Appendix S2 Methods and results of analyses testing whether distributions of species’
vegetation types are related to commonness and origin, and habitat variables.
Distribution of vegetation types for common and rare alien, and common and rare native
species
Common species in our dataset may be common because their suitable habitats are common,
whilst rare species may be restricted to habitats that are rare. To test this, we first obtained for
each of the species data on the vegetation types in which it occurs in Switzerland from
Landolt et al. (2010) and Delarze & Gonseth (2008). Then, using data on the distribution of
vegetation types in Switzerland (Hegg et al. 1993), we calculated for each species its potential
maximum distribution as the number of km2-grid cells in Switzerland known to contain at
least one of the vegetation types in which the species has been observed. Here ‘vegetation
type’ refers to a phyto-sociologically defined plant community (Ellenberg & Klötzli 1972;
Aeschimann et al. 2004). Some lakeshore vegetation types (Bidentalia tripartitae, Bidention
tripartitae, Phragmito-Magnocaricetea, Glycerio-Sparganion) were not mapped individually.
For species occurring in these communities, the maximum number of km2-grid cells with
lakeshore habitat in Switzerland was used. Data were available for 39 out of 41 species, (data
were not available for the alien species Rudbeckia hirta and Eryngium giganteum).
We used Kruskal-Wallis tests to analyse the differences in distribution of vegetation
types between alien and native common and alien and native rare species. One analysis
compared the number of km2-grid cells of alien versus native species, and a second compared
common and rare species. The number of km2-grid cells containing a vegetation type differed
significantly between alien and native species (χ2 = 7.282, P = 0.007); alien species had a
median of 14,522 km2 cells containing a vegetation type compared to a median of 3,025 km2
cells for native species (Fig. S1). Similarly, common species also had significantly more km2grid cells containing a vegetation type (median = 11,819 km2) than rare species (median =
3,025 km2; χ2 = 6.938, P = 0.008; Fig. 1). Thus, common and alien species occur in vegetation
types that are more widespread than the vegetation types containing rare and native species,
respectively
Fig. S1 The number of km2-grid cells in Switzerland containing at least one vegetation type
for species which are alien and common, alien and rare, native and common and native and
rare in Switzerland (n=39). Bold line indicates median number of km2-grid cells per group;
box represents 25%-75% interquartile range; whiskers encompass 1.5 times the interquartile
range.
Distribution of vegetation types in relation to habitat variation and environmental indices
Differences in habitat associations and environmental conditions among species may result in
more widespread habitats being available for some species compared to other species with
differing requirements. To assess whether species habitat differences were associated with
distribution of vegetation types (i.e. whether certain environmental conditions are more
common), we used Kendall’s tau correlation on the number of km2-grid cells containing at
least one vegetation type per species and the environmental variable indices. We also used
Kendall’s tau correlation to test the association between the distribution of vegetation types
per species and the NMDS axes describing habitats.
The number of km2-grid cells in Switzerland containing at least one vegetation type
per species was positively associated with the soil nutrient and hemeroby indices, but was not
significantly associated with any of the other environmental indices (Table S1). The three
NMDS axes were not significantly correlated with the distribution of vegetation types (Table
S1). Therefore, whilst species occurring in nutrient-rich, human-modified habitats may have
more widespread vegetation types, habitat dissimilarity of species overall did not appear to be
strongly related to the potential distribution of species. In other words, it is unlikely that in our
study habitat dissimilarity resulted in common species occurring in more widespread
vegetation types than rare species).
Table S1. Kendall’s tau correlation coefficients for relationships between the number of km2grid cells in Switzerland containing at least one vegetation type per species, values from the
Flora Indicativa, and the NMDS axes 1-3 for the 39 species with available data.
No. km2-grid cells containing at least
one vegetation type
NMDS1
0.048 ns
NMDS2
0.189 ns
NMDS3
-0.210 ns
Temperature
-0.029 ns
Soil nutrients
0.302 *
Light
-0.129 ns
Soil moisture
-0.164 ns
Hemeroby
0.312 *
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Delarze, R. & Gonseth, Y. (2008) Lebensraüme der Schweiz: Ökologie-GefährdungKennarten. 2nd edition. Ott-Verlag, Bern, Switzerland.
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