Key Words ants, biological invasion, indirect effects, interspecific competitions Abstract Invasions by non-native ants are an ecologically destructive phenomenon affecting both continental and island ecosystems throughout the world. Invasive ants often become highly abundant in their introduced range and can outnumber native ants. These numerical disparities underlie the competitive asymmetry between invasive ants and native ants and result from a complex interplay of behavioral, ecological, and genetic factors. Reductions in the diversity and abundance of native ants resulting from ant invasions give rise to a variety of direct and indirect effects on non-ant taxa. Invasive ants compete with and prey upon a diversity of other organisms, including some vertebrates, and may enter into or disrupt mutualistic interactions with numerous plants and other insects. Experimental studies and research focused on the native range ecology of invasive ants will be especially valuable contributions to this field of study. Table 1 lists characteristics of six of the most widespread, abundant, and damaging invasive ants. A great disparity exists with respect to how much is known about each of these species. For example, the red imported fire ant (S. invicta) ranks as one of the most well studied social insects (Ross & Keller 1995, Tschinkel 1998, whereas Anoplolepis gracilipes remains poorly studied by comparison, so much so that its native range is not even known. As an inevitable result of this difference, we devote more attention to the relatively well-known S. invicta and the Argentine ant (Linepithema humile) than to other species but caution against drawing the conclusion that other invasive ants resemble L. humile and S. invicta or pose less TABLE ECOLOGICAL EFFECTS OF ANT INVASIONS 183 INVASIVE ANTS AND THEIR GENERAL CHARACTERISTICS

Despite the severe ecological and economic damage caused by introduced species, factors that allow invaders to become successful often remain elusive. Of invasive taxa, ants are among the most widespread and harmful. Highly invasive ants are often unicolonial, forming supercolonies in which workers and queens mix freely among physically separate nests. By reducing costs associated with territoriality, unicolonial species can attain high worker densities, allowing them to achieve interspecific dominance. Here we examine the behavior and population genetics of the invasive Argentine ant (Linepithema humile) in its native and introduced ranges, and we provide a mechanism to explain its success as an invader. Using microsatellite markers, we show that a population bottleneck has reduced the genetic diversity of introduced populations. This loss is associated with reduced intraspecific aggression among spatially separate nests, and leads to the formation of interspecifically dominant supercolonies. In contrast, native populations are more genetically variable and exhibit pronounced intraspecific aggression. Although reductions in genetic diversity are generally considered detrimental, these findings provide an example of how a genetic bottleneck can lead to widespread ecological success. In addition, these results provide insights into the origin and evolution of unicoloniality, which is often considered a challenge to kin selection theory.

Invading organisms may spread through local movements (giving rise to a diffusion-like process) and by long-distance jumps, which are often human-mediated. The local spread of invading organisms has been fit with varying success to models that couple local population growth with diffusive spread, but to date no quantitative estimates exist for the relative importance of local dispersal relative to humanmediated long-distance jumps. Using a combination of literature review, museum records, and personal surveys, we reconstruct the invasion history of the Argentine ant (Linepithema humile), a widespread invasive species, at three spatial scales. Although the inherent dispersal abilities of Argentine ants are limited, in the last century, human-mediated dispersal has resulted in the establishment of this species on six continents and on many oceanic islands. Humanmediated jump dispersal has also been the primary mode of spread at a continental scale within the United States. The spread of the Argentine ant involves two discrete modes. Maximum distances spread by colonies undergoing budding reproduction averaged 150 m͞year, whereas annual jump-dispersal distances averaged three orders of magnitude higher. Invasions that involve multiple dispersal processes, such as those documented here, are undoubtedly common. Detailed data on invasion dynamics are necessary to improve the predictive power of future modeling efforts.stratified diffusion ͉ invasive ants ͉ Linepithema humile ͉ rate of invasion

