Ecologia das interações entre formigas e plantas com nectários extraflorais em uma savana neotropical: variação temporal na rede de interações e seus efeitos sobre a herbivoria foliar

Abstract

Studies of interactions between ants and plants with extrafloral nectaries are common in the literature, however, due to the great diversity of species involved and the variation in the results of interactions, little is known about the structure and dynamics of these relationships within the community. Thus, this study aimed to evaluate the network structure formed by ants and plants with extrafloral nectaries and the result of interactions for the most representative module of the cerrado plant community in a period of two years (from September 2008 to August 2010). The plant species studied were Caryocar brasiliense, Lafoensia pacari, Ouratea hexasperma, O. spectabilis, Qualea grandiflora, Q. multiflora, Q. parviflora, Stryphnodendron adstringens and S. polyphyllum. In total, 34 ant species were associated with nine plant species. The interactions network formed by these two groups of organisms exhibited similar characteristics to other mutualistic networks, such as nestedness, asymmetry in specialization and low specialty. This pattern held during the two years of study, although there were changes in number of associations and the ant species present in the network. The peak of associations between plants and ants occurred at the beginning of the rainy season, coinciding with the months of activity of the extrafloral nectaries of all species studied. In this period, the protection of ants against leaf herbivory was more efficient compared with the period of inactivity of nectaries. Experiments of ant exclusion showed that protection against leaf herbivory was effective for all species in the two years of study. However, for some species analyzed individually, the reduction of herbivory was not significant. This fact may be related to the attractiveness of plants to ants, represented by the quality of extrafloral nectar, which varied between plant species and among periods of the day (morning, afternoon, evening). Temporal partition of the species of ants foraging on plants was observed in this study, suggesting the existence of resource competition among species. Our results indicate that the ant-plant interactions with extrafloral nectaries in Cerrado form complex associations that are strongly structured, and that despite temporal variations, it retains the benefit to the associated plants. The study of a module of the network of interactions was important for the application of experimental manipulation and can be considered an important tool for studies of interactions in communities.