there may be a considerable time lag, spanning decades in 
some cases, before the impact is apparent (e.g., many 
invasive alien trees) (well established) {1.5}. Such time lags 
can lead to people not perceiving the ongoing slow changes 
in their environment, including the impacts of invasive alien 
species (well established) {1.5.2}. There can also be 
significant time lags in the response of invasive alien species 
to various drivers because the underlying processes that 
facilitate biological invasions operate at varying temporal 
scales (short- to long-term) (well established) {1.5, 3.2.3.1, 
3.6.3}. 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 (3.3.5.1). For example, in the western United 
States, the invasive alien Carcinus maenas (European shore 
crab) reduced the abundance of a native clam, releasing 
another alien species, Gemma gemma (the amethyst gem 
clam), from competition, allowing it to become 
superabundant and to spread, after having been locally 
distributed and at low abundance for over 50 years (well 
established) {3.3.5.1}. Patterns in the numbers of alien 
species seen today reflect the drivers of decades ago (i.e., 
invasion debt) (established but incomplete) {3.1.1, 3.1.5}. 
Consequently, past and ongoing amplification of drivers may 
lead to a long legacy of future invasive alien species as, for 
example, the number of new alien species that become 
invasive increases over time (i.e., invasion debt) (established 
but incomplete) {2.3.1.5, 3.1.5, 3.6.3}.
 <Section-header> C. Invasive alien species and their negative impacts can 
be prevented and mitigated through effective management </Section-header> 

31SUMMARY FOR POLICYMAKERS
19SUMMARY FOR POLICYMAKERS
BACKGROUND
C. Invasive alien species and their negative impacts can 
be prevented and mitigated through effective management