Chapter 4: Impacts of invasive alien species on nature, nature's contributions to people, and good quality of life.

Executive summary
1. Invasive alien species impact nature at all ecological levels, from native individuals, populations, species, to communities and ecosystems (well established) {4.3.1}. Although some invasive alien species can have both positive and negative impacts (well established) {4.3, 4.4, 4.5, 4.6}, the overall negative impacts of invasive alien species far exceed any positive impacts on nature and humans (established but incomplete) {4.3.1, 4.4.1, 4.5.1, 4.6.1, 4.6.3}. Almost three-quarters (71 per cent) of the documented impacts on nature adversely affect native species (well established) {4.3.1}. The magnitude of impacts of invasive alien species varies depending on the geographic and environmental context (well established) {4.3.1, 4.3.2, 4.3.3, 4.4.1, 4.4.2, 4.4.3, 4.5.1, 4.5.2, 4.5.3, 4.6.1, 4.6.2, 4.6.3, 4.6.4, 4.6.5}. The most commonly observed impacts on nature are changes in ecosystem properties, reductions in the performance of native species and declines in local populations of both plants and animals (well established) {4.3.1.3}. The most frequently observed mechanisms of impacts are competition, physical and chemical changes of the invaded ecosystems and trophic interactions through predation and herbivory (well established) {4.3.1.3}. In terrestrial ecosystems, most studies of impacts on nature are documented from plants and occur in forests, grasslands and human-dominated habitats (well established) {4.3.2.1}. Few impacts on nature are documented from very cold (tundra and high mountain habitats), very dry (deserts and xeric shrub lands) or flooded terrestrial habitats (wetlands – peatlands, mires, bogs) (well established) {4.3.1}. No impacts have been documented in the cryosphere and the deep-sea (established but incomplete) {4.3.1, Table 4.2}. The magnitude of negative impacts of invasive alien species often varies with the invaded biomes and species, and impacts are sometimes exacerbated or attenuated by the interaction of invasive alien species with other drivers such as climate change, changes in land- and sea-use, or pollution (established but incomplete) {4.3.1, Box 4.5}. The number of documented impacts of invasive alien species has risen in parallel with the documented number of alien species (established but incomplete) {4.3.1}. About 7 per cent of alien plants, 17 per cent of alien vertebrates, 23 per cent of alien invertebrates, and 12 per cent of alien microbes are known to be invasive, but their numbers are likely an underestimate (established but incomplete) {4.2}.
2. Invasive alien species have contributed to local or global extinctions of native species (well established) {4.3.1, Box 4.4}. Of all invasive alien species with documented impacts, 6 per cent (218 invasive alien species) have been associated with the local extinction of at least one native species (established but incomplete) {4.3.1}. Invasive alien species are a significant factor that directly or indirectly caused 60 per cent of documented global animal and plant extinctions (established but incomplete) {Box 4.4} and have caused 1,215 documented local extinctions of 255 native species across all taxa (established but incomplete) {4.3.1}. These local extinctions have been documented in marine (23.2 per cent) freshwater (14.5 per cent) and terrestrial realms (62.1 per cent) (well established) {4.3.1}. Invasive alien animals (vertebrates 51 per cent, invertebrates 32.5 per cent) are more often implicated in causing local extinctions than invasive alien plants (15.3 per cent) and microbes (1.2 per cent) (well established) {4.3.1}.
3. Impacts of invasive alien species are more harmful to isolated ecosystems, such as islands, than elsewhere (established but incomplete) {4.3.1.1}. Documented negative impacts on native species on islands are far more frequent than positive impacts (40.5 per cent vs. 4.5 per cent) (well established) {4.3.1.1}. Of the global extinctions caused by invasive alien species, the overwhelming majority occurred on islands and other isolated ecosystems (established but incomplete) {4.3.1, Box 4.4}. Local extinctions are more frequently documented from islands than from non-island locations (9.2 per cent vs. 4.0 per cent) (well established) {4.3.1}. Of the top ten invasive alien species documented to have caused local extinctions on islands, five are domesticated or synanthropic species: Rattus spp. (rats), Capra hircus (goats), Mus musculus (house mouse), Felis catus (cat), but also other vertebrates such as Anas platyrhynchos (mallard) (well established) {4.3.1.1}.
4. Invasive alien species pose a substantial threat to the conservation of native biodiversity, landscapes and seascapes in protected areas (established but incomplete) {4.3.1.2}. Invasive alien species impact areas protected for nature conservation, with impacts of similar magnitude and frequency occurring both inside and outside protected areas (established but incomplete) {4.3.1.2}. Impacts on nature in protected areas constitute 19.3 per cent of the total number of documented impacts on nature (established but incomplete) {4.3.1.2}. Reports of negative impacts on native species in protected areas are far more frequent than positive impacts (33.2 per cent vs. 6.3 per cent) (established but incomplete) {4.3.1.2}.
5. Invasive alien species cause impacts on all categories of nature’s contributions to people (well established) {4.4}. A large majority (80 per cent) of documented impacts on nature’s contribution to people are negative and harm people by decreasing ecosystem services (well established) {4.4.1}. The most commonly observed negative impact of invasive alien species to nature’s contributions to people is a reduction of human food supply (well established) {4.4.1}, which is caused by all taxa, in all regions and realms (well established) {4.4.2, 4.4.3}. Other important impacts of invasive alien species on nature’s contributions to people are on habitat maintenance (16 per cent records) and on the provision of materials, companionship and labour (14 per cent records). In terrestrial systems, the most common invasive alien species causing impacts are plants, particularly in cultivated areas and in temperate and boreal forests (well established) {4.4.2.1}. In inland waters, 70 per cent of the documented impacts on nature’s contributions to people are from inland surface waters and water bodies/freshwater (well established) {4.4.2.2}, and most of them are caused by invasive alien vertebrates (well established) {4.4.2.2}. In marine systems, the impacts are mostly caused by invasive alien invertebrates and predominate in shelf ecosystems (well established) {4.4.2.3}.
