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 scales surrogates related to various aspects of human activities,as mentioned above, are the only possibility for studying this issue.