Considerable strides have been made in the new domain of plant invasion ecology.

The three key questions are:

Scientists continue to rely heavily on the track record of various species as aliens in order to predict their invasiveness (explaining why some species are more invasive than others), rather than on mechanistic understanding of invasions. Problems with this approach include the fact that only a small proportion of the world’s flora has been widely planted as aliens, and for long enough, to gauge their invasiveness. Fewer than half of the currently invasive plant species have a track record of being invasive elsewhere.

Species known to have invaded many parts of the world are obviously more likely to invade in other areas, but no prediction can be made for species that have no history of widespread introduction over long periods, or evidence of their invasiveness. Nonetheless, it seems that risk assessments based largely on the experience of other invasive species are still the most pragmatic approach in predicting invasiblity worldwide. In any event, continuing global changes will complicate any predictions.

There is slow progreas towards a more mechanistic understanding of invasiveness, contributing to the debate on whether it is feasible or desirable to aim for standardized criteria for all weed lists, given that such lists serve different purposes in different parts of the world. Formal protocols for assessing the risk of invasiveness are not widely applied. Australia and New Zealand are the only countries using risk assessment models in day-to-day decision-making when evaluating applications for permission to introduce new species.