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Global trade and commerce have led to the unintended establishment of more than 200 ant species  beyond their natural geographic ranges. Most of these ants are hitchhikers in nursery plants, freight and machinery, etc., and a subset of them have moved around the globe and achieved broad cosmopolitan distributions. These are often referred to as tramp ants, which broadly speaking is defined as exotic (non-native) ants transported with cargo by ships, trains, or other means. The first list of tramp ants was compiled more than a century ago, and comprised a total of 14 species. The number has since been extended to 42 . Among the better known of these are the Argentine ant (Linepithema humile), fire ants (Solenopsis invicta and S. geminata), the yellow crazy ant (Anoplolepis gracilipes), the longhorn crazy ant (Paratrechina longicornis), the little fire ant (Wasmannia auropunctata), and the big-headed ant (Pheidole megacephala). These species frequently become dominant in introduced communities and reach pest status due to their tremendous ecological, economic, and agricultural impacts .
Success of Tramp Ants: What Do Different Species Have in Common?
Tramp ants often form polydomous colonies occupying socially connected yet spatially separated nests . Virtually all of these species have a polygynous (multiple queens) social structure with a notable exception involving the fire ant S. invicta, in which two social forms (monogyne and polygyne) occur in most invasive populations. The presence of multiple queens in a tramp ant colony significantly increases colony reproductive potential and often results in larger colony sizes compared with monogyne colonies. New colonies largely or solely arise via colony budding, a process whereby a queen accompanied by hundreds of workers travels on foot to start a new nest in a location not far from their natal nest . The chance of successful establishment of such colony fragments containing workers is higher than those founded solely by queens that start from nothing . Such colonies also may persist and survive longer despite their relatively localized dispersal. Continual colony founding via this mode can eventually lead to formation of a dense, expansive, continuous population of many nests with little or no recognizable colony boundaries – a supercolony [10, 20].
A supercolony is characterized by lack of aggression among workers from different nests. However, clear boundaries do exist between different supercolonies. Some supercolonies are massive both in terms of the number of workers and their geographic spread . One of the most striking examples is a single Argentine ant supercolony in Europe that spans more than 6,000 km from Italy to Spain. This supercolony, despite its massive size, is characterized by the absence of any visible boundary among physically separated nests and an apparent loss of intraspecific aggression among workers from different nests throughout the range. Supercolony formation likely plays a role in invasion and ecological success of Argentine ants despite the fact their population characteristics pose something of an evolutionary paradox , as they appear to violate kin selection theory. Several studies suggest that the very high worker numbers attained as a result of the lack of aggression among neighboring nests makes them better at outcompeting native ants that do display aggression among colonies. This drastic alteration in social organization also allows Argentine ants to invest more resources toward foraging and reproduction .
Several tramp ant species have unique reproduction systems that differ from the haplodiploid sex determination that is general in the Hymenoptera. As an example, double-clonal reproduction of queens (thelytoky) and males and sexual reproduction by workers have been reported in non-native populations of the longhorn crazy ant, little fire ant, and Vollenhovia emeryi. This reproductive system has been suggested as a potential adaptive mechanism associated with the invasion success of these ants as a result of a complete separation of the maternal and paternal genomes that allows the ants to avoid the usual costs associated with inbreeding. Whether this mode of reproduction exists in native populations of the longhorn crazy ant is unknown, largely because the native range of this species is still uncertain. Nevertheless, double-clonal reproduction found in the native range of the little fire ant is regarded as a preadaptive trait for invasion.
The outstanding ecological success of several tramp ant species is attributed in part to their mutualistic relationships with sap-feeding hemipterans (scale insect, mealybugs, aphids, etc.) that produce honeydew . Honeydew, which is rich in carbohydrates, serves as an important food source for many ants (not just tramp species). In exchange for honeydew, the ants reward their hemipteran partners with frequent tending and protection from predators, parasitoids and, in some cases, pathogens . This trophobiosis between honeydew-producing insects and tramp ants may allow the formation of unusually high densities of both partners [17, 18], potentially leading to severe effects for agriculture, community structure, and ecosystem services. For example, the high densities and elevated colony growth of the fire ant S. invicta in the USA are attributed partly to a greater availability of honeydew. Another striking example involves a mutualism between the yellow crazy ant and invasive scale insects on Christmas Island in the Indian Ocean, where the ants have reached extremely high densities that have resulted in significant alteration of the rainforest landscape and the loss of multiple native species .
