Emerging invasive species: global rises resulting from increased accessibility of new sources
Thousands of species have been introduced into regions outside their native ranges by humans, and many have become permanent additions to local faunas and floras. The number of these established alien species has markedly increased worldwide during the past two centuries. Our ability to predict the identity of future invasive alien species is largely based on knowledge of prior invasion history. Therefore, emerging alien species — those never encountered as aliens before — pose a significant challenge to assessing biosecurity risk. Consequently, understanding the temporal trends, origins and spread of emerging alien species is pivotal to improving prevention and risk assessment tools.
The observed growth in the numbers of alien species has been largely attributed to increases in the drivers of alien species introductions, such as import volumes and human mobility, and rising establishment rates due to land degradation. However, the number of alien species in a region may also be affected by changes in the accessibility of source pools of species in their native range. For example, ‘historical’ alien bird introductions (AD 1500–1903) were largely driven by European colonial expansion and thus mostly drawn from birds originating in Europe and European colonies. Conversely, ‘modern’ bird introductions (1983–2000) primarily relate to introductions via the pet bird trade and are species native to regions close to key trade hubs. These new source pools provide many new potential alien species as old source pools start to deplete (the proportion of new alien species in a source pool declines with every newly introduced species), thereby maintaining the rate of alien species establishments in new regions.
Disentangling the factors underpinning the introduction of alien species will improve our understanding of past invasion dynamics and result in better-informed predictions of future invasion risks. To do this, Darren Ward and his colleagues created a database of more than 45 000 first records of just over 16 000 alien species to investigate the occurrences of emerging alien species worldwide. A ‘first record’ is the date an alien species was first recorded as present in a country. Included in the database were vascular plants, vertebrates (mammals, birds and fish) and invertebrates (insects, crustaceans and molluscs).
The team found that the distribution of alien species was highly skewed, with the majority of species (58%) having just a single first record (one country) in the database. By contrast, 26 species had more than 50 first records, with the top five being the domestic pigeon (first records in 197 countries), longhorn crazy ant (134), big-headed ant (92), house sparrow (87), and European rabbit (82). The vascular plant with the highest number of first records was Canadian horseweed (40).
Even after many centuries of invasions, the rate of emergence of new alien species is still high. For example, the team showed that during 2000–2005 one-quarter of first records in a country were of species that had not been previously recorded elsewhere as alien.
The number of emerging invasive species cannot be solely explained by increases in well-known drivers, such as the amount of imported commodities from historically important source regions. Instead, these dynamics reflect new, expanding trade networks, along with environmental change. Overall, up to 16% of all species on Earth (depending on the taxonomic group) qualify as potential alien species.
These results suggest there remains a high proportion of emerging alien species yet to be encountered, with future impacts that are difficult to predict. Biosecurity risk assessment approaches that rely less on prior invasion history will need to be prioritised.
This work was funded by the Ministry of Business, Innovation and Employment.
Darren Ward (Manaaki Whenua - Landcare Research, and the School of Biological Sciences, University of Auckland)
Seebens H et al. 2018. Global rise in emerging alien species results from increased accessibility of new source pools. Proceedings of the National Academy of Sciences 115(10): E2264–E2273. doi: 10.1073/pnas.1719429115