Roberth Fagundes

Individual-based ant-plant networks: diurnal-nocturnal structure and species-area relationship.

Despite the importance and increasing knowledge of ecological networks, sampling effort and intrapopulation variation has been widely overlooked. Using continuous daily sampling of ants visiting three plant species in the Brazilian Neotropical savanna, we evaluated for the first time the topological structure over 24 h and species-area relationships (based on the number of extrafloral nectaries available) in individual-based ant-plant networks. We observed that diurnal and nocturnal ant-plant networks exhibited the same pattern of interactions: a nested and non-modular pattern and an average level of network specialization. Despite the high similarity in the ants’ composition between the two collection periods, ant species found in the central core of highly interacting species totally changed between diurnal and nocturnal sampling for all plant species. In other words, this “night-turnover” suggests that the ecological dynamics of these ant-plant interactions can be temporally partitioned (day and night) at a small spatial scale. Thus, it is possible that in some cases processes shaping mutualistic networks formed by protective ants and plants may be underestimated by diurnal sampling alone. Moreover, we did not observe any effect of the number of extrafloral nectaries on ant richness and their foraging on such plants in any of the studied ant-plant networks. We hypothesize that competitively superior ants could monopolize individual plants and allow the coexistence of only a few other ant species, however, other alternative hypotheses are also discussed. Thus, sampling period and species-area relationship produces basic information that increases our confidence in how individual-based ant-plant networks are structured, and the need to consider nocturnal records in ant-plant network sampling design so as to decrease inappropriate inferences.

Loss and gains in ant–plant interactions mediated by extrafloral nectar: fidelity, cheats, and lies

All mutualistic plant–animal interactions are mediated by costs and benefits in relationships where resources (from plants) are exchanged by services (from animals). The most common trading coin that plants offer to pay for animal services is nectar; the main servers are hymenopterans. Extrafloral nectar (EFN) is produced in almost all aboveground plant parts not directly related with pollination, and their true function has long been an issue of discussion among naturalists and will be our main subject. The protective function of extrafloral nectaries (EFNs) is reviewed and considered with an alternative hypothesis, presenting not only ants, but also spiders and wasps as potential and effective agents in these protective interactions. Despite their likely relevance, the phenological variation (mainly sequential flowering and resprouting) of host plants mediating these interactions have been generally ignored. We discuss how the outcomes of each ant–EFN bearing plant interaction vary depending on physical and biotic changes in interacting organisms (internal factors such as phenology and species identity) as well as in their environments (external factors such as climatic variation), all of which may modify the character of each interaction. We propose that ant–EFN bearing plant interactions serve an excellent and unique model to test the “Geographic Mosaic Theory” of coevolution providing us a more clear view of how evolution has structured these plant–animal ecological networks.

Effects of the Trophobiont Herbivore Calloconophora pugionata (Hemiptera) on Ant Fauna Associated with Myrcia obovata (Myrtaceae) in a Montane Tropical Forest

Many studies have investigated the mechanisms behind the structure of arboreal ant assemblages. In this study, the objective was to evaluate the effect of availability of honeydew-producing colonies of Calloconophora pugionata (Membracidae) on the structure of ant assemblages associated with the host plant Myrcia obovata (Myrtaceae) in an Atlantic forest of Minas Gerais (Brazil). Our experiment consisted in a gradual exclusion of hemipteran colonies out of the host plant crown and further record of the ant assemblage response (species richness, composition, and occurrence) to the presence and density of treehopper colonies. The hypothesis was that an increase in the number of trophobiont herbivores results in an increase in tending ant occurrence but a reduction in ant species diversity. Results corroborated our main hypothesis: membracids had a positive effect on the occurrence of ants but negative on species richness. Overall insect occurrence was also reduced with increasing in C. pugionata colonies, probably due to strengthening dominant ant species territory sizes and intensification of patrolling.

The bamboo Merostachys fischeriana (Bambusoideae: Bambuseae) as a canopy habitat for ants of neotropical montane forest.

Although Merostachys fischeriana is very abundant in the Brazilian Atlantic Rainforest, little attention has been paid to the biological interactions with other animals. The present study describes some of the interactions between ants and this bamboo species. The experiment was carried out in a fragment of a montane tropical forest in the Parque Estadual do Itacolomi, near Ouro Preto, MG, Brazil. Thirty culms of bamboo were randomly collected. The ants were obtained by direct collection from nodes and internodes. Morphometric variables of the bamboo were recorded for characterization of potential ant habitat. Merostachys fischeriana grows in rosettes as a thin bamboo (average = 1,0 cm; se = 0,27; n = 20) and is tall enough to reach the upper canopy of this low forest (average = 9,1 m; se = 2,72; n = 20). Fifteen ant species were sampled. Brachymyrmex heeri Forel was the most abundant in the nodes, while Camponotus crassus Mayr (Hymenoptera: Formicidae) was the most abundant in the internodes. The composition of the species that inhabit the internodes was different from the composition in the node (Q-test: Q = 3,76; P = 0,05). The level of occupation was defined by the number of holes (F = 10,33; P < 0,01), the number of internodes in the canopy (F = 6,84; P = 0,01) and the length of the culm (F = 7,52; P = 0,01). The plants morphology allowed the occurrence of additional species of ants in the canopy and influenced the composition of the entire ant assemblage.

Food source availability and interspecific dominance as structural mechanisms of ant-plant-hemipteran multitrophic networks

Extrafloral nectar of plants and honeydew of hemipterans is a food source extensively explored by ants. Although basically a sugary liquid food, nectar and honeydew are composed of different nutrients and offered in distinct ways; thus, ants must interact differently with plants and hemipterans. In this study we assessed the availability and dominance of nectar of extrafloral nectaries and honeydew of sap-sucking hemipterans (i.e., sugar-based resources) as mechanisms regulating interaction frequency and structuring ant-plant-hemipteran networks. We studied 12 plant species (240 shrubs, 20 per species) and 12 hemipteran species (240 aggregations, 20 per species) that interacted with 26 ant species in an area of Rupestrian Fields (Rocky Montane Savannah), Brazil. We observed that the 7 ant species that collected honeydew were a subset of the 25 ant species feeding on nectar, but the highly interacted species Camponotus crassus was the same for both subnetworks. The ant-plant subnetwork exhibited a nested pattern of interaction with a low degree of specialization, while the ant-hemipteran subnetwork exhibited lower nestedness but higher specialization. We found a positive relationship between the offer of EFNs and the number of interactions with ants, probably resulting from reduced competition in plants with high availability of EFNs. However, hemipteran species that were most abundant did not interact with more species of ants, probably because of the numerical dominance of the species tending all hemipteran aggregations, regardless of size. However, segregation between ant species was higher than expected by chance for both plants and hemipterans, confirming a deterministic factor (i.e., competition between ant species) regulating the frequency of interactions. In summary, the availability of ENFs seems to be an important mechanism regulating ant-plant interactions, while numerical dominance seems to be an important mechanism structuring ant-hemipteran interactions.