Invasion History, Human Commerce, and the Bridgehead Effect
Tramp ants have been particularly successful at spreading around the world largely because of human-mediated long-distance dispersal [3, 22, 25]. For instance, historical and customs interception records show that many tramp species most likely are transported as cargo stowaways [12, 14]. An analysis of thousands of border interception records from ports of the USA and New Zealand from the past decades showed that most intercepted ants, surprisingly, come from other invasive areas and not from their native ranges . The finding that established invasive populations often serve as the source of new invasions – known as the bridgehead effect – is much more prevalent than previously thought. Similarly, extensive genetic data in an earlier study of fire ants revealed that the primary source of all newly invaded areas (e.g., Australia, Taiwan, Hong Kong, China) was the widespread invasive North American population, rather than populations in the native South American range .
Detailed population genetic analyses, along with historic and interception records, have also unraveled the introduction patterns of other tramp ants, including those species with complex histories and widespread distributions. As one example, a recent genetic study of the tropical fire ant S. geminata provided evidence that Asian populations were introduced approximately 480 years ago from an area in southern Mexico, a finding that corresponds well with global Spanish trade routes of the sixteenth and seventeenth centuries . A recent meta-analysis study also showed that the timing of other major ant invasions almost perfectly correlates with two major historical waves of human globalization (approximately 1850–1914 and 1960–present) during last 200 years .
The genetic footprint of human activities on tramp ant dispersal patterns can be used to predict the invasion risks of tramp ants or, more specifically, the potential pathways of species introductions. Integration of data from population genetic analyses and trade records between the USA and China has demonstrated that the volume of trade from fire ant-infested areas is a predictor of the potential of different fire ant populations a source of outbreaks . Empirical data to support the utilization of trade records to prioritize quarantine efforts against tramp ants are now being accumulated, including from recent invasions of S. invicta in Japan. This ant was intercepted primarily in seaports that received the highest amounts of merchandize (expressed as the number of cargo containers) from China. A similar trend also holds for invasion of Argentine ants in Japan. Thus, perhaps not surprisingly, propagule pressure  is a key predictor for establishment of tramp ants in new areas.
Many tramp ant studies have focused on the two most highly invasive, economically important ants, namely, the red imported fire ant S. invicta and the Argentine ant L. humile. These are arguably the best studied ant species in the world and serve as models for studying patterns of tramp ant invasions and establishment. Unfortunately, other tramp ant species are often overlooked because they generally are not as devastating, although research efforts on other species have increased in recent years. Nonetheless, the histories and pathways of the spread and the invasive potential of many tramp ants remain unclear, possibly impeding the discovery of novel mechanisms underlying invasion success. Additional studies of numerous tramp ant species are warranted and critical for at least two reasons. First, ants are arguably the most diverse group of social insects, with high variation in life histories, ecology, reproductive modes, and numerous other traits. Tramp ants are no exception. Data from different species of varying biology and ecology are of great value in developing preventive risk assessment frameworks and rapid response actions.
Second, a number of studies of several invasive social insects, including ants, have documented genetic, behavioral, and life history changes following introduction into new areas. Identification of the sources and invasion routes of tramp ants provide crucial baseline knowledge for both of these important avenues of research. Comparisons of genetic, ecological, and behavioral differences between the native and introduced populations can potentially lead to identification of additional traits associated with invasion success [5, 6, 15, 19, 21, 23, 24]. Fortunately, current genetic/genomic technologies provide greater power and higher resolution, which means that studies aimed at identification of the native range and reconstruction of invasion routes for a given tramp ant species are now feasible. Additional studies of other invasive ants will further increase our understanding of patterns and processes of rapid evolution and adaptation to new environments. Such studies are especially important, given the massive numbers of species facing changing environmental conditions as a result of ongoing global climate change.
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