L' Annee biologique最新文献

Tropical arboreal ants are distributed in a mosaic pattern in the canopy of forests and tree crop plantations each of them characterised by their status of dominance. One can distinguish ‘dominant’ species, characterised by extremely populous societies and highly developed interspecific as well as intraspecific territorial behaviour. They tolerate on their territory nonterritorial and less populous species classified as ‘non-dominants’. Nonetheless, many species do exist whose status is intermediary. Usually, they behave like non-dominant species but are able, under certain conditions, to defend a territory. They are cited as ‘sub-dominant’. According to the chemical trapping method employed by researchers, the structure of mosaics have most often been studied using an index of dominance, characterised by the number of negative or positive associations between one species and the others. This index only covers the relative presence or absence of the different species on the same trees. It only gives a punctual statement on the structure of the mosaic without any notion of evolution of the mosaic in time. It does not take into account the behavioural intra- and interspecific interactions. Aggressive interactions between species depend on genetic and environmental factors. Many studies have shown that aggressiveness is closely related to a mechanism of interindividual discrimination, permitting an individual to discriminate between nestmates and non-nestmates. This colonial recognition is based on the existence of a ‘colonial odour’ as a result of a blending of ‘individual odours’. Each individual odour is due to cuticular hydrocarbons which play the role of a contact pheromone. The colonial odour also depends on the environmental odour of the nest. Aggressiveness which results from this mechanism of recognition can be expressed through different mechanisms such as territorial behaviour, dominance hierarchy, and ritualised aggressive behaviour. Territorial behaviour is the expression of a strong intraspecific aggressiveness, by which workers of a colony defend an area of their vital domain against neighbouring conspecifics. In arboreal ant mosaics, dominance hierarchy can exist between dominant ants, and should explain the overturning of dominant ants in time. Ritualised behaviours were observed under intra- and interspecific low-aggressiveness conditions and allow to economise the loss of one or several workers during fights whose issue are uncertain. Their systematic study would greatly facilitate understanding of the evolution of arboreal mosaics.

Due to the brood that it contains, the nest of social wasps constitutes a major source of protein for eventual predators. In the tropics, ants are generally considered as the most efficacious among them. In the face of such predation, wasps have developed over the course of evolution a great number of strategies. In addition to guarding and defensive behaviours displayed by workers, social wasps have nests whose structure is a response to the predation pressure exerted by ants. Thus, the presence of a petiole attaching the nest to a natural support itself sometimes selected because of its limited accessibility constitutes a frequent example of architectural defence. The effectiveness of the petiole is often improved through the repeated application of repellent substances on its surface. In the same way, wasps that found their new societies through swarming construct nests protected by an envelope. In the American tropics, army ants which hunt on the ground and can rapidly take over a tree, regularly pillage wasp nests. Because no defensive strategy is efficacious against such predators, the choice of nest site becomes decisive. Paradoxically, it is by nesting in close proximity to an arboreal ant nest that certain wasp species have been able to find a way of responding to the threat exerted by other ants. These ants, which tolerate the presence of the wasp nest, are capable of efficaciously protecting the tree or the branch sheltering the nest.

In communities of tree-nesting ants in tropical rain forests, energy-rich resources from plants enable ants to achieve high densities and permit the evolution of energy-intensive strategies of prédation. Ants are more abundant, and can maintain populations of phytophagous insects at lower densities than if ant populations were limited simply by insect prey. A large proportion of woody plants in tropical forests are involved in such loose protection mutualisms. Many epiphytes are also involved in loose associations with ants, in which benefits to plants may include nutrition and seed dispersal as well as protection. From such opportunistic interactions numerous symbiotic mutualisms have evolved, in which ants and plants are more intimately and often more specifically associated. The diversity of these symbiotic interactions between ants and plants make these associations good models for examining many general questions in ecology. Plants that have evolved specialised structures (‘ant-domatia’) to house ants are termed ‘myrmécophytes’ or ‘ant-plants’, and their specialised associates are termed ‘plant-ants’. In these symbioses, plants and ants have coevolved. Ant colonies that provide increased benefits to the host plant enhance its growth and survival, thus receiving more benefits from it, and vice versa. Selection favours mutualistic traits, and interests of the two partners tend to converge. However, because these associations are horizontally transmitted, neither partner obtains benefits from reproduction of the other. Because reproduction draws away resources from growth (from which the partner benefits), it introduces conflicts of interest between ants and plants, and several examples show the importance of such conflicts in the dynamics of coevolution. Antplant coevolution has produced parasites as well as mutualists. Much is still unknown on the evolutionary ecology of these symbioses. Mechanisms of interaction at the chemical level (chemical ecology) are little explored. The functioning of ant-plant associations at the level of populations and communities is poorly understood, and information in this domain is crucial for the conservation of these intricate symbioses in forests increasingly subjected to disturbance and fragmentation.

Arboreal ants constitute one of the largest animal groups in the tropical forest canopy not just in terms of their individual number and overall biomass, but also in terms of diversity. Because the arboreal habitat has particular characteristics which clearly differentiate it from the ground, the ants' ecological success depends on developing adaptations designed to overcome the constraints peculiar to this environment in order to best exploit its resources. Although it is not always possible to identify all of the characteristics common to all arboreal ants species, those species said to be ‘dominant’ share morphological, physiological and behavioural traits which differentiate them from other ants. The most important difference is the type of food sources exploited. These dominant species feed principally on the nectar of the extra-floral nectaries of plants or on the honeydew of Homoptera. These products are extremely rich in sugary substances and low in nitrogen. The exploitation of these food sources has, then, repercussions on the growth and reproduction of ant colonies, given the importance of nitrogen as a limiting factor. Obtaining the necessary amount of nitrogen involves exploiting large volumes of liquid. From a purely morphological point of view, it is interesting to note that the digestive system of most of the dominant ant species has a modified proventriculus and a thin cuticle which means less need for proteins. These modifications permit individual foragers to transport large volumes of liquid by holding them in their crops in order to distribute them to the rest of the colony. The venom of many of these species also differs from that of other ants in that it lacks nitrogen. Dominant species have, thus, developed adaptations related to their feeding habits, lowering the need for nitrogen in individuals and, in this way, permitting the greater development of the colony. The exploitation of this type of food source is also facilitated by the type of nests constructed by the ants. Indeed, the elaboration of a nest independently of any structure provided by the plant offers numerous advantages, despite the important energetic costs involved. These types of nests allow the ant to (1) ‘choose’ the nest site and thus to install the nest on plants providing the best resources (extra-floral nectaries or Homoptera attendance) and (2) exploit a greater number of resources with lower energy costs by placing parts of the colonies near food sources. In addition, even if part of the sugary food substances are destined for the synthesis of molecules necessary to the survival and proper functioning of individuals, there is a large surplus. This surplus might indirectly reinforce these species' defensive systems on both an inter- as well as intraspecific level by serving as an energy source enabling individuals to have high level of activity and aggressiveness. The remaining arboreal species, which — in terms of diversit