Paulo S. D. Silva

Decreasing abundance of leaf-cutting ants across a chronosequence of advancing Atlantic forest regeneration

Leaf-cutting ants (species of Atta and Acromyrmex ) are dominant herbivores and play a key role as ecosystem engineers of tropical and subtropical America (Fowler et al . 1989, Weber 1972). Not only are they among the most polyphagous and voracious herbivorous insects, cutting up to 15% y −1 of the leaf standing crop (Urbas et al . 2007, Wirth et al . 2003), but also they strongly affect the light environment and the nature of plant assemblages via ant-nest-mediated disturbances (Farji-Brener & Illes 2000, Hull-Sanders & Howard 2003, Moutinho et al . 2003). Some leaf-cutting ant species have turned into an omnipresent feature of present-day neotropical landscapes and a wealth of studies has documented their abundance to drastically increase with increasing agricultural land use, disturbance and deforestation/fragmentation (Fowler et al . 1986, Jaffe 1986, Terborgh et al . 2001, Vasconcelos & Cherrett 1995, Wirth et al . 2007). In view of their ecosystem engineering capacity and the ever-increasing conversion of tropical forests into agricultural landscapes (Wright 2005), it has been concluded that disturbance-driven accumulation of Atta colonies leads to far-reaching and deleterious consequences in present-day neotropical landscapes (Wirth et al . 2008). But what about the opposite scenario of regenerating forests? Is disturbance-mediated hyper-abundance of leaf-cutting ants a reversible phenomenon? We believe that this question is highly relevant because (1) knowledge of the dynamics of leaf-cutting ant populations during forest regeneration is lacking and (2) natural secondary succession has become a widespread phenomenon after land is abandoned or temporarily fallowed (Wright 2005). In the Brazilian Amazon during the 1990s, for example, secondary forests have reclaimed 31% of the once deforested land (Perz & Skole 2003).

Harvesting of Protium heptaphyllum (Aubl.) March. seeds (Burseraceae) by the leaf-cutting ant Atta sexdens L. promotes seed aggregation and seedling mortality

The role played by leaf-cutting ants as seed dispersers of non-myrmecochorous plants remains poorly understood. Here we document the harvesting of Protium heptaphyllum (Aubl.) March. seeds (Burseraceae) by the leaf-cutting ant Atta sexdens L. and its consequences for (1) seed deposition pattern; (2) seed germination; and (3) seedling mortality. The study was carried out at Dois Irmaos, a 390 ha reserve of Atlantic forest, northeast Brazil. Ant-seed harvesting on the ground was detected in 18.5% of all fruiting trees and ants harvested 41.1% ± 19.7% of the seed crop (mean ± s). In average, ants piled seeds 3.4 ± 2.2 m away from the trunk of parent trees and seed density in these piles reached 128.8 ± 138.8 seeds 0.25 m2 during the peak of seed discarding by ants. During a 13 month period, mean seedling mortality varied from 0.54% up to 10.6% in ant-made seed piles vs. 0.05-4.2% in control samples, what resulted in a total seedling mortality of 97.7% vs. 81%. Ants systematically cut seedling epicotyls, accounting for 55% of seedling mortality in seed piles, whereas only 14 seedlings (4.2%) were cut by ants in the control samples. Our results suggest that seed harvesting by A. sexdens (1) affects approximately 20% of fruiting P. heptaphyllum trees and their seed crops; (2) promotes short-distance seed dispersal and high levels of seed aggregation; and (3) reduces seedling survival beneath parents.

