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 m(2)) 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.

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).

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.

-(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 nonmyrmecochorous 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 Irmãos, 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 m 2 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.Key words -Atlantic forest, Atta sexdens, leaf-cutting ants, Protium heptaphyllum, seed dispersal RESUMO -(Coleta de sementes de Protium heptaphyllum (Aubl.) March. (Burseraceae) pela formiga cortadeira Atta sexdens L. promove agregação de sementes e mortalidade de plântulas). O papel das formigas cortadeiras como dispersoras de sementes de plantas não-mirmecocóricas permanece pouco conhecido. Neste artigo nós documentamos a remoção de sementes de Protium heptaphyllum (Aubl.) March. (Burseraceae) pela formiga cortadeira Atta sexdens L. e suas conseqüências em termos de (1) padrão de deposição de sementes; (2) germinação; e (3) mortalidade de plântulas. O estudo foi desenvolvido em Dois Irmãos, uma reserva de 390 ha de Floresta Atlântica no Nordeste do Brasil. As formigas coletaram 41,1% ± 19,7% (média ± s) da produção total de sementes e a remoção foi verificada em 18,5% de todas as árvores que frutificaram. Em média, as formigas amontoaram sementes a 3,4 ± 2,2 m de distância dos troncos das plantas-mãe e a densidade de sementes nas pilhas alcançou 128,8 ± 138,8 sementes 0,25 m 2 durante o pico de descarte pelas formigas. Durante o período de 13 meses, a mortalidade média das plântulas variou entre 0,54% até 10,6% nas pilhas formadas pelas formigas vs. 0,05%-4,2% nas amostras controles, o que resultou numa mortalidade total de 97,7% vs. 81% respectivamente. As formigas cortaram sistematicamente o epicótilo das plântulas, respondendo por 55% da mortalidad...