Search

How workers within an ant colony perceive and enforce colony boundaries is a defining biological feature of an ant species. Ants fall along a spectrum of social organizations ranging from single-queen, single nest societies to species with multi-queen societies in which workers exhibit colony-specific, altruistic behaviors towards non-nestmate workers from distant locations. Defining where an ant species falls along this spectrum is critical for understanding its basic ecology. Herein we quantify queen numbers, describe intraspecific aggression, and characterize the distribution of colony sizes for tawny crazy ant (Nylanderia fulva) populations in native range areas in South America as well as in their introduced range in the Southeastern United States. In both ranges, multi-queen nests are common. In the introduced range, aggressive behaviors are absent at all spatial scales tested, indicating that within the population in the Southeastern United States N. fulva is unicolonial. However, this contrasts strongly with intraspecific aggression in its South American native range. In the native range, intraspecific aggression between ants from different nests is common and ritualized. Aggression is typically one-sided and follows a stereotyped sequence of escalating behaviors that stops before actual fighting occurs. Spatial patterns of non-aggressive nest aggregation and the transitivity of non-aggressive interactions demonstrate that results of neutral arena assays usefully delineate colony boundaries. In the native range, both the spatial extent of colonies and the average number of queens encountered per nest differ between sites. This intercontinental comparison presents the first description of intraspecific aggressive behavior for this invasive ant and characterizes the variation in colony organization in the native-range, a pre-requisite to a full understanding of the origins of unicoloniality in its introduced range.

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.

The once-extensive tallgrass prairie community of North America has been reduced to small rem- nants, many of which are surrounded by intensive corn (Zea mays) agriculture. We investigated adult corn- rootworm beetles (Chrysomelidae: Diabrotica spp.), important pests of corn, on sunflowers (Asteraceae: He- lianthus spp.) in prairie remnants in southeast Minnesota. Large numbers of beetles invaded the prairie from surrounding corn fields in late summer. D. barberi and D. virgifera were captured on sticky traps in all loca- tions in the prairie, but abundance was much greater near the edge adjacent to corn. We observed D, barberi (but not D. virgifera) feeding extensively on sunflower pollen and occasionally on other flower parts, such as petak. Sunflowers located nearer corn fields sustained morefloral damage than those farther from om. To de- termine the effect of beetle &mage on seed set, we enclosed sunflower heads in bags with either zero, two, or four D. barberi adults. Seed set was reduced in heads enclosed with D. barberi Thus, this agriculturalpest may interfere with the successful reproduction of sunflowers and possibly otherprairie composites thatflower in late summer. Given the small size of mostprairie remnants and the abundance of thisflowerrfeeding beetle in land- scapes dominated by corn agriculture, D. barberi may affect the sustainability ofprairie plantpopulations.

Biological invasions represent communities in flux. Although stasis is never the rule in nature, biological interactions in communities usually occur within a framework of shared ecological and evolutionary history. Consequently, biological invasions represent unique opportunities to study dynamics that can otherwise be difficult to observe (Elton 1958). Invasive ants are excellent organisms with which to pursue this goal, in part because ants as a group play a variety of important ecological roles within biological communities (Holldobler and Wilson 1990). Ant invasions hold much potential for improving an understanding of ecological processes in general, as well as of interactions more specific to myrmecology. For instance, the dynamics that exist during ant invasions may reveal the traits that promote behavioural or ecological dominance. Furthermore, highly successful invasive ant species are often less conspicuous in their native ranges, so identification of the factors responsible for their greater prominence in introduced areas can provide insight into the more typical workings of ant communities, may highlight intrinsic differences between communities that have formed in separate biogeographic regions, and may suggest possible methods of control. Understanding ant invasion processes and causes of success thus has both basic and applied relevance. Human activities have introduced many ant species to new biogeographic regions (McGlynn 1999b; Chapter 13). While most introduced ant species have limited success in spreading away from the human-modified habitats in which they usually first arrive, a subset can invade nearby, or even distant, undisturbed natural habitats. This distinction is not always hard and fast, as virtually all species exhibit different degrees of invasiveness and ecological dominance in different locales, and sometimes even under similar conditions in the same locale (e.g. Abbott et al. 2007). Notwithstanding, the following species are the most consistent in their ability to penetrate natural ecosystems and affect the composition or abundance of native species within them (Holway et al. 2002a): the Argentine ant (Linepithema humile), the red imported fire ant (Solenopsis invicta), the tropical fire ant (S. geminata), the big-headed ant (Pheidole megacephala), the little fire ant (Wasmannia auropunctata), and the yellow crazy ant (Anoplolepis gracilipes) (see Plate 14). We focus on these species because of their pronounced invasive tendencies, and also because they have been studied the most intensively, especially with regard to the factors that control spread and underlie dominance. In fact, the majority of our knowledge on these topics comes from studies on just two species: S. invicta and L. humile. This represents both a major weakness and a clear avenue for progress in the field, pointing to a need for more complete information on the ecologies of other invasive ant species, as well as their close relatives that fail to become invasive (Chapter 13; see also Section 14.4). This limitation must be acknowledged at the outset, as it impinges on our under

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 ( Linepithema humile ) from southern California to infer how the trophic position of this widespread invasive species changes over time as native ant species are displaced. We couple this longitudinal analysis with a biregional comparison of stable isotope data (δ 15 N) on ants from Argentina (native range) and California (introduced range) to quantify ( i ) how the trophic position of L. humile differs between native and introduced populations, and ( ii ) how relative trophic position as estimated by δ 15 N values of Argentine ants compare with those of other ants at the same site. Both long-term and biregional comparisons indicate that the Argentine ant's relative trophic position is reduced at sites with a longer history of occupation. Over the course of 8 years, the relative trophic position of L. humile remained high at the leading edge of an invasion front but declined, on average, behind the front as native ants disappeared. Relative to native populations, where L. humile is among the most carnivorous of ants, Argentine ants from California occupied lower trophic positions. These results support the hypothesis that Argentine ants shift their diet after establishment as a result of resource depletion and increasing reliance on plant-based resources, especially honeydew-producing Hemiptera. Our results demonstrate the value of long-term and biregional data in uncovering ecological effects of invasions.

Flooding impacts ground nesting ant colonies by destroying the infrastructure housing and organizing societal function. Here, we report the convergent evolution in distantly related ant species of a behavioral trait that minimizes costs of flooding: the construction of earthen levees around nest entrances. In a South American floodplain ecosystem, we observed five ant species constructing prominent earthen berms encircling nest entrances shortly after large rainfall events. In four of these species, experimental flooding of nests demonstrated that earthen berms sufficed to prevent floodwaters from entering the below ground portions of the nest. Additional manipulations revealed that levee breaching caused, pronounced, and extended reductions in food collection for two distantly related species. Foraging was preempted by the allocation of workers to repair the internal structure of the nest. These findings represent convergent evolution of a functionally important nest construction behavior in response to comparable selective forces.