Atta Nests Research Articles

Nest Architecture and Colony Growth of Atta bisphaerica Grass-Cutting Ants.

Atta grass-cutting ants (Formicidae: Myrmicinae: Attini) are found in the Cerrado biome and build giant nests with hundreds or thousands of large chambers. We assessed for Atta bisphaerica grass-cutting ants whether the total volume of fungus chambers and other nest parameters grow at close or similar proportions to worker numbers in the colony. Data on fungus garden biomass, population, external area, number of entrance holes, number of fungus chambers, total fungus chambers volume, and nest depth were obtained during colony growth/nest development. Our results reveal that the fungus garden biomass, external area, and total fungus chamber volume increased at rates similar to the increase in the number of workers. The total volume of chambers, and external area increased at a similar proportion to the increase in number of workers, probably due to the fungus garden allocation needs of the colony. The number of fungus chambers, number of entrance holes, and nest depth increased less than 4-fold for every 10-fold increase in the worker number. In addition, the height of the fungus chambers increased as the width increased, a pattern not observed for tunnel height and length, and the chamber volume increased according to worker number. Thus, this study demonstrates for A. bisphaerica that nest development in terms of chamber volume is similar to the increase in number of colony workers, and this contributes to a better understanding of Atta nest architecture.

Waste deposition in leaf-cutting ants is guided by olfactory cues from waste.

Social insects often use olfactory cues from their environment to coordinate colony tasks. We investigated whether leaf-cutting ants use volatiles as cues to guide the deposition of their copious amounts of colony refuse. In the laboratory, we quantified the relocation of a small pile of colony waste by workers of Atta laevigata towards volatiles offered at each side of the pile as a binary choice, consisting of either waste volatiles, fungus volatiles, or no volatiles. Fungus volatiles alone did not evoke relocation of waste. Waste volatiles alone, by contrast, led to a strong relocation of waste particles towards them. When fungus and waste volatiles were tested against each other, waste particles were also relocated towards waste volatiles, and in a high percentage of assays completely moved away from the source of fungus volatiles as compared to the previous series. We suggest that deposition and accumulation of large amounts of refuse in single external heaps or a few huge underground waste chambers of Atta nests is due to both olfactory preferences and stigmergic responses towards waste volatiles by waste-carrying workers.

Species turnover regulates leaf‐cutter ant densities in environmental gradients across the Brazilian Cerrado

AbstractEnvironmental changes resulting from Atta leaf‐cutter ant activities characterize them as ecosystem engineers of ecological and economic importance in multiple habitats. Although Atta effects depend strongly on colony density, there has been limited and inconsistent information available on their abundance and interspecific dominance in natural areas and across environmental gradients in the Brazilian Cerrado. Our study bridges this gap by testing the hypothesis that Atta nest density at genus level is high and relatively constant across locations regardless of vegetation type due to strong effects of dominant species turnover. To do so, we conducted a natural experiment surveying Atta nest abundance in five different vegetation types representing the environmental gradient from forest to savannah habitats found in the Cerrado. Our linear transect surveys covered 48.9 km in four well‐preserved areas across two Brazilian states in which we recorded 124 colonies. Of these, 84 nests belonged to A. laevigata and 40 to A. sexdens, the two dominant Atta species in the region. We also found no nests in 30% of the sampled area, which when combined with strong variation in density measured per transect indicates colonies tend to be aggregated in the landscape. Furthermore, we observed a strong dominant species turnover from forest habitats where 90% of nests belonged to A. sexdens to savannah habitats where 92% of nests were A. laevigata. Turnover effect was reflected in the absence of a significant difference in density between locations and vegetation types, indicating that density at genus level remains high and relatively constant regardless of nest aggregation. Our findings allow larger scale inferences about Atta effects and reveal new insights of their dynamics into natural areas that could affect plant species distribution and contribute to spatial heterogeneity of vegetation, having important implications for Cerrado conservation.

Performance and fate of tree seedlings on and around nests of the leaf-cutting ant Atta cephalotes: Ecological filters in a fragmented forest

Habitat fragmentation is currently the most pervasive anthropogenic disturbance in tropical forests and some species of leaf‐cutting ants of the genus Atta (dominant herbivores in the neotropics) have become hyper‐abundant in forest edges where their nests directly impact up to 6% of the forest area. Yet, their impacts on the regeneration dynamics of fragmented forests remain poorly investigated. Here we examine the potential of Atta cephalotes nests to function as ecological filters impacting tree recruitment. Growth, survival and biomass partitioning of experimentally planted seedlings (six tree species) were examined at eight spatially independent A. cephalotes colonies in a large Atlantic Forest fragment. Seedling performance and fate (leaf numbers and damage) were monitored up to 27 months across three habitats (nest centre, nest edge and forest understorey). Plants at illuminated nest centres showed twice the gross leaf gain as understorey individuals. Simultaneously, seedlings of all species lost many more leaves at nests than in the forest understorey, causing a negative net leaf gain. Net leaf gain in the shaded understorey ranged from zero (Licania and Thyrsodium species) to substantial growth for Copaifera and Virola, and intermediate levels little above zero for Protium and Pouteria. Also seedling survival differed across habitats and species, being typically low in the centre and at the edge of nests where seedlings were often completely defoliated by the ants. Lastly, seedling survival increased strongly with seed size at nest edges while there was no such correlation in the forest. Our results suggest that Atta nests operate as ecological filters by creating a specific disturbance regime that differs from other disturbances in tropical forests. Apparently, Atta nests favour large‐seeded tree species with resprouting abilities and the potential to profit from a moderate, nest‐mediated increase in light availability.

