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Abstract: The release from bottom‐up control is a key process promoting the proliferation of leaf‐cutting ants (LCA) along neotropical rainforest edges. Considering the preference of LCA for drought‐stressed plants, edge‐induced drought‐stress in plants could be one of the mechanisms behind this bottom‐up response. We hypothesized that plants growing along the forest edge suffer higher levels of drought stress, which makes them a more suitable food resource for LCA and affects LCA colony density in a Brazilian Atlantic rainforest remnant. Additionally, we investigated whether this effect is stronger in north‐facing edges because the radiation load is higher in this edge aspect in the southern hemisphere. We examined LCA colony density and plant relative water content (RWC) by relating them to edge distance and edge orientation. We recorded 57 LCA colonies in which the density showed an approximately six‐fold increase within the first 50 m of the forest compared with distances > 150 m. North‐facing edges presented an approximately four‐fold higher colony density compared with other edge aspects. Among the 296 plant individuals sampled, the average RWC was unrelated to edge distance, although plants growing near north‐facing edges had approximately 10% lower RWC values compared with in south‐facing edges. We also found that, considering edge orientation, the lower the average RWC, the higher the LCA colony density. We conclude that drought stress‐induced osmoregulation of plants varies with edge orientation and also that LCA may profit from changes in leaf quality in terms of an increased population density along edges, where plant individuals face higher drought stress levels. [ABSTRACT FROM AUTHOR]

Major shifts in the availability of palatable plant resources are of key relevance to the ecology of leaf-cutting ants in human-modified landscapes. However, our knowledge is still limited regarding the ability of these ants to adjust their foraging strategy to dynamic environments. Here, we examine a set of forest stand attributes acting as modulating forces for the spatiotemporal architecture of foraging trail networks developed by Atta cephalotes L. ( Hymenoptera: Formicidae: Attini). During a 12-month period, we mapped the foraging systems of 12 colonies located in Atlantic forest patches with differing size, regeneration age, and abundance of pioneer plants, and examined the variation in five trail system attributes (number of trails, branching points, leaf sources, linear foraging distance, and trail complexity) in response to these patch-related variables. Both the month-to-month differences (depicted in annual trail maps) and the steadily accumulating number of trails, trail-branching points, leaf sources, and linear foraging distance illustrated the dynamic nature of spatial foraging and trail complexity. Most measures of trail architecture correlated positively with the number of pioneer trees across the secondary forest patches, but no effects from patch age and size were observed (except for number of leaf sources). Trail system complexity (measured as fractal dimension; Df index) varied from 1.114 to 1.277 along the 12 months through which ant foraging was monitored, with a marginal trend to increase with the abundance of pioneer stems. Our results suggest that some leaf-cutting ant species are able to generate highly flexible trail networks (via fine-tuned adjustment of foraging patterns), allowing them to profit from the continuous emergence/recruitment of palatable resources. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The role played by abandoned nests of leaf-cutting ants ( Atta spp.) as a small-scale disturbance regime that affects plant recruitment, species coexistence and forest regeneration remains poorly investigated. Here we examine whether abandoned nests of Atta cephalotes serve as regeneration niches and operate as particular plant recruitment habitats, favouring forest regeneration after ant activities cease and leading to the establishment of taxonomically/ecologically distinct plant assemblages. Soil properties, canopy openness, light availability and regenerating plant assemblages were evaluated across 18 nests and adjacent control plots in a large remnant of Atlantic Forest in north-east Brazil from December 2004 to December 2005. Surprisingly, nests and control plots exhibited very similar light environments irrespective of nest age, but nest soils exhibited substantial reductions in carbon content (1.45 ± 0.24 vs. 1.79 ± 0.13%) and organic matter (2.50 ± 0.41 vs. 3.08 ± 0.23%), and proved to be much more resistant to penetration (30.57 ± 6.08 vs. 39.48 ± 7.53 mm). Functional signature of regenerating plant assemblages exhibited little variation across both habitat types, as they were dominated by pioneer, small-seeded and vertebrate-dispersed species. However, abandoned nests exhibited less dense, impoverished and more homogeneous regenerating plant assemblages at local and landscape scale; they clearly lacked nest-dependent plant species and represented floristic subsets of the flora inhabiting the undisturbed forest. This recruitment bottleneck was transient in the long term because nest-related effects ameliorated in older nests. Our results suggest that, unlike treefall gaps, abandoned nests represent temporary (relatively long-lasting) islands of unsuitable substrate that reduce plant recruitment, retard forest regeneration, and fail in providing a special regeneration niche able to promote species coexistence and plant diversity. [ABSTRACT FROM AUTHOR]

