Estimates and measures of propagule pressure are needed to
define the invasibility of communities/ecosystems/regions.
Numbers
of recorded aliens do not allow for robust comparisons
of invasibility. However, given the complexity of modes of
introduction pathways into different regions and the variety of
factors that determine whether a propagule reaches a certain
community/habitat, it seems that at biogeographical and global
scalessurrogatesrelatedtovariousaspectsofhumanactivities,as
mentioned above, are the only possibility for studying this issue.