The effect of abandoned leaf-cutting ant nests on plant assemblage composition in a tropical rainforest of Costa Rica

ABSTRACT Leaf-cutting ant nests represent a potential source of disturbance within Neotropical forests that might favour selective establishment of some plant species. To test this hypothesis, I counted the number and determined the composition of plant species and individuals established in 10 abandoned Atta cephalotes nests and adjacent understory plots in an old-growth forest of Costa Rica. Specifically, I evaluated whether abandoned leaf-cutting ant nests differentially affect plant assemblages according to their 1) regeneration status, 2) seed size, and 3) dispersal mode. No differences were found in the relative abundance of species (and individuals) with different regeneration status, seed size, and dispersal mode between understory plots and abandoned Atta nests. Moreover, entire sites (abandoned ant nests and their nearby understory plots) were frequently grouped together according to those characteristics, suggesting that local effects were stronger than the effect of nests. Four non-mutually ex...

Production of Minima Workers by Gynes of Atta colombica Guérin-Ménéville (Hymenoptera: Formicidae: Attini) that Lack a Fungal Pellet

Abstract Early studies of Atta nest founding showed that gynes exhibit claustral nest foundation, cultivate fungal gardens with fecal secretions, and nourish their larvae with eggs. These studies also showed that gynes sometimes lose their fungal pellets, or the fungal garden fails before workers emerge, apparently dooming the incipient colony. Here we report that Atta colombica foundresses maintained in the laboratory can produce workers even though they lack fungal pellets to initiate gardens. If such behavior occurs in nature, it raises the possibility that workers might re-acquire a fungal symbiont after nest establishment, and potentially rescue failing colonies.

Combat between large derived societies: A subterranean army ant established as a predator of mature leaf-cutting ant colonies

Mature colonies of Atta leaf-cutting ants are dominant herbivores throughout the Neotropics. Although young colonies have natural enemies, mature colonies, which live in extensive nests containing millions of workers, currently have no recognised predators. New World army ants (Ecitoninae) are specialist social predators of other ants, and the army ant Nomamyrmex esenbeckii, a primarily subterranean species, is known to prey upon young Atta colonies. Here we present the results of the first long-term study of the predator-prey interaction between N. esenbeckii and Atta. Our study establishes the army ant N. esenbeckii as the only known predator capable of successfully attacking and killing mature as well as young colonies of Atta leaf-cutting ants. In natural raids, and experimental tests, Atta rapidly recruited their largest workers (majors) as a specific defensive response to N. esenbeckii raiders and both taxa used their largest individuals in the frontline of battles. The deployment and behaviour of these large workers demonstrates a size-related division of labour and agrees with the predictions of Lanchester’s Linear Law of Combat. Both taxa also used cooperative combat ‘teams’ to overwhelm large combatants from the other side. The success of N. esenbeckii raids varied greatly, such that they were prevented from entering Atta nests in the least successful raids, and completely overran Atta colonies in the most successful raids. The speed and magnitude of the defensive response of mature Atta colonies was key in determining the level of success of N. esenbeckii raids.

Do leaf‐cutting ant nests make “bottom–up” gaps in neotropical rain forests?: a critical review of the evidence

We discuss the role of leaf cutting ant nests (Atta sp.) on the formation of understory “bottom–up” gaps in tropical rain forests. We (i) describe the characteristics of Atta nests that qualify them as promoters of understory gaps; (ii) review the effects of Atta nests on the dynamics of systems other than tropical forests; and (iii) discuss the hypotheses that in neotropical rain forests Atta nests affect plant productivity, and also influence distribution and community composition of the flora. Finally, we discuss the importance of Atta nests in the dynamics of forests and compare their effects with the better‐known effects of treefall gaps. Atta nests modify soil structure, soil nutrient content, light intensity at ground level, seed bank composition and understory structure. The presence of the nest and associated refuse also accelerates the soil nutrient cycles. The size, high frequency, turnover rate and duration of these nests could make them a good substrate for particular species of plants. In spite of their potential importance, little information exists on the effects of Atta nests on plant productivity and/or plant composition in neotropical rain forests.

