Aim

The introduction of aquatic non-indigenous species (ANS) has become a major driver for global changes in species biogeography. We examined spatial patterns and temporal trends of ANS detections since 1965 to inform conservation policy and management. 

Location

Global

Methods

We assembled an extensive dataset of first records of detection of ANS (1965-2015) across 49 aquatic ecosystems, including the i) year of first collection, ii) population status and iii) potential pathway(s) of introduction. Data were analysed at global and regional levels to assess patterns of detection rate, richness, and transport pathways. 

Results

An annual mean of 43 (± 16 S.D.) primary detections of ANS occurred – one new detection every 8.4 days for 50 years. The global rate of detections was relatively stable during 1965-1995, but increased rapidly after this time, peaking at roughly 66 primary detections per year during 2005-2010, then declining marginally. Detection rates were variable within and across regions through time. Arthropods, molluscs and fishes were the most frequently reported ANS. Most ANS were likely introduced as stowaways in ships’ ballast water or biofouling, although direct evidence is typically absent.  

Main conclusions

This synthesis highlights the magnitude of recent ANS detections, yet almost certainly represents an underestimate as many ANS go unreported due to limited search effort and diminishing taxonomic expertise. Temporal rates of detection are also confounded by reporting lags, likely contributing to the lower detection rate observed in recent years. There is a critical need to implement standardized, repeated methods across regions and taxa to improve the quality of global-scale comparisons and sustain core measures over longer timescales. It will be fundamental to fill in knowledge gaps given that invasion data representing broad regions of the world's oceans are not yet readily available and to maintain knowledge pipelines for adaptive management.  

We assembled an extensive dataset of primary detection events of ANS (i.e. the first recorded collection of each species in each region) across global (primarily coastal marine, but also estuarine and freshwater) aquatic ecosystems for the period 1965-2015 using online ANS databases as primary data sources, including AquaNIS (http://www.corpi.ku.lt/databases/index.php/aquanis/), GLANSIS (https://www.glerl.noaa.gov/glansis/), Marine Biosecurity Porthole (https://www.marinebiosecurity.org.nz/) and NEMESIS  (http://invasions.si.edu/nemesis/). In addition, we conducted a literature search using the Web of Science to locate published datasets not available online, using the search terms “nonindigenous” or “non-indigenous” or “nonnative” or “non-native” or “alien” AND “aquatic” or “marine” AND “database” or “dataset” or “list” or “inventory”. Datasets were included only if dedicated, expert research on ANS had been conducted such that an up-to-date comprehensive and reliable inventory exists; datasets comprised of only a single taxonomic group, or not listing dates of first collection, or covering a shorter time period than this analysis were not included. 

At least one co-author with extensive regional knowledge of ANS carefully reviewed and edited each regional dataset for accuracy, checking scientific peer-reviewed publications, reports, books and personal collections to confirm: i) year of first collection; ii) current population status and; iii) potential pathway(s) of introduction (as known up to July 15, 2020).  Level of Certainty was not critically reviewed.Records were compiled only for ANS collected from the natural environment while those reported exclusively on or within pathways were excluded (e.g., taxa sampled during surveys of ships’ ballast water and biofouling, tsunami debris and other marine litter). Only species considered fully aquatic were included including marine stenohaline, marine euryhaline, diadromous and freshwater-euryhaline species, but excluding, for example, shoreline plants and aquatic birds. Freshwater stenohaline species were included for the Laurentian Great Lakes and Baltic Sea but excluded from all other estuarine and marine ecosystems as being ‘inland’ introductions. We also excluded records for cryptogenic species (whose status as indigenous or non-indigenous is unresolved) and taxa poorly studied or otherwise presenting challenges for taxonomic identification and assessment of historical biogeographic origin (e.g., fungi, protists, parasitic and free-living flatworms, viruses and microbes).  Note that Level of Certainty was not assigned in a uniform manner and may be unreliable, except for the 'Direct Evidence' category.