006;

Co-ordinating Editor: H. Bruelheide.
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