Leaf‐cutting ant preferences for five native tropical plantation tree species growing under different light conditions

AbstractTo examine the susceptibility of five Costa Rican tree species to leaf‐cutter ants (Atta cephalotes L.: Formicidae, Attini), young and mature leaves from trees that were grown in a plantation under full sun and partial shade were offered to six leaf‐cutter colonies located in full sun and six in partial shade. In addition to offering leaf disks to the ants, we offered large pieces of leaves to assess the effect of cutting leaves on food choices. Leaf‐cutters responded differently to each plant species, preferring Virola koschyni Warburg (Myristicaceae) and Hyeronima alchorneoides Allemao (Euphorbiaceae) over Stryphnodendrum microstachyum Poeppig & Endlicher (Mimosoideae), Pentaclethra macroloba Willdenow (Fabaceae) and Vochysia ferruginea Martius (Vochysiaceae). In agreement with previous studies, interspecific differences among the tree species in water and saponin content appeared to account for the observed ant preferences among the five tree species: leaf‐cutter ants preferred leaves and disks with more water and less saponins and the five tree species varied significantly in these traits. An observed positive correlation between preference and phenolic/nitrogen ratio appears to be spurious, and is instead due to a negative correlation between water content and nitrogen content. For the first time, Atta nest location has been shown to affect consumption: nests located in partial shade removed more leaf material than those from the sun plots. However, nest location had no effect on preference ranking of the tree species tested. Pieces of leaves from all the tree species grown in partial shade were significantly more removed than those trees grown in full sun. Leaf age, toughness, leaf specific weights, ant activity, and colony were not correlated with food choices. Because of the observed preferences, the five tree species should not be considered as equal candidates for plantation purposes. However, complete characterization of the candidate status of tree species for plantations in the neotropics must include information on the ability of such species to tolerate pest attacks in addition to their natural defenses to attack.

Changes in leaf‐cutting ant populations (Formicidae: Attini) after the clearing of mature forest in Brazilian Amazonia

This study investigates how clearing of mature forest for agriculture and ranching in Amazonia affects leaf‐cutting ant populations (Atta and Acromyrmex). Data on ant nest densities in mature forest, and in young (2–5 years) and older (6–18 years) second growth forests was obtained from five sites in the central and eastern regions of Brazilian Amazonia. Clearing of mature forest strongly increased the density of nests and altered the species present. Acromyrmex nest densities were up to 20 times higher in young secondary than in mature forests, whereas the density of Atta nests increased up to 30 times. Nest densities declined with growth of secondary forest, but nest densities in older secondary were still higher than in mature forests. Secondary forests were usually dominated by species of leafcutting ant not originally present in the mature forest. Evidence is presented to show that deforestation in Amazonia is increasing the geographical range of some leaf‐cutting ant species.

On Nest Densities of the Leaf-Cutting Ant Atta cephalotes in Tropical Primary Forest

Atta nest densities in six undisturbed primary rain forest ecosystems in the Orinoco-Amazon basin have been estimated. The only species encountered was Atta cephalotes which occurred at a maximal density of 0.045 nests per ha. Three possible explanations for these low densities in contrast to those found in disturbed habitats are discussed. The number of clearings in the forest is proposed to be the limiting factor for colonization of new forest sites by A. cephalotes.

The association ofAmphisbaena alba(Reptilia: Amphisbaenia) with the leaf-cutting antAtta cephalotesin Trinidad

Evidence suggests that Amphisbaena alba is a facultative inquiline of nests of the leaf-cutting ant Atta cephalotes in Trinidad. The evidence is culled from three main sources: (i) direct excavation of Atta nests; (ii) the ability of A. alba to follow Atta trail pheromones; (iii) the analysis of A. alba intestinal or faecal contents. When inside ant nests, A. alba may feed primarily on arthropods, notably beetles, which are themselves inquilines in ant nests. Studies on the life-cycle of a pentastomid parasite of A. alba, which depends for transmission on ant and beetle inquilines, also indicates that A. alba and Atta are intimately associated. The literature suggesting a regular association between ants (and termites) and amphisbaenians in general is reviewed.

The Impact of the Leaf Cutter Ant Atta Colombica on the Energy Flow of a Tropical West Forest

The patterns, quantity, and activities associated with the leaf—cutting of Atta colombica were studied August 20—27, 1971, in a lowland tropical wet forest at Osa Peninsula, Costa Rica (Lat. 08 42'N Long. 83 29'W). The study nest had an area of 44.8 m2 and covered 1.4 ha of forest floor. Daily, on a m2 of forest floor, the study nest had inputs of 10.17 leaf fragments with an area of 0.0108 m2, a weight of 0.0813 g, an ash content of 0.0039 g, and a potential energy of 0.3455 Kcalories. The ants return to the forest floor 0.0525 g of refuse with a potential energy of 0.0764 Kcalories and an ash content of 0.0212 g/m2 day. It was calculated that the leaf—cutting activity of ants reduced the gross production of the forest by 1.76 Kcal/m2 day but accelerated net production by at least 1.80 Kcal/m2 day through the return of ash rich in phosphorus to the forest floor. The size of the Atta nest may be determined by the balance of the energy input to the nest and the cost of obtaining, carrying (concentrating), and distributing the potential energy into the nest. Of a work force of 12,000 ants/m2 of trail, 75% were not carrying leaves and were assumed to be doing trail maintenance work. Rainfall and litter fall were the main obstacles of leaf transport, which was about 70% efficient. The ant's energy allocation for maintenance, which limits growth, and the establishment of reward feedbacks to their energy producers have implications for man's urban system development.