Leaf-cutting ants alter seedling assemblages across second-growth stands of Brazilian Atlantic forest

Secondary forests promote an increased abundance of leaf-cutting ants (LCA) but the consequences on forest dynamics have been poorly addressed. Here we examine seedling assemblage attributes, seed germination and seedling survival across foraging zones of Atta cephalotes colonies inhabiting 15 second-growth patches (25–47 y old) of Atlantic forest. Seeds and seedlings were monitored within foraging zones and control sites over 1 y, including spots around ant nests. Overall, 1862 seedlings from 108 plant species were recorded. Seedling density decreased by 53% in foraging-zone plots (3.31 ± 0.23 seedlings m −2 ) when compared with control plots (7.02 ± 0.44 seedlings m −2 ) and a similar decrease was observed for species richness. Ant-induced alterations in the seedling assemblage were further indicated by segregation between foraging-zone and control plots (NMDS), habitat effects on species similarity (ANOSIM), and indicator tree species associated with control plots. While seed germination and seedling survivorship were uncorrelated to either nest distance or age of second-growth stands (with the exception of Tapirira guianensis ), defoliation by LCAs was a significant cause of seedling mortality. Our results suggest that LCAs interfere with successional trajectories of Atlantic forest as foraging zones filter seedling establishment, supporting less-dense, impoverished and convergent seedling assemblages.

Foraging in highly dynamic environments: leaf-cutting ants adjust foraging trail networks to pioneer plant availability

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.

Foraging activity of leaf-cutting ants changes light availability and plant assemblage in Atlantic forest

1. Leaf‐cutting ants (LCAs) have often been denoted as ecosystem engineers because of their multifarious effects on the vegetation, particularly via nest‐driven environmental changes. However, the non‐trophic impacts of LCAs on forest dynamics via foliage harvesting across sizeable foraging zones (so‐called associated ecosystem engineering) are still poorly investigated.

Natural History and Ecological Correlates of Fungus-Growing Ants (Formicidae: Attini) in the Neotropical Cerrado Savanna

ABSTRACT Fungus-growing ants (Formicidae: Attini) comprise a diverse and ecologically important group in Neotropical habitats. Compared with leaf-cutters, however, relatively little is known about the biology of less conspicuous attine species. Here, we compare nest size and structure, colony size and demographic composition, and worker size and polymorphism among the genera Cyphomyrmex, Mycetarotes, Mycocepurus, Myrmicocrypta, Sericomyrmex, and Trachymyrmex. In total, 25 ant colonies (one species per genus) were investigated at one site in the Brazilian savanna. Results indicate a consistent variation in nest size and structural complexity (architecture), colony and worker size, and a tendency to polymorphism among the studied genera. In addition, nest mound volume was found to be a good predictor of both worker number and volume of the fungus garden. Based on morphometric analyses, Sericomyrmex and Trachymyrmex together formed a distinct group from the other genera. The transition from the ancestral agricultural system toward the derived leaf-cutting habit also is followed by remarkable changes in nest size and architecture, colony size, and worker size and polymorphism. Our results support other recent studies that consider Sericomyrmex and Trachymyrmex as possessing transitional habits, distinct both from species that cultivate fungus by using mostly nonplant items (insect feces and corpses) as well as from typical leaf-cutters Atta and Acromyrmex. This is the first study to detect correlations of nest traits with worker number and size of fungus garden in the less conspicuous attines. Results highlight the importance of combining data on natural history and morphometry to understand the evolutionary history of fungus-growing ants.

Attractiveness of fallen fleshy fruits to ants depends on previous handling by frugivores

Abstract: Interactions between ants and vertebrate-dispersed fleshy fruits are common on the floor of tropical forests. These interactions are considered beneficial to seeds if ants clean the seed (which may prevent fungal contamination and/or increase germination success) or act as secondary fine-scale dispersers (depositing the seed in nearby nutrient-enriched nest sites). Benefits to seeds, however, may be constrained by limited ant access to fruits. Here, we experimentally tested whether ant access to fallen fleshy fruits of Psychotria suterella (Rubiaceae) can be affected by previous handling (including ingestion) by birds. We found that mandibulated fruits and fruits embedded in feces are more prone to be visited by ants compared to entirely intact fruits. Moreover, the number of ant species per station was higher for vertebrate-handled fruit categories. We suggest that ongoing vertebrate loss in tropical forests may affect this highly opportunistic and potentially mutualistic interaction.

