DOI: 10.5061/dryad.34tmpg4kr
title: Invasive ant and trade flows from continents to countries worldwide

description: A major goal of invasion biology is to understand global species flows
 between donor and recipient regions. Our current view of such flows
 assumes that species are moved directly from their native to their
 introduced range. However, if introduced populations serve as bridgehead
 population that generate additional introductions, tracing
 intercontinental flows between donor and recipient regions misrepresents
 the introduction history. Our aim was to assess to what extent bridgehead
 effects distort our view of global species flows. We separately mapped
 "flows" of 252 alien ant species established on one to six
 continents, representing a gradient of relatively certain to completely
 unreliable flows. In 83% of countries, more than 50% of alien ants were
 established on six continents, indicating that flows to these countries
 are unreliable. Flows of species established on a single continent were
 linked to global trade flows, while flows including cosmopolitan species
 were not linked to global trade. It is crucial to account for bridgehead
 effects when assessing the biogeography and intercontinental flows of
 alien species. This is urgent for improving our understanding of how
 species are moved around the planet.
description: <strong>Species distributions and
 flows</strong><br> To determine the number of alien ant
 species that are established in each country, we used the geo-referenced
 database Antmaps (an authoritative database maintained and updated
 regularly by experts based on new records from the peer-reviewed
 scientific literature). The Antmaps database includes information on the
 native and alien ranges of 252 ant species (https://antmaps.org/?). We did
 not consider occurrence records that may be dubious (needing taxonomic
 verification). We kept both indoor and outdoor locations because all parts
 of the species’ distribution are the consequence of human-mediated
 dispersal. Populations that occurred at indoor locations were also a
 possible source of new invasions, for example if material such as potted
 plants and soil are moved from an indoor location to a different location.
 The aim of our analyses was not to distinguish between factors (climate,
 habitat) filtering out species at the establishment stage of the invasion
 process, but to understand what drives global species movements. As all
 species records are a reflection of global species flows, we kept all
 records for the analyses presented in the main part of the
 manuscript. We delimited the countries and continents
 based on the administrative database GADM version 3.6. For mapping, we
 used the Mollweide projection. We defined a species “flow” as the number
 of species introduced from one region to another region. To calculate the
 species flows from donor to recipient regions, we defined the species’
 native range as all countries containing native populations according to
 Antmaps. For species whose native range covers more than one continent, we
 weighted the flow from each of the continents by the number of political
 regions where the species is native (i.e., non-overlapping country or
 sub-country polygons, representing states, counties or islands and which
 are more homogenous in size than entire countries.
 <strong>Countries</strong><br> In total, 173
 countries worldwide host alien ant species. To compare species flows, we
 focused on the 41 countries which had both species exotic in only one
 continent and species exotic in several continents. In that way, we were
 able to compare the different species flows for all alien species
 (hereafter ALL species) or species exotic in one continent (Exo1) or two
 (Exo2), three (Exo3), four (Exo4), five (Exo5) or all continents except
 Antarctica (Exo6). <strong>Interception
 data</strong><br> We have sourced previously published
 interception records for the United States and New Zealand from 1914-2013
 (described in detail in Bertelsmeier et al. 2018, PNAS). In total, this
 dataset contains 69 alien ant species intercepted on cargo, goods, mail
 and baggage and has information on the country of origin for each
 interception, and therefore allows calculating the proportion of secondary
 interceptions for each species (i.e., the proportion of all interceptions
 of a species which come from a country where the species is not
 native). <strong>Trade
 data</strong><br> Most biological invasions arise via
 human-mediated transport, allowing species to establish in new geographic
 regions. In particular, accidental transport with traded commodities is an
 important dispersal pathway for insects in general and especially ants. We
 used general import flows to represent global flows of potential transport
 vectors. To calculate import flows to all countries, we used cumulative
 import data from 1998 to 2017 extracted from the UN Comtrade Database
 (United Nations Commodity Trade Statistics
 Database, http://comtrade.un.org/db/ (accessed May 2019)). This dataset
 contains dyadic trade flows between pairs of countries, given in US
 dollars per year. Such comprehensive data is not available for earlier
 periods; as most imports over the last two centuries have occurred during
 this recent period of globalization, we expect these relatively recent
 imports to have left their footprint on the flows of ants. Because no
 import data was available for four previously defined administrative units
 (Puerto Rico, Christmas Island, Norfolk Island and Marshall Islands), they
 were excluded from this analysis. The flows to each of the remaining 37
 countries were standardized by dividing the flows by the total imports to
 each country in order to study variations in the proportions of geographic
 origins of the flows (and not the absolute quantities).