Leaf-cutting ants ( Atta spp.) have become a topical issue in Neotropical ecology, particularly because they are reaching hyper-abundance due to escalating levels of fragmentation in recent years. Yet, despite intensive research on their role as dominant herbivores, there is still insufficient documentation on the impacts of their large, long-lived nests on plant assemblage structure and ecosystem functioning. Our study aimed at investigating the magnitude, nature, and spatial extent of nest influence by assessing 11 attributes of ant nest, canopy structure, light environment and sapling assemblage for 20 colonies in four plots along nest-understorey gradients in a large remnant of Atlantic forest. We also monitored the performance of seeds and seedlings of Chrysophyllum viride, an abundant shade-tolerant species. Previously unrecognized canopy gaps above ant nests (0.04–87.9 m2) occurred in 95% of all colonies surveyed. Overall, canopy openness and light availability at least doubled in ant nest plots compared with distant understorey plots. These drastic changes in the light environment paralleled those in plant assemblage: sapling density almost tripled (mean ± SE: 0.42 ± 0.1 saplings m−2) and sapling species richness doubled (0.16 ± 0.02 species m−2) in distant plots, as did shade-tolerant species. After a 1-year period, only 33 ± 15.6% of the seeds germinated and all seedlings died on nests, whereas seed germination reached 68 ± 5.1% in distant plots and 66.4 ± 7.6% of their seedlings survived after 12 months. Therefore, plot location was the most significant explanatory variable for predictable and conspicuous changes in the light environment and structure of sapling assemblages. Our findings greatly extend knowledge on the role played by leaf-cutting ants as ecosystem engineers by demonstrating that ant nest-mediated disturbance promotes environmental modifications in tens of meters around nests and is thus, strong enough to drive plant recruitment and consequently alter both the floristic and functional signature of plant assemblages. [ABSTRACT FROM AUTHOR]

Leaf-cutting ants (LCAs) profoundly benefit from edge creation in Neotropical forests, where they act as a keystone species and disturbance agent. In view of their poorly explored population dynamics, the question arises whether high densities of LCAs are a transitional or a persisting phenomenon. We studied the temporal variation of LCA colony densities at the edge of the Brazilian Atlantic forest. At physically stable edges of an old forest fragment, densities of Atta cephalotes and Atta sexdens (11 and five times higher in a 50 m edge zone in comparison with the forest interior) persisted over a 4-yr interval (2001–2005) with no significant difference in densities between years. Species-specific per colony growth rates ranged from 12 to −5 percent/yr, suggesting that populations were approximately at equilibrium. High rates of colony turnover (little less than 50% in 4 yr) indicated an average colony life span of about 7 yr—a life expectancy considerably lower than previous estimates for Atta colonies. Stable, hyper-abundant populations of LCAs accord with the constantly high availability of palatable pioneer vegetation (the preferred food source of LCAs) at forest edges and are expected to persist in time as long as forests are characterized by high edge to interior ratios, with potentially long-lasting consequences for the ecosystem. [ABSTRACT FROM AUTHOR]

Previous studies have shown that leaf-cutting ant populations benefit greatly from living in or near the edges of the Brazilian Atlantic forest. One of the mechanisms responsible for this rise in population density is an edge-mediated increase of pioneer plants, resulting in increased food availability for the ants (i.e., less bottom-up control). Here, we hypothesized that the release from natural enemies (i.e., less top-down control) may also contribute to the phenomenon. We investigated whether parasitism of phorid flies on leaf-cutting ants decreases in colonies located along the forest edge vs. the interior of a large tract of Atlantic forest in northeastern Brazil. For this, we assessed abundance and rates of oviposition attack by phorids in bimonthly intervals over a period of 1 year in 10 adult colonies of Atta cephalotes (L.) (Hymenoptera: Formicidae: Myrmicinae), five at the forest edge and five in the forest interior. The number of phorids attracted by ants at edge colonies was 40% lower than that at interior colonies. The temporal variation in phorid attraction was also significant, with approximately 35% fewer flies in the dry months as compared to the rainy months. As a result of lower phorid abundance, ant workers of edge colonies suffered three times fewer oviposition attacks than those of interior colonies. There was a tendency for fewer attacks during dry months, but the difference in the temporal variation was not significant. Our findings suggest that edge creation contributes to increased leaf-cutting ant abundance, not only via the attenuation of bottom-up forces, but also through an environmentally triggered depression of parasitoid abundance/efficiency, possibly because of adverse environmental conditions in edge habitats. [ABSTRACT FROM AUTHOR]