(Figure SPM.5). Many invasive alien species have been 
intentionally introduced outside their natural range around 
the world for their perceived benefits without consideration 
or knowledge of their negative impacts, but there have 
also been many unintentional introductions, including as 
contaminants of traded goods and stowaways in shipments. 
Indirect drivers of change, particularly those associated 
with economic activities, of which international trade is the 
most important, are increasingly facilitating transport and 
introduction, the early stages of biological invasion. Direct 
drivers, particularly land- and sea-use change and climate 
change, are increasingly important later in the biological 
invasion process, facilitating the establishment and spread 
of invasive alien species, with fragmented ecosystems being 
more vulnerable to invasive alien species. Transport and 
utility infrastructures in terrestrial and aquatic environments 
can create corridors that facilitate the spread of invasive 
alien species, including into remote, undisturbed and 
protected areas. For some invasive alien species, the spread 
is immediate, but others only begin to spread long after 
first introduction, meaning that currently observed threats 
of invasive alien species can lead to underestimation of the 
magnitude of the future impact. Invasive alien species may 
increase in numbers after a long period at low density as 
a result of changes in interactions with other species, for 
example as a result of the introduction of a missing dispersal 
agent or the removal of a competitor.
SUMMARY FOR POLICYMAKERS
The thematic assessment report on
INVASIVE ALIEN SPECIES
AND THEIR CONTROL
B. Globally, invasive alien species 
and their impacts are increasing 
rapidly and are predicted to 
continue rising in the future