How leaf-cutting ants impact forests: drastic nest effects on light environment and plant assemblages.

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.

Diversidade de formigas epigéicas (Hymenoptera: Formicidae) em capões do Pantanal Sul Matogrossense: relações entre riqueza de espécies e complexidade estrutural da área / Ant diversity (Hymenoptera: Formicidae) from capões in Brazilian Pantanal: relationship between species richness and structural complexity

A riqueza de formigas de capões do Pantanal brasileiro foi amostrada e relacionada com a complexidade estrutural das áreas. As formigas foram coletadas com armadilhas de solo do tipo pitfall em 28 capões na Fazenda Rio Negro, Município de Aquidauana, MS. A complexidade estrutural dos capões foi avaliada através de estimativa da densidade da vegetação e da espessura da serapilheira próximo às armadilhas. Setenta e uma espécies foram encontradas, pertencentes a 26 gêneros e sete subfamílias, das quais Ectatomma edentatum Roger (Formicidae: Ectatomminae) e uma espécie de Pheidole foram as mais freqüentes. Somente foi encontrada relação positiva entre a riqueza da comunidade de formigas e a densidade da vegetação herbácea, corroborando a idéia de que a heterogeneidade do ambiente é um fator determinante para a coexistência e diminuição de competição entre espécies.

Species richness of epigeic ants was surveyed in forest islands named capões of Brazilian Pantanal and related with their structural complexity. The ants were collected using pitfall traps in 28 capões from Rio Negro Farm, in Aquidauana municipality, Mato-Grosso do Sul state, Brazil. The structural complexity of capões was evaluated by measuring vegetation density and litter quantity near the pit-fall traps. Seventy-one species, distributed in 26 genera and seven sub-families were found. Ectatomma edentatum Roger (Formicidae: Ectatomminae) and one species of Pheidole were the most frequent species. Species richness was positively correlated only with herbaceous vegetation density of capões, supporting the idea that the increase in environmental heterogeneity diminishes species competition, allowing species co-occurrence.

[Ant diversity (Hymenoptera: Formicidae) from capões in Brazilian Pantanal: relationship between species richness and structural complexity]. / Diversidade de formigas epigéicas (Hymenoptera: Formicidae) em capões do Pantanal Sul Matogrossense: relações entre riqueza de espécies e complexidade estrutural da área.

Species richness of epigeic ants was surveyed in forest islands named capões of Brazilian Pantanal and related with their structural complexity. The ants were collected using pitfall traps in 28 capões from Rio Negro Farm, in Aquidauana municipality, Mato-Grosso do Sul state, Brazil. The structural complexity of capões was evaluated by measuring vegetation density and litter quantity near the pit-fall traps. Seventy-one species, distributed in 26 genera and seven sub-families were found. Ectatomma edentatum Roger (Formicidae: Ectatomminae) and one species of Pheidole were the most frequent species. Species richness was positively correlated only with herbaceous vegetation density of capões, supporting the idea that the increase in environmental heterogeneity diminishes species competition, allowing species co-occurrence.

Occurrence of Atta cephalotes (L) (Hymenoptera: Formicidae) in Alagoas, Northeastern Brazil / Occurrence of Atta cephalotes (L) (Hymenoptera: Formicidae) in Alagoas, Northeastern Brazil

Atta cephalotes (L.) presents a wide distribution in the Neotropical region. In Brazil, this species is disjunctly distributed in the Amazon region and in the states of Maranhão, Pernambuco and southern Bahia, all of which belong to Northeast Region. Here we report on a survey of Atta colonies in 42 remnants of the Atlantic rainforest in Alagoas and Pernambuco. Our results include new records of A. cephalotes in the state of Alagoas and thereby extend the known distribution area of its southern populations in Northeast Brazil. In addition, the species was strongly associated with well-conserved forest remnants, indicating that it is highly sensitive to disturbance. We suggest that, in the fragmented scenario of the northeastern Atlantic forest, A. cephalotes may be replaced by A. sexdens, which was present throughout all survey sites.

Atta cephalotes (L.) apresenta ampla distribuição na região Neotropical. No Brasil, foi registrada na região Amazônica e nos estados do Maranhão, Pernambuco e sul da Bahia, todos pertencentes à Região Nordeste. Neste trabalho é apresentado o registro de A. cephalotes em 42 remanescentes de Floresta Atlântica nordestina em Alagoas e Pernambuco. Os resultados incluem novos registros de A. cephalotes em Alagoas, ampliando a distribuição da espécie no Nordeste. Além disso, mostram que a espécie é fortemente associada a remanescentes de floresta bem conservados, indicando alta sensitividade a perturbações. É sugerido que, no cenário fragmentado da Floresta Atlântica nordestina, A. cephalotes pode estar sendo substituída por A. sexdens que esteve presente em todas as áreas amostradas.