Behavioral plasticity mediates asymmetric competition between invasive wasps and native ants.

One of the most successful invasive species is the common wasp, Vespula vulgaris. We recently reported how foragers of this species have adopted previously unknown interference behavior when competing for food with native ants. Picking their opponents up in their mandibles, flying backward and dropping them some distance away from the disputed resource, wasps were shown to efficiently deal with a yet aggressive competitor and to modulate this behavior according to circumstances. Here we further discuss the nature and functioning of this unusual strategy. We first highlight the questions this interaction raises regarding the competitive advantages offered by asymmetries in body size and flight ability. Then, we argue that this study system illustrates the important role of behavioral plasticity in biological invasions; not only in the success of invaders but also in the ability of native species to coexist with these invaders.

A novel interference behaviour: invasive wasps remove ants from resources and drop them from a height.

This study reports a novel form of interference behaviour between the invasive wasp Vespula vulgaris and the New Zealand native ant Prolasius advenus. By videotaping interactions at bait stations, we found that wasps commonly remove ant competitors from food resources by picking up the workers in their mandibles, flying backward and dropping them unharmed some distance from the food. Both the frequency and the efficiency of the wasp behaviour significantly increased with the abundance of ant competitors. Ant removals were the most common interference events initiated by wasps when ants were numerous, while intraspecific conflicts among wasps were prominent when few ants were present. The 'ant-dropping' behaviour emphasizes how asymmetry in body sizes between competitors can lead to a pronounced form of interference, related to asymmetric locomotion modes.

Dynamics of the association between a long-lived understory myrmecophyte and its specific associated ants.

Myrmecophytic symbioses are widespread in tropical ecosystems and their diversity makes them useful tools for understanding the origin and evolution of mutualisms. Obligate ant-plants, or myrmecophytes, provide a nesting place, and, often, food to a limited number of plant-ant species. In exchange, plant-ants protect their host plants from herbivores, competitors and pathogens, and can provide them with nutrients. Although most studies to date have highlighted a similar global pattern of interactions in these systems, little is known about the temporal structuring and dynamics of most of these associations. In this study we focused on the association between the understory myrmecophyte Hirtella physophora (Chrysobalanaceae) and its obligate ant partner Allomerus decemarticulatus (Myrmicinae). An examination of the life histories and growth rates of both partners demonstrated that this plant species has a much longer lifespan (up to about 350 years) than its associated ant colonies (up to about 21 years). The size of the ant colonies and their reproductive success were strongly limited by the available nesting space provided by the host plants. Moreover, the resident ants positively affected the vegetative growth of their host plant, but had a negative effect on its reproduction by reducing the number of flowers and fruits by more than 50%. Altogether our results are important to understanding the evolutionary dynamics of ant-plant symbioses. The highly specialized interaction between long-lived plants and ants with a shorter lifespan produces an asymmetry in the evolutionary rates of the interaction which, in return, can affect the degree to which the interests of the two partners converge.

Predation and aggressiveness in host plant protection: a generalization using ants from the genus Azteca.

In studying the ant genus Azteca, a Neotropical group of arboreal species, we aimed to determine the extent to which the ants use predation and/or aggressiveness to protect their host plants from defoliating insects. We compared a territorially dominant, carton-nester, Azteca chartifex, and three plant-ant species. Azteca alfari and Azteca ovaticeps are associated with the myrmecophyte Cecropia (Cecropiaceae) and their colonies shelter in its hollow branches; whereas Azteca bequaerti is associated with Tococa guianensis (Melastomataceae) and its colonies shelter in leaf pouches situated at the base of the laminas. Whereas A. bequaerti workers react to the vibrations transmitted by the lamina when an alien insect lands on a leaf making it unnecessary for them to patrol their plant, the workers of the three other species rather discover prey by contact. The workers of all four species use a predatory behaviour involving spread-eagling alien insects after recruiting nestmates at short range, and, in some cases, at long range. Because A. alfari and A. ovaticeps discard part of the insects they kill, we deduced that the workers' predatory behaviour and territorial aggressiveness combine in the biotic defence of their host tree.

