THE THEMATIC ASSESSMENT REPORT ON INVASIVE ALIEN SPECIES AND THEIR CONTROL
22SUMMARY FOR POLICYMAKERS
majority of documented global extinctions attributed mainly 
to invasive alien species occurring on islands, local 
extinctions account for 9 per cent of documented impacts 
of invasive alien species on islands, in contrast to 4 per 
cent on mainlands (well established) {4.3.1.1}. For 
example, Boiga irregularis (brown tree snake) caused the 
global extinction of Myiagra freycineti (Guam flycatcher) 
and local extinction or serious population reduction for 
many other resident bird species in Guam (well 
established) {4.3.1}. Islands are also vulnerable to climate 
change, which can increase the rate of establishment and 
spread of many invasive alien species (well established)
{Box 2.5}. Many invasive alien species on islands only 
occupy a small portion of their predicted range and are 
likely to expand further (established but incomplete) {Box 
2.5}. The number of alien plants exceeds the total number 
of native plants on more than one quarter of islands (well 
established) {Box 2.5}. Invasive alien species have been 
reported in areas protected for nature conservation, some 
remote areas (e.g., high mountains), and also in tundra and 
deserts, which emphasizes that these areas, despite being 
protected for nature conservation or remote, are also 
vulnerable to the negative impacts of invasive alien species 
(well established) {Box 2.4, 4.3.1.2, 4.3.2.1}. Fifty-three 
invasive alien species have caused the local extinctions of 
240 native species in protected areas globally (established 
but incomplete) {4.3.1.2}. The invasive alien Rattus rattus 
(black rat) has been documented as the only cause of the 
global extinction of Nesoryzomys darwini and 
Nesoryzomys indefessus (rice rats), which were endemic to 
the protected areas of the Galapagos Islands (well 
established) {4.3.1}.
A4 Invasive alien species adversely affect the full 
range of nature's contributions to people, imposing 
an economic burden (well established) {4.4.1}. 
Some alien species have been intentionally introduced for 
their benefits to people, often without consideration or 
knowledge of their negative impacts (well established)
{3.3.1}. However, nearly 80 per cent of the documented 
impacts of invasive alien species on nature's contributions to 
people are negative (well established) {4.4.1}. Reduction in 
food supply is by far the most frequently reported impact 
across all taxa and regions (well established) {4.4.1, 4.6.2}. 
In terrestrial systems, invasive alien plants are the taxonomic 
group most frequently reported as having a negative impact, 
particularly in cultivated areas and temperate and boreal 
forests (well established) {4.4.2.1}. For example, in northwestern Europe, Picea sitchensis (Sitka spruce) severely 
alters habitats such as coastal heathlands and mires, which 
are important habitats for threatened and endangered 
plants, birds and other species, and for local cultural 
heritage (well established) {4.3.2.1}. In coastal areas, 
invasive alien invertebrates are the most frequently reported 
taxonomic group with an impact on nature's contributions to 
people, particularly provision of food (well established)
{4.4.2.3}. For example, Carcinus maenas (European shore 
crab) has had an impact on commercial shellfish beds in 
New England and Canada, Asterias amurensis (northern 
Pacific seastar) and Ciona intestinalis (sea vase) have 
negatively affected mariculture and fisheries along the 
Korean coast, and Mytilopsis sallei (Caribbean false mussel) 
has displaced native clams and oysters that are locally 
important fishery resources in India (well established)
{4.4.2.3}. In 2019, global annual costs of biological invasions 
were estimated to exceed US$423 billion, with variations 
across regions, but this is likely to be a gross underestimate 
(Figure SPM.3) (established but incomplete) {Box 4.13}. 
Ninety-two per cent of this cost is attributed to the damage 
that the invasive alien species have caused to nature's 
contributions to people and good quality of life; only 8 per 
cent is related to the management expenditures for 
biological invasions (established but incomplete) {Box 4.13}. 
Economic benefits are often gained by a few people or 
sectors while costs, often long-term ones, are borne by 
many others (established but incomplete)
{3.2.3.5, 4.2.1, 6.2.2(6)}.
A5 Invasive alien species overwhelmingly 
undermine good quality of life (established but 
incomplete) {4.5, 4.6.3}. Invasive alien species can 
threaten livelihoods, water and food security, economies 
and human health (e.g., causing diseases, allergies and 
physical injuries) (Figure SPM.3) (well established) {4.5.1, 
4.5.1.3}, with 85 per cent of the documented impacts of 
invasive alien species on good quality of life being negative
(Figure SPM.3) (well established) {4.5.1}. Invasive alien 
species can also serve as vectors for infectious zoonotic 
diseases that can lead to epidemics, such as malaria, 
dengue fever, chikungunya, Zika, yellow fever and West Nile 
fever, which are transmitted by invasive mosquito species 
(e.g., Aedes albopictus and Aedes aegyptii) (well 
established) {Box 1.14, 4.5.1.3}. Invasive alien plants can 
impact human health directly, particularly through the 
production of highly allergenic pollen, for example, Prosopis 
juliflora (mesquite) and Ambrosia artemisiifolia (common 
ragweed) (well established) {4.5.1.3}. Indigenous Peoples 
and local communities, ethnic minorities, migrants, poor 
rural and urban communities are disproportionately 
impacted by invasive alien vector-borne diseases 
(established but incomplete) {4.5.1}. Although there is 
limited research on the interplay between gender relations 
and invasive alien species (established but incomplete)
{4.5.1, 4.7.2}, there is some evidence of inequities and 
marginalization in gender- and age-specific activities where 
invasive alien species impede access to natural resources 
or require management (established but incomplete) {4.5.1, 
5.2, 5.2.1, 5.5.5}. For example, in Lake Victoria artisanal 
fisheries mainly involving men have declined following the 
introduction, establishment and spread of the invasive alien 
plant Pontederia crassipes (water hyacinth), which has led 
to the depletion of tilapia (established but incomplete)
SUMMARY FOR POLICYMAKERS
19SUMMARY FOR POLICYMAKERS
BACKGROUND
Figure SPM 2 Examples of invasive alien species with a negative impact on nature (green), 
and, in some cases, nature's contributions to people (yellow) and/or good 
quality of life (teal). 