6. Impacts of invasive alien species on human health vary from nuisance to poisoning, disease and death (well established) {4.5.1}. Zoonotic diseases transmitted by invasive mosquitos inflict misery, chronic disease and death (well established) {4.5.1.3}. Invasive alien plants can be highly allergenic or phytotoxic (well established) {4.5}. Several invasive ant species have been documented as causing serious allergic or toxic reactions (well established) {4.5.1.3}. Health impacts caused by venomous and poisonous invasive alien marine species have frequently been documented in the Mediterranean Sea (well established) {4.5.1.3}.
7. Global cumulative damages due to invasive alien species totalled more than US$ 1.738 trillion between 1970 and 2020 (established but incomplete) {Box 4.13}. In 2017 alone, documented aggregate global costs of biological invasions were estimated to reach US$162.7 billion, exceeding the 2017 gross domestic product of 52 of the 54 countries on the African continent, and more than twenty times higher than the combined total funds available in 2017 for the World Health Organization and the United Nations (established, but incomplete) {Box 4.13}. In 2019, global annual costs of biological invasions were estimated to exceed $423 billion, with variations across regions, but this is likely a gross underestimation (established but incomplete) {Box 4.13}. North America (53 per cent) and Asia (13 per cent) were associated with the highest documented costs, which is partly driven by cost data incompleteness for most taxa and regions of the world (well established) {Box 4.13}. Agriculture is the economic sector most frequently documented as affected by invasive alien species and specifically by insects which are often categorized as pests (established but incomplete) {Box 4.13}.
8. Invasive alien species cause impacts on good quality of life that affect the opportunities for people to live a fulfilled life (established but incomplete) {4.5}. The majority of the 3,783 documented impacts on good quality of life are documented as negative for people (about 85 per cent) (established but incomplete) {4.5.1}. Most negative impacts (56 per cent) on good quality of life are the result of changes to “material and immaterial assets” by invasive alien species (established but incomplete) {4.5.1, 4.5.2, 4.5.3}. Invertebrates are documented as causing the highest number of negative impacts on good quality of life (51 per cent of negative impacts) (established but incomplete) {4.5.3}. Conversely, plants (responsible for 42 per cent of positive impacts) are more likely to result in positive impacts on good quality of life (established but incomplete) {4.5.3}. Negative and positive impacts on society are most often documented in Asia-Pacific (41 per cent of negative impacts and 53 per cent of positive impacts), and in cultivated areas (29 per cent of negative impacts and 26 per cent of positive impacts) (established but incomplete) {4.5.2.1, 4.5.3}. Although there is very little systematic research on gender differences in impacts of invasive alien species, the available data suggest that some invasive alien species may cause gender-differentiated impacts (established but incomplete) {4.5.1}.
9. Indigenous Peoples and local communities report more negative than positive impacts caused by invasive alien species, especially on water resources, human health and health of livestock and access to traditional lands (well established) {4.6.1}. Indigenous Peoples and local communities report ten times more negative than positive impacts caused by invasive alien species on nature (92 per cent negative, 8 per cent positive) (well established) {4.6.1}. Impacts on nature, often affect the deep kinship connection that many Indigenous Peoples and local communities have with nature (well established) {4.6.3}. When considering nature’s contributions to people, reports are more balanced (55 per cent negative to 45 per cent positive) (well established) {4.6.2}. Two-thirds (68 per cent) of the impacts on the good quality of life of Indigenous Peoples and local communities have been documented as negative, compared to one-third (32 per cent) that have been documented as positive (well established) {4.6.3}. Invasive alien species have frequently been documented to cause the loss of access to and mobility within traditional lands, leading to harder labour requirements (well-established) {4.6.3}. Negative impacts on the health of Indigenous Peoples and local communities can be direct (e.g., injury) and indirect, including general feelings of despair and stress. Some invasive alien species can provide some benefits, including income and development of local industry (well established) {4.6.3, 4.6.4}, but Indigenous Peoples and local communities highlight that seemingly positive impacts are not often considered wholly positive by their communities, especially when communities had little agency or choice in responding to the invasive alien species (well established) {4.6.2, 4.6.3, 4.6.4}. There are many cases where Indigenous Peoples and local communities have adapted to the negative impacts of invasive alien species (well established) (4.6.3). Whilst more negative impacts have been documented on cultural values and practices, involvement of Indigenous Peoples and local communities in the use and management of invasive alien species is, in some cases, also documented as an opportunity for skills development and knowledge transfer (established but incomplete) {4.6.5}.
10. There are substantial geographic and taxonomic gaps in the documentation, quantification and understanding of impacts (established but incomplete) {4.7.2}. The quality and quantity of information available on impacts of invasive alien species for different taxa, units of analysis, regions and realms differ greatly, and research efforts are unevenly distributed across regions, temporal scales, and taxa (well established) {4.7.2}. These biases can be observed across all realms, especially in marine ecosystems, where the extent and timing of research efforts on marine invasive alien species lag behind terrestrial studies (established but incomplete) {4.7.2}. About 95 per cent of the sources listed in the dataset are in English, severely underrepresenting studies only available in non-anglophone sources (well established) {4.7.2}.