THE THEMATIC ASSESSMENT REPORT ON INVASIVE ALIEN SPECIES AND THEIR CONTROL
31SUMMARY FOR POLICYMAKERS
subsequent feedback facilitating the establishment and 
spread of other invasive alien species (well established)
{3.4.2}. Indirect drivers also interact with one another. For 
example, sociocultural changes may lead to increased rates 
of infrastructure development through urbanization, and 
these interactions may further influence the rate and 
magnitude of change in land- and sea-use and other direct 
drivers that may in turn facilitate biological invasions (well 
established) {3.2.1}. Feedback and non-linear relationships 
among interacting drivers are likely to be exacerbated with 
ongoing and concurrent amplification of drivers (established 
but incomplete) {3.1.1, 3.5, 3.6.3, Box 4.5}, potentially 
leading to numbers of invasive alien species never previously 
encountered (established but incomplete) {2.6.1}.
B14 Negative impacts of invasive alien species can 
occur long after first introduction, and currently 
observed threats from invasive alien species can 
lead to an underestimation of the magnitude of the 
future impact (well established) {1.4.4, 2.2.1}. There 
are often time lags in detection and reporting of newly 
introduced invasive alien species (well established) {2.2.1}. 
Some invasive alien species spread very rapidly, while others 
take longer to spread and fully occupy their potential ranges 
(well established) {2.2.1, 2.2.3}. For some invasive alien 
species, the impact is immediate and continues into the 
long-term (e.g., fast-spreading pathogens such as Zika virus 
and Batrachochytrium dendrobatidis (chytrid fungus), and 
fast-spreading predators such as lionfish), while for others 
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}.
C. Invasive alien species and their negative impacts can 
be prevented and mitigated through effective management
C15 Management of invasive alien species has 
been successful in many contexts (Figure SPM.6, 
Table SPM.1) (well established) {5.5.1, 5.5.2, 5.5.3, 
5.5.4, 5.5.5, 5.5.6}. There are three options for preventing 
or reducing the number and negative impacts of invasive 
alien species:
Pathway management, based on the analysis of preborder, border and post-border risks, can prevent the 
movement and spread of invasive alien species through 
surveillance and the implementation of biosecurity 
response measures (well established) {5.3.1.1, 5.5.1, 
5.5.2}.
Species-based management at a local or landscape 
level, which includes surveillance, early detection and 
rapid response, eradication, containment and widespread 
control (including biological control), and can be 
applied throughout the biological invasion process (well 
established) {5.3.1.2, 5.5.2, 5.5.3, 5.5.4, 5.5.5}.
Site- or ecosystem-based management, which can 
both protect and restore native species and ecosystems 
(well established) {5.3.1.3, 5.5.6}.
The use of individual species-based and site-based 
approaches for the management of multiple invasive alien 
interacting, and non-additive effects of drivers, with differences in the overall importance of drivers across stages. While all drivers 
can potentially influence each biological invasion stage, indirect drivers, particularly those associated with economic growth, are more 
important in facilitating the transport and introduction stages {3.6.2}. In contrast, direct drivers, particularly land- and sea-use and 
climate change, are proportionally more important in facilitating the later stages of biological invasion {3.6.2}. 
31SUMMARY FOR POLICYMAKERS
19SUMMARY FOR POLICYMAKERS
BACKGROUND
TIC ASSESSMENT REPORT ON INVASIVE ALIEN SPECIES AND THEIR CONTRO