Nest site selection and induced response in a dominant arboreal ant species.

It is well known that arboreal ants, both territorially dominant species and plant ants (e.g., species associated with myrmecophytes or plants housing them in hollow structures), protect their host trees from defoliators. Nevertheless, the presence of an induced defense, suggested by the fact that the workers discovering a leaf wound recruit nestmates, is only known for plant ants. Based on the results from a field study, we show here (1) that colonies of Azteca chartifex, a territorially dominant, neotropical arboreal ant species, mostly selected Goupia glabra (Goupiaceae) trees in which to build their principal carton nests and (2) that plant signals induced workers to recruit nestmates, which patrol the leaves, likely providing the plant with a biotic defense. Furthermore, the number of recruited workers was clearly higher on G. glabra, their most frequently selected host tree species, than on other tree species. These results show that contrary to what was previously believed, induced responses are also found in territorially dominant arboreal ants and so are not limited to the specific associations between myrmecophytes and plant ants.

Indirect defense in a highly specific ant-plant mutualism.

Although associations between myrmecophytes and their plant ants are recognized as a particularly effective form of protective mutualism, their functioning remains incompletely understood. This field study examined the ant-plant Hirtella physophora and its obligate ant associate Allomerus decemarticulatus. We formulated two hypotheses on the highly specific nature of this association: (1) Ant presence should be correlated with a marked reduction in the amount of herbivory on the plant foliage; (2) ant activity should be consistent with the "optimal defense" theory predicting that the most vulnerable and valuable parts of the plant are the best defended. We validated the first hypothesis by demonstrating that for ant-excluded plants, expanding leaves, but also newly matured ones in the long term, suffered significantly more herbivore damage than ant-inhabited plants. We showed that A. decemarticulatus workers represent both constitutive and inducible defenses for their host, by patrolling its foliage and rapidly recruiting nestmates to foliar wounds. On examining how these activities change according to the leaves' developmental stage, we found that the number of patrolling ants dramatically decreased as the leaves matured, while leaf wounds induced ant recruitment regardless of the leaf's age. The resulting level of these indirect defenses was roughly proportional to leaf vulnerability and value during its development, thus validating our second hypothesis predicting optimal protection. This led us to discuss the factors influencing ant activity on the plant's surface. Our study emphasizes the importance of studying both the constitutive and inducible components of indirect defense when evaluating its efficacy and optimality.

Ant species diversity in the 'Grands Causses' (Aveyron, France): In search of sampling methods adapted to temperate climates.

This study aimed at showing the applicability of using a combination of four sampling methods (i.e., Winkler extractors, pitfall traps, baiting and manual collection), something most often conducted in the tropics, to create an inventory of ant species diversity in temperate environments. We recorded a total of 33 ant species in the Grands Causses by comparing three vegetal formations: a steppic lawn ('causse' sensu stricto), which was the most species-rich (29 species), followed by an oak grove (22 species) and a pine forest (17 species). No sampling method alone is efficient enough to provide an adequate sampling, but their combination permits one to make a suitable inventory of the myrmecofauna and to obtain information on the ecology of these ant species.

Coexistence between Cyphomyrmex ants and dominant populations of Wasmannia auropunctata.

The little fire ant Wasmannia auropunctata is able to develop highly dominant populations in disturbed areas of its native range, with a resulting negative impact on ant diversity. We report here on the tolerance of such populations towards several fungus-growing ants of the genus Cyphomyrmex (rimosus complex) in French Guiana. This tolerance is surprising given the usually high interspecific aggressiveness of W. auropunctata when dominant. In order to understand the mechanisms behind such proximity, aggressiveness tests were performed between workers of the different species. These behavioural assays revealed a great passivity in Cyphomyrmex workers during confrontations with W. auropunctata workers. We also found that the aggressiveness between W. auropunctata and two Cyphomyrmex species was more intense between distant nests than between adjacent ones. This dear-enemy phenomenon may result from a process of habituation contributing to the ants' ability to coexist over the long term.