Ant assemblages are almost all related with the vegetation composition and so can provide us important information for conservation strategies, which are especially relevant to an environmentally protected area. We sampled the ant fauna in three different phytophysionomies in order to verify if the composition of ant species is different among the areas, especially because one of the areas is a Rocky Field and there is little information about the ant fauna in this habitat. A total of 8730 individuals were registered and an NMDS analysis showed that the ant assemblies are different at the three phytophysionomies (Rocky Field, Riparian Forest, and Secondary Forest). This study shows that the species that compose the ant assemblies in different phytophysionomies are a reflex of the environment, supporting the hypothesis that the vegetational composition results in different compositions in the ant assembly. Vegetal composition is determinant in the formation of the litter and consequently in the occurrence of ant species that depend on this layer of organic matter for nesting and foraging.
2005). This assumption is strikingly evident for leaf-cutting ants who travel along trails hundreds of meters long (Lewis et al., 2008). Walking long distances through a trail system is an intrinsic feature of leaf-cutting ants. The main function of this trail system is to guide foragers between the nest and the resource patch (Shepherd, 1982; Fowler & Stiles, 1980). Foraging trails arise from the recruitment process, which involves outbound scouts, who are the first workers to leave the nest seeking food. Once they find it, the scouts return to the nest laying chemical cues, which leads forager workers to the resource (Jaffe & Howse, 1979). After collecting leaves, foragers return to the nest also laying the trail pheromone. As a result, a positive feedback occurs, in which the more intense the pheromone trails, the more ants that are recruited and so on (Sumpter, 2006). This huge flow of individuals can
During foraging, thousands of leaf-cutting ant workers travel along high traffic foraging trails which, when narrow, reduce the leaf delivery rate due to the reduction in workers' travel-speed. On the other hand, high worker traffic promotes head-on encounters which are supposed to mediate worker task allocation and so could constitute a cue which induces traffic reduction. Very small workers along trails, for example, could change their task between marking the trail chemically to hitchhiking. Since they assume the hitchhiker function even in the absence of phorid parasitoids, one can suppose that hitchhiker behavior could be a strategy mediated by head-on encounters to avoid the high density of workers. Thus, we studied how the variation of worker density on the trail influences the hitchhiker frequency, testing the hypothesis that very small workers climb on the transported leaves to reduce trail traffic. Therefore, five Acromyrmex subterraneus colonies were linked to a foraging area by trails of different width (1.5 or 3 cm). We counted the number of hitchhikers and the outbound worker flow. The frequency of hitchhikers increased along narrow trails, and also due to outbound workers in both trail widths. Regardless of outbound foraging flow being comparable in both trail widths, the narrower ones had high density of workers leading to a presumed increase in head-on encounters. Head-on encounter rates cause a reduction in travel speed and, furthermore, are regulatory factors of task-allocation. Thus, high density trails lead to an increase in the rate of head-on encounters which could constitute as a stimulus to task-allocation of very small workers to the function of hitchhiker to avoid traffic jams.
The structure of an ant assemblage can be influenced by various factors, the main ones being mutualism, competition, parasitism and predation. Competition plays an important role in the structure of the assemblage, since competitive interactions could control the access of different species to resources (food and nesting sites), thus determining the coexistence among species within an assemblage. Evidence which supports the role of competition as a structuring factor of ant assemblages comes from observation of physical and chemical aggression among species to protect resources and territorial limits (Parr & Gibb, 2010). The high competitive potential of each ant species arises from the fact that most of them are omnivorous. This increases the competition effect
Seeds of different plant species constitute an alternative but also significant substrate that leaf-cutting ants use to cultivate their fungus garden. However, how they are processed inside the nest and if their use implies differential allocation of worker size classes are still poorly known. Using laboratory colonies of Acromyrmex subterraneus (Forel) as a model, the behaviors related to the processing of three different seeds (sesame, guava, and grape) as fungus substrate were listed. At the same time, we measured how each worker size class contributed to the execution of these behaviors by registering their respective frequency. It was found that medium-sized (1.2 > head width < 1.6 mm) and minimum-sized (head width <1.1 mm) workers assumed the role of incorporation for sesame and grape seeds, respectively. Major-sized workers (head width >1.7 mm) were concentrated on licking and holding guava seeds. Tegument removal was the only task observed that differs between treatment of seeds and treatment of leaves before their incorporation, as described in the literature. It was verified that different species of seeds imply a differential allocation of worker size classes and the inclusion or exclusion of some tasks from the behavioral repertoire. Regardless of the substrate type, leaf-cutting ant workers follow a coordinated and specialized procedure to cultivate the fungus garden but always maintain a high degree of cooperation.
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