Assessing the Impact of Deforestation of the Atlantic Rainforest on Ant-Fruit Interactions: A Field Experiment Using Synthetic Fruits

Ants frequently interact with fleshy fruits on the ground of tropical forests. This interaction is regarded as mutualistic because seeds benefit from enhanced germination and dispersal to nutrient-rich microsites, whereas ants benefit from consuming the nutritious pulp/aril. Considering that the process of deforestation affects many attributes of the ecosystem such as species abundance and composition, and interspecific interactions, we asked whether the interaction between ants and fallen fleshy fruits in the Brazilian Atlantic forest differs between human-created fragments and undisturbed forests. We controlled diaspore type and quantity by using synthetic fruits (a plastic ‘seed’ covered by a lipid-rich ‘pulp’), which were comparable to lipid-rich fruits. Eight independent areas (four undisturbed forests, and four disturbed forest fragments) were used in the field experiment, in which we recorded the attracted ant species, ant behaviour, and fruit removal distance. Fruits in undisturbed forest sites attracted a higher number of species than those in disturbed forests. Moreover, the occurrence of large, fruit-carrying ponerine ants (Pachycondyla, Odontomachus; 1.1 to 1.4 cm) was higher in undisturbed forests. Large species (≥3 mm) of Pheidole (Myrmicinae), also able to remove fruits, did not differ between forest types. Following these changes in species occurrence, fruit displacement was more frequent in undisturbed than in disturbed forests. Moreover, displacement distances were also greater in the undisturbed forests. Our data suggest that fallen fleshy fruits interacting with ants face different fates depending on the conservation status of the forest. Together with the severe loss of their primary dispersers in human-disturbed tropical forest sites, vertebrate-dispersed fruits may also be deprived of potential ant-derived benefits in these habitats due to shifts in the composition of interacting ant species. Our data illustrate the use of synthetic fruits to better understand the ecology of ant-fruit interactions in variable ecological settings, including human-disturbed landscapes.

Does inundation risk affect leaf-cutting ant distribution? A study along a topographic gradient of a Costa Rican tropical wet forest

Successional state of forest and availability of pioneer plants are recognized factors affecting densities of leaf-cutting ants. However little is known about how abiotic factors can shape nest distributions. We investigated the effect of topography, soil, forest successional state and inundation risk on nest density and size of Atta cephalotes colonies along streams in a tropical wet forest in Costa Rica. In each forest type, we surveyed 12 sites, each site comprising five transects (10 × 100 m) varying in topography and proximity to streambeds. We found no difference regarding nest size or density between forest types or soil consociation. Nest density varied with topographic environment, with significantly higher nest density on slope tops (farther from streambeds) and without colonies in valley bottoms (closer to streambeds). Nests found in areas affected by the last great inundation before our study were scarcer and smaller than nests in non-flooded areas. We showed that inundation events favour an accumulation of Atta colonies towards higher sites, where they are also allowed to become larger and may survive longer. Inundation risk may be a strong force shaping the distribution of leaf-cutting ant nests in tropical floodplain forests, even concealing the relevance of successional state of forest.

Practical Urban: The Urban Furniture and Its Relationship with the City

This article explains the concepts of city and urban practices—phenomena of urban activity—that directly influence the deployment of street furniture and, above all, the importance given to them by the population, with regard to its true functions, identity, meaning (symbology), uses and appropriations. It is vitally important for the study also understand the urban furniture relation to the design of cities, is to complement the public space, is the way interferes with the urban landscape. You have to understand how society is shown in front of herself and the world itself that surrounds it, and what are the devices that make city living when connect—through uses and customs—as this is the vital forces of individuals and community space practices created by the tactics of the inhabitants to allow its ambiance, wellness, safety and comfort, sensations often perceived by the set of elements that constitute the urban furniture of cities.