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Traits Associated with Invasiveness in Alien Plants: Where Do we Stand? 107
alien flora is beyond the researcher’s control, as she/he must rely on data that
have been collated by others.
The size of the region needs to be appropriate for the questions asked, but
larger areas (>100,000 km 2 , the scale of political regions) are preferable, since
species inventories are usually compiled for political regions, are biogeographically
arbitrary, and are repeatable (see discussion in Cadotte et al.
2006b). The vast majority of studies (16 in Table 7.2) were conducted at the
regional scale, with only a few (Thébaud and Simberloff 2001; Sutherland
2004) addressing the problems at a continental scale. The study by Hamilton
et al. (2005) is the only one that compares the effect of studied variables
between regional and continental scales. In most studies, cross-species comparisons
treating species as independent data points were conducted without
explicit consideration of phylogenetic relatedness among species (Cadotte et
al. 2006b). However, incorporating phylogenetic information can elucidate the
extent to which changes in invasiveness may be correlated with changes in
other traits through a particular phylogeny (Harvey and Pagel 1991; Cadotte
et al. 2006b). Using phylogenetic corrections may, or may not provide different
results (Harvey et al. 1995). However, the same results with and without
using phylogenetic corrections indicates that throughout the phylogeny of
alien species there have been multiple and independent correlated evolutionary
divergences between invasion success and the trait examined (as found,
e.g., by Hamilton et al. 2005 for seed mass and specific leaf area). The list of
studies that used phylogenetic corrections (coded F2 in Table 7.2) clearly
indicates that the frequency of its application has been increasing recently,
presumably with the gradual improvement in availability of phylogenetic
trees for multispecies assemblages. Alternatively, phylogenetic bias can be
reduced by comparing invading and non-invading congeners (Goodwin et al.
1999).
The variety of methods, approaches, scales, and measures used in comparative
multispecies studies of species invasiveness makes it dangerous to draw
generalizations without taking the character of individual studies into
account.What can be thus inferred about species traits and their effects on the
invasiveness of plant species? Is the message consistent?
7.2.3 Main Findings of Comparative Multispecies Studies (1995–2005)
Although the multispecies studies test different hypotheses, simply because
the inventories and databases contain different information (Cadotte et al.
2006b), some traits have been tested frequently enough for a pattern to
emerge (Table 7.2 and see below).
Growth form (usually separating species into annual, biennial, perennial,
shrubs, and trees) and life form (following Raunkiaer’s scheme) are the most
frequently analyzed traits – obviously because these data are readily available.