Although the ecological effects of invasions often become obvious soon after introduced species become established, more gradual effects may take years to manifest and can thus require long-term data for quantification. We analyzed an 8-year record of stable isotope data on Argentine ants ( biological invasions ͉ stable isotopes ͉ food webs B iological invasions threaten biodiversity and drain economic resources. Despite the importance of species introductions, the short-term and small-scale nature of most invasive species research is a recognized limitation of this field (1). Additionally, confounding environmental factors can obscure links between the spread of invasive species and decline of natives (2). Spatiotemporal fluctuations in the population sizes of introduced species (3) that result from changes in resource use or availability (4) can greatly alter the extent to which invaders disrupt ecosystems. Because a better understanding of such variation will inform both ecological theory and management strategies, there is an urgent need for long-term studies as well as for research that investigates ecological interactions in the native ranges of introduced species.One underappreciated source of variation regarding introduced species concerns dietary flexibility and shifts in trophic position between native and introduced populations. This form of ecological plasticity may enhance invasion success in a number of ways. Species capable of extracting required nutrients from multiple trophic levels might establish in a broader range of environments compared with more specialized consumers. Furthermore, theory predicts diminishing biomass at higher trophic levels, suggesting that species feeding at lower trophic levels might attain greater abundance (5).In this study we examine the direct effects of Argentine ant (Linepithema humile) invasions on native ant diversity and then quantify how dietary flexibility in this widespread invader affects spatiotemporal variation in its trophic position. Invasive ants provide an ideal system to test how trophic flexibility contributes to invasion success. With many species introductions, it can be difficult to separate the effects of the invader from covarying factors, such as habitat disturbance, that might also negatively affect natives (2, 6). Because Argentine ants aggressively displace above-ground foraging native ants (7), changes in the diet of L. humile that occur during and just after invasion can be linked to native ant displacement. Second, invasive ants, such as L. humile, are highly omnivorous and frequently form nonspecialized associations with honeydew-producing Hemiptera (7-10). Greater use of honeydew and other plant-based resources in introduced populations might result in a decrease in trophic position (relative to that of native populations), such that invasive ants would persist at higher densities than if they were acting as carnivores (7,11,12). Similar arguments have been proposed to explain the great abundance of ants in tropical rainforest canopies ...

Mutualisms play key roles in the functioning of ecosystems. However, reciprocally beneficial interactions that involve introduced species also can enhance invasion success and in doing so compromise ecosystem integrity. For example, the growth and competitive ability of introduced plant species can increase when fungal or microbial associates provide limiting nutrients. Mutualisms also may aid animal invasions, but how such systems may promote invasion success has received relatively little attention. Here we examine how access to food-for-protection mutualisms involving the red imported fire ant ( Solenopsis invicta ) aids the success of this prominent invader. Intense interspecific competition in its native Argentina constrained the ability of S. invicta to benefit from honeydew-producing Hemiptera (and other accessible sources of carbohydrates), whereas S. invicta dominated these resources in its introduced range in the United States. Consistent with this strong pattern, nitrogen isotopic data revealed that fire ants from populations in the United States occupy a lower trophic position than fire ants from Argentina. Laboratory and field experiments demonstrated that honeydew elevated colony growth, a crucial determinant of competitive performance, even when insect prey were not limiting. Carbohydrates, obtained largely through mutualistic partnerships with other organisms, thus represent critical resources that may aid the success of this widespread invasive species. These results illustrate the potential for mutualistic interactions to play a fundamental role in the establishment and spread of animal invasions.

A wide variety of animal communities are organized into interspecific dominance hierarchies associated with the control and harvest of food resources. Interspecific dominance relationships are commonly found to be linear. However, dominance relations within communities can form a continuum ranging from intransitive networks to transitive, linear dominance hierarchies. How interference competition affects community structure depends on the configuration of the dominance interactions among the species. This study explores how resource size and the trait-mediated indirect effect (TMIE) specialist phorid fly parasitoids exert on interference competition, affect the transitive nature of competitive interactions in an assemblage of woodland ants. I quantify the linearity of networks of interactions associated with large and small food resources in the presence and absence of phorid parasitoids. Two distinct, significantly linear dominance hierarchies exist within the ant assemblage depending on the size of the disputed resource. However, the presence of phorid fly parasitoids eliminates the linearity of both dominance hierarchies. The host's phorid defense behaviors reduce the competitive asymmetries between the host and its subdominant competitors, increasing the indeterminacy in the outcome of competitive interactions. Thus, both resource size variation and phorid-induced TMIEs appear to facilitate coexistence in assemblages of scavenging ants.

Summary 1.Trade-offs underpin local species coexistence. Trade-offs between interference and exploitative competitive ability provide a mechanism for explaining species coexistence within guilds that exploit overlapping resources. 2. Omnivorous, leaf litter ants exploit a shared food base and occur in species-rich assemblages. In these assemblages, species that excel at usurping food items from other species are poor at finding food items first. In assemblages where some members are attacked by phorid fly parasitoids, host species face an additional trade-off between defending themselves against parasitic attack and maximizing their competitive abilities. Host species thus face two trade-offs that interact via the trait-mediated indirect interaction generated by phorid defence behaviour. 3. In this study we test for the existence of these trade-offs and evaluate the predictions of a model for how they interact in an assemblage of woodland ants in which two behaviourally dominant members are attacked by phorid fly parasitoids as they attempt to harvest food resources. 4. The major findings are that unparasitized species in the assemblage follow a dominancediscovery trade-off curve. When not subject to attack by phorid flies, host species violate that trade-off by finding resources too quickly for their level of behavioural dominance. In contrast, when attacked by their phorid parasitoids, the host species dominance drops such that they fall into the assemblage trade-off. 5. These results match the predictions of the balance of terror model, which derives the optimal host response to parasitism, indicating that the host species balance the competing fitness costs of reduced competitive dominance and loss of workers to parasitism. This result supports the view that understanding the structure of ecological communities requires incorporating the indirect effects created by trait plasticity.