THE THEMATIC ASSESSMENT REPORT ON INVASIVE ALIEN SPECIES AND THEIR CONTROL
20SUMMARY FOR POLICYMAKERS
BACKGROUND
A. Invasive alien species are a major threat to nature, 
nature's contributions to people, and good quality of life
A1 More than 37,000 established alien species, 
including more than 3,500 invasive alien species 
with documented impacts, have been recorded 
worldwide (well established) {2.1.4, 4.2}. Alien 
species (plants, animals, fungi and microorganisms, 
including pathogens) are being introduced globally at an 
unprecedented rate; currently, approximately 200 new alien 
species are recorded every year (well established) {2.2.1}. 
Invasive alien species represent a subset of alien species, 
consisting of those that have established and spread and 
are known to have a negative impact on nature and, in 
some cases, people (Figure SPM.1). Although their 
numbers are likely to be underestimated and expected to 
increase, to date 1,061 alien plants (6 per cent of all 
established alien plants), 1,852 alien invertebrates (22 per 
cent), 461 alien vertebrates (14 per cent) and 141 alien 
microbes (11 per cent) are known to be invasive globally 
(established but incomplete) {4.2}. Although some invasive 
alien species can provide benefits for people (e.g., through 
provision of food and fibre), those benefits do not mitigate 
or undo their negative impacts on nature, nature's 
contributions to people, and good quality of life across all 
regions and taxa globally (well established) {1.3.4, 4.1.2, 
4.3, 4.4, 4.5}. In addition to their impacts on nature, about 
16 per cent of invasive alien species have negative impacts 
on nature's contributions to people, and about 7 per cent 
on good quality of life (Figure SPM.2) (established but 
incomplete) {4.2}. Based on data and information included 
in this assessment, most impacts are reported in the 
Americas (34 per cent), Europe and Central Asia (31 per 
cent) and Asia-Pacific (25 per cent), with fewer reported in 
Africa (7 per cent) (established but incomplete) {4.2}. 
Twenty per cent of all impacts are reported from islands 
(established but incomplete) {4.2}. A disproportionate 
number of documented negative impacts have been 
reported from the terrestrial realm (75 per cent), especially 
temperate and boreal forests and woodlands and cultivated 
areas (including agricultural land) (established but 
incomplete) {Table 4.2}. About one quarter of the 
documented negative impacts have been reported from 
aquatic realms (freshwater: 14 per cent; marine: 10 per 
cent), especially from inland surface waters/waterbodies 
and shelf ecosystems (established but incomplete)
{Table 4.2}.
A2 Invasive alien species are a major direct driver 
of change, causing biodiversity loss, including local 
and global species extinctions (Figures SPM.2 
and 3) (well established) {4.3.1}. Invasive alien species 
have contributed solely or alongside other drivers of change 
to 60 per cent of recorded global animal and plant 
extinctions (established but incomplete) {Box 4.4, 4.3.1}, 
while invasive alien species are the only driver attributed to 
16 per cent of documented global extinctions (established 
but incomplete) {Box 4.4}. The majority of documented 
global extinctions (90 per cent) with invasive alien species as 
one of the major causes are reported from islands
(established but incomplete) {Box 4.4}. At least 218 invasive 
alien species have caused 1,215 documented local 
extinctions of native species across all taxa (Figure SPM.3)
(established but incomplete) {4.3.1}. Invasive alien species 
harm native species most often by changing ecosystem 
properties (27 per cent), for example soil and water 
characteristics, and through competition between species 
(24 per cent), predation (18 per cent) and herbivory (12 per 
cent) (established but incomplete) {4.3.1.3}. The majority of 
reports of the impacts of invasive alien species on native 
species document negative effects (85 per cent), primarily 
negatively impacting the growth, survival and reproduction 
of individuals, which lead to local population declines and 
local and global extinctions (well established) {4.3.1}. Some 
invasive alien species have a profound ecological impact 
that spans various levels, from individual species and 
communities to whole ecosystems, resulting in complex, 
undesirable and in some cases irreversible outcomes when 
the system has crossed a threshold beyond which 
ecosystem restoration is not possible (well established) {Box 
1.5, Box 4.12, 4.3.3}. For example, Castor canadensis
(North American beaver) and Magallana gigas (Pacific 
oyster) change ecosystem properties by transforming 
habitats, with cascading effects on a myriad of native 
species (well established) {4.3.2.1, Box 4.11}. On Christmas 
Island, the arrival of the invasive alien Anoplolepis gracilipes
(yellow crazy ant) caused the decline of the native Christmas 
Island Gecarcoidea natalis (red crabs), which resulted in the 
population explosion of the invasive alien Lissachatina fulica
(giant African land snail) (well established) {3.3.5.1}. 
Increased biotic homogenization (or loss of uniqueness) of 
biological communities is a major negative impact of invasive 
alien species (well established) {1.3.4}. The magnitude of the 
negative impacts of invasive alien species on nature 
depends on the context, and the factors that determine the 
highest magnitudes of impact are not well understood 
(established but incomplete) {Box 4.9, 4.3.2.1, 4.7.1}. For 
SUMMARY FOR POLICYMAKERS
19SUMMARY FOR POLICYMAKERS
BACKGROUND
D. Ambitious progress to 
manage biological invasions8
can be achieved with integrated 
governance