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. The Argentine ant (Linepithema humile) is a widespread invasive species that competitively displaces native ants throughout its introduced range. Although this pattern of displacement is well known, its underlying mechanisms remain little studied. To gain a more detailed understanding of this widespread competitive displacement, I compared the exploitative and interference abilities of the Argentine ant with those of seven species of native ants it displaces in riparian woodlands in northern California. I performed four different manipulative field experiments; each measured different aspects of the competitive ability of the eight species of ants in this study. The main goals of this study were to identify the mechanisms responsible for the Argentine ant's strong competitive ability, to determine if native ants are subject to species-specific trade-offs in exploitative and interference ability typically present among coexisting ants, and if so, to assess whether Argentine ants are subject to this trade-off as well.Argentine ants located and recruited to baits as quickly or more quickly than did native ants-both in areas where Argentine ants and native ants occurred together (i.e., at the edge of invasion fronts) and where they occurred separately (i.e., away from invasion fronts). Along the edge of invasion fronts, Argentine ants also controlled a greater proportion of baits than did native ants. In one-on-one interactions, individual Argentine ant workers experienced mixed success in overcoming individual workers of the seven native ant species.When fighting against native ants, Argentine ants used both physical aggression and chemical defensive compounds, although the latter mechanism was more often successful in deterring opponents. Chemical defensive compounds produced by Argentine ants were repellent but appeared no more so than those of native ants. Although Argentine ant workers were not able to overcome native ant workers consistently, Argentine ant colonies succeeded in displacing most native ant colonies from baits. The discrepancy between worker-level and colony-level interference ability suggests that numerical advantages are key to the Argentine ant's proficiency at interference competition.Like ants in other communities, the native ants in this study were subject to a competitive trade-off in which interference ability and exploitative ability were negatively correlated. In contrast, Argentine ants were proficient at both exploitative and interference competition relative to the native ants they displaced and are thus removed from this trade-off. T...

The western honey bee () is the most frequent floral visitor of crops worldwide, but quantitative knowledge of its role as a pollinator outside of managed habitats is largely lacking. Here we use a global dataset of 80 published plant-pollinator interaction networks as well as pollinator effectiveness measures from 34 plant species to assess the importance of in natural habitats. is the most frequent floral visitor in natural habitats worldwide, averaging 13% of floral visits across all networks (range 0-85%), with 5% of plant species recorded as being exclusively visited by For 33% of the networks and 49% of plant species, however, visitation was never observed, illustrating that many flowering plant taxa and assemblages remain dependent on non- visitors for pollination. visitation was higher in warmer, less variable climates and on mainland rather than island sites, but did not differ between its native and introduced ranges. With respect to single-visit pollination effectiveness, did not differ from the average non- floral visitor, though it was generally less effective than the most effective non- visitor. Our results argue for a deeper understanding of how , and potential future changes in its range and abundance, shape the ecology, evolution, and conservation of plants, pollinators, and their interactions in natural habitats.

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The Argentine ant (Linepithema humile) is a widespread invasive species that competitively displaces native ants throughout its introduced range. Although this pattern of displacement is well known, its underlying mechanisms remain little studied. To gain a more detailed understanding of this widespread competitive displacement, I compared the exploitative and interference abilities of the Argentine ant with those of seven species of native ants it displaces in riparian woodlands in northern California. I performed four different manipulative field experiments; each measured different aspects of the competitive ability of the eight species of ants in this study. The main goals of this study were to identify the mechanisms responsible for the Argentine ant's strong competitive ability, to determine if native ants are subject to species-specific trade-offs in exploitative and interference ability typically present among coexisting ants, and if so, to assess whether Argentine ants are subject to this trade-off as well.Argentine ants located and recruited to baits as quickly or more quickly than did native ants-both in areas where Argentine ants and native ants occurred together (i.e., at the edge of invasion fronts) and where they occurred separately (i.e., away from invasion fronts). Along the edge of invasion fronts, Argentine ants also controlled a greater proportion of baits than did native ants. In one-on-one interactions, individual Argentine ant workers experienced mixed success in overcoming individual workers of the seven native ant species. When fighting against native ants, Argentine ants used both physical aggression and chemical defensive compounds, although the latter mechanism was more often successful in deterring opponents. Chemical defensive compounds produced by Argentine ants were repellent but appeared no more so than those of native ants. Although Argentine ant workers were not able to overcome native ant workers consistently, Argentine ant colonies succeeded in displacing most native ant colonies from baits. The discrepancy between worker-level and colony-level interference ability suggests that numerical advantages are key to the Argentine ant's proficiency at interference competition.Like ants in other communities, the native ants in this study were subject to a competitive trade-off in which interference ability and exploitative ability were negatively correlated. In contrast, Argentine ants were proficient at both exploitative and interference competition relative to the native ants they displaced and are thus removed from this trade-off. These findings imply that Argentine ants secure a majority of available food resources where this species comes into contact with native ants. Argentine ants may be able to break the competitive trade-off constraining native ants because of their unique colony structure and because they have escaped their natural enemies. The observation that Argentine ants are uncoupled from the competitive trade-off constraining native ants may provide a general explana...

Despite the innumerable ecological problems and large economic costs associated with biological invasions, the proximate causes of invasion success are often poorly understood. Here, evidence is provided that reduced intraspecific aggression and the concomitant abandonment of territorial behavior unique to introduced populations of the Argentine ant contribute to the elevated population densities directly responsible for its widespread success as an invader. In the laboratory, nonaggressive pairs of colonies experienced lower mortality and greater foraging activity relative to aggressive pairs. These differences translated into higher rates of resource retrieval, greater brood production, and larger worker populations.