Richness and Composition of Ground-dwelling Ants in Tropical Rainforest and Surrounding Landscapes in the Colombian Inter-Andean Valley.

Tropical rainforests are characterized by having high structural complexity, stratification, and species diversity. In Colombia, tropical rainforests are critically endangered with only 24% of their area remaining. Forest fragments are often valued based on the presence of vertebrate taxa despite that small habitat remnants may still harbor diverse invertebrate communities. We surveyed the ant fauna associated with rainforest fragments and their surrounding landscape elements (including mature forests, flooded forests, gallery forests, live fences, and pastures) in the Magdalena River watershed. Pitfall traps and litter samples were used to estimate ant richness and diversity, and to compare ant composition among landscape elements. We found 135 species from 42 genera, representing 16% of the species and 43% of the genera known for Colombia. Our surveys also uncovered 11 new ant records for the Colombian inter-Andean region and 2 new records for the country of Colombia: Mycocepurus curvispinosus (Mackay) and Rhopalothrix isthmica (Weber). The highest species richness was found in forest-covered sites, and richness and diversity was lower in the disturbed landscapes surrounding the forest patches. Species composition varied significantly between all habitat types, but was most similar between forest types suggesting that a loss of structural complexity has the greatest effect on ant communities. Across our study sites, ten species showed the greatest response to habitat type and could qualify as indicator taxa for this region. We conclude by discussing the value of conserving even small forests in this landscape due to their ability to retain high diversity of ants.

Caste determination in a polymorphic social insect: nutritional, social, and genetic factors.

We examined how dietary, social, and genetic factors affect individual size and caste in the Florida harvester ant Pogonomyrmex badius, which has three discrete female castes. The diet that a larva consumed, as indicated by delta(13)C, delta(15)N, and C:N, varied with caste. Both N content and estimated trophic position of dietary input was higher for major than for minor workers and was highest for gynes (reproductive females). The size and resources of a colony affected the size of only minor workers, not that of gynes and major workers. Approximately 19% of patrilines showed a bias in which female caste they produced. There were significant genetic effects on female size, and the average sizes of a major worker and a gyne produced by a patriline were correlated, but neither was correlated with minor worker size. Thus, genetic factors influence both caste and size within caste. We conclude that environmental, social, and genetic variation interact to create morphological and physiological variation among females in P. badius. However, the relative importance of each type of factor affecting caste determination is caste specific.

Genetics and behavior of a colonizing species: the invasive Argentine ant.

Baker and Stebbins's 1965 book The Genetics of Colonizing Species aimed to draw together scientists from a variety of disciplines to provide a conceptual framework for the study of species introductions. A goal of their volume was to examine how studies on biological invasions could be used to provide insight into basic research questions as well as to develop practical strategies for control. In this article, we attempt to follow the goals of Baker and Stebbins by reviewing work on the genetics and behavior of a widespread colonizing species, the Argentine ant (Linepithema humile). Specifically, we examine the evolutionary changes that have taken place as a result of this species being introduced into new environments and synthesize recent research on Argentine ants from the perspective of population genetics, recognition systems, and the mechanisms that may underlie their ecological success.

An experimental study of competition between fire ants and Argentine ants in their native range.

An understanding of why introduced species achieve ecological success in novel environments often requires information about the factors that limit the abundance of these taxa in their native ranges. Although numerous recent studies have evaluated the importance of natural enemies in this context, relatively few have examined how ecological success may result from differences in the magnitude of interference competition between communities in the native and introduced ranges of nonnative species. Here we examine how native-range competitive environments may relate to invasion success for two important invasive species, the red imported fire ant (Solenopsis invicta) and the Argentine ant (Linepithema humile), in a region of native-range sympatry. At two study sites in northern Argentina, we used stable-isotope analysis, a variety of observational approaches, and two different reciprocal removal experiments to test (1) whether S. invicta competes asymmetrically with L. humile (as suggested by the 20th century pattern of replacement in the southeastern United States) and (2) the extent to which these two species achieve behavioral and numerical dominance. Stable-isotope analysis and activity surveys indicated that S. invicta and L. humile are both omnivores and forage during broadly overlapping portions of the diel cycle. Short-term removal experiments at baits revealed no competitive asymmetry between S. invicta and L. humile. Longer-term colony removal experiments illustrated that S. invicta and L. humile experience an approximately equal competitive release upon removal of the other. Our results indicate that neither S. invicta nor L. humile achieves the same degree of behavioral or ecological dominance where they co-occur in native populations as they do in areas where either is common in their introduced range. These results strongly suggest that interspecific competition is an important limiting factor for both S. invicta and L. humile in South America.

Multifunctionality and mechanical origins: ballistic jaw propulsion in trap-jaw ants.

Extreme animal movements are usually associated with a single, high-performance behavior. However, the remarkably rapid mandible strikes of the trap-jaw ant, Odontomachus bauri, can yield multiple functional outcomes. Here we investigate the biomechanics of mandible strikes in O. bauri and find that the extreme mandible movements serve two distinct functions: predation and propulsion. During predatory strikes, O. bauri mandibles close at speeds ranging from 35 to 64 m.s-1 within an average duration of 0.13 ms, far surpassing the speeds of other documented ballistic predatory appendages in the animal kingdom. The high speeds of the mandibles assist in capturing prey, while the extreme accelerations result in instantaneous mandible strike forces that can exceed 300 times the ant's body weight. Consequently, an O. bauri mandible strike directed against the substrate produces sufficient propulsive power to launch the ant into the air. Changing head orientation and strike surfaces allow O. bauri to use the trap-jaw mechanism to capture prey, eject intruders, or jump to safety. This use of a single, simple mechanical system to generate a suite of profoundly different behavioral functions offers insights into the morphological origins of novelties in feeding and locomotion.

Relationships among native and introduced populations of the Argentine ant (Linepithema humile) and the source of introduced populations.

The Argentine ant (Linepithema humile) is a damaging invasive species that has become established in many Mediterranean-type ecosystems worldwide. To identify likely sources of introduced populations we examined the relationships among native Linepithema populations from Argentina and Brazil and introduced populations of L. humile using mitochondrial cytochrome b sequence data and nuclear microsatellite allele frequencies. The mitochondrial phylogeny revealed that the populations in Brazil were only distantly related to both the introduced populations and the native populations in Argentina, and confirmed that populations in Brazil, previously identified as L. humile, are likely a different species. The microsatellite-based analysis provided resolution among native and introduced populations of L. humile that could not be resolved using the mitochondrial sequences. In the native range, colonies that were geographically close to one another tended to be genetically similar, whereas more distant colonies were genetically different. Most samples from the introduced range were genetically similar, although some exceptions were noted. Most introduced populations were similar to native populations from the southern Rio Parana and were particularly similar to a population from Rosario, Argentina. These findings implicate populations from the southern Rio Parana as the most likely source of introduced populations. Moreover, these data suggest that current efforts to identify natural enemies of the Argentine ant for biological control should focus on native populations in the southern Rio Parana watershed.

Patterns of spread in biological invasions dominated by long-distance jump dispersal: Insights from Argentine ants.

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 human-mediated 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. Human-mediated 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 m/year, 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.

Reduced genetic variation and the success of an invasive species.

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.