Invasive species threaten ecosystems, habitats and other species (Bellard, Cassey and Blackburn 2016). They are usually non-native (invasive alien species) but can also include expanding native populations (Nackley et al. 2017). The annual rate of first records of non-native species has increased during the last 200 years and the increase in numbers does not show any sign of saturation, meaning that efforts to mitigate invasions have not been effective (Seebens et al. 2017). The ecological impacts of invasive species are felt through direct and indirect competition, predation, habitat degradation, hybridization, and their role as disease agents and vectors – also a threat to human health and food security (Figure 6.6) (Strayer 2010; Paini et al. 2016).
Invasive plants can impact the provisioning of key ecosystem services, such as access to clean water, by the congestion and eutrophication of waterways, degradation of catchment areas, and viability of pasture and rangeland (Packer et al. 2017). Invertebrate species that have become invasive may pose an even greater risk. The population expansion of the invasive zebra mussel in the North American Great Lakes was so great that it impeded water flow of municipal water supplies and hydroelectric companies (Rapai 2016). Invasive pests, such as the gypsy moth, emerald ash borer and hemlock woolly adelgid in North America, have both large biodiversity and economic impacts (Aukema et al. 2011). Invasive insect vectors can also facilitate the spread of parasites and emerging infectious diseases (Rabitsch, Essl and Schindler 2017), including chikungunya, dengue and Zika, which are vectored by mosquitoes (Akiner et al. 2016). Invasive vertebrates present grave danger on islands (Spatz et al. 2017), where they may be the major driver of biodiversity loss (Leadley et al. 2014; Doherty et al. 2016).
The economic costs, both direct and indirect (e.g. costs of control efforts), amount to many billions of dollars annually (for regional estimates see Kettunen et al. 2008; Pejchar and Mooney 2009; van Wilgen et al. 2012). The cost of restoring lost ecosystem services following invasion of the Laurentian Great Lakes by the spiny water flea was estimated to be between US$86.5 million and US$163 million (Walsh, Carpenter and Vander Zanden 2016). These costs do not reflect the additional environmental and societal/cultural impacts of invasive species.
Major routes for species invasion include deliberate release, escape and accidental introductions via trade, tourism and ship ballast water (CBD 2014; Early et al. 2016). Good governance may decrease invasion risk from trade (Brenton-Rule, Barbieri and Lester 2016), whereas climate change may facilitate increased spread by opening up new niche space (Wolkovich et al. 2013) and lowering barriers to establishment, especially in more extreme environments (Duffy et al. 2017). Loss of native biodiversity is likely to enhance invasion risk, while rising temperatures in cold regions increase the likelihood of establishment (Molina-Montenegro et al. 2012; Cuba-Díaz et al. 2013; Chown et al. 2017). Future threats are posed by increased transport in the Arctic with the decrease in sea ice, commercial use of microbes in crop production, horizontal gene transfer from genetically modified organisms, and the emergence of invasive microbial pathogens (Ricciardi et al. 2017).