A distribuiÃÃo das formigas cortadeiras (Atta chephalotes) e seu papel na regeneraÃÃo de um trecho de Floresta AtlÃntica Nordestina

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

The leaf-cutting ants are considered important herbivores in Neotropical primary forests because they promote spatial and temporal heterogeneity of resources such as light, nutrients and soil. Despite their known importance, their distribution in early secondary forests and their role on forest regeneration are still poorly studied. This work was developed in secondary forest patches of a Brazilian Atlantic forest remnant (8Â30âS, 35Â50âW) and its main objectives were: (1) to investigate the density of Atta cephalotes colonies and the size of their foraging areas in secondary forest patches following a 25-47.5 yr-old chronosequence; and (2) to evaluate the influence of these ants on seedling recruitment and establishment. At the first part of this thesis, we investigated the density of leaf-cutting ant colonies in 14 forest patches and accompanied, during one year, the foraging area size of 12 colonies. At the second part of this work, the composition and diversity of seedlings were analyzed in 12 1m2-plots randomly distributed in the foraging areas of 15 colonies and their respective control areas (nearby areas without activity of leaf-cutting ants). Moreover, seed germination and seedling mortality of three plant species (Tapirira guianensis - Anacardiaceae; Pouteria sp. - Sapotaceae; Simarouba amara â Simaroubaceae) were experimentally investigated at five different distances (3, 6, 9, 12 and 15 m from the colony border) of the same 15 colonies. According to our results, ant colonies density decreased and foraging area size increased with the increasing of forest patches age. Foraging areas size also showed a positive response to plant species richness. Based on our results, we estimated that forest patches with less than 30 years, where 19 of the 26 colonies were found, leaf-cutting ants could be able to exploit more than 89% of the total forest area (11,15 de 12,51 ha). Considering the results of the second chapter, seedlings diversity was lower inside the foraging areas (mean Shannon-Wienerâs diversity index: Hâ= 0.27 Â DP 0.23 bits/individual) than in the control areas (Hâ= 0.44 Â 0.61 bits/individual). Moreover, some species did occur only in the control areas, what indicates a taxonomic segregation. Only the species T. guianensis showed significant variation in seed germination in relation to the factors considered; we observed higher seed germination at greater distances (15 m) of the colony and in forest patches with regeneration age of 30-40 years. All the species presented significant differences in relation to their mortality causes. The principal mortality factor for T. guianensis seedlings was herbivory by other organisms, specially galling insects, while seedlings of the other two species were encountered mainly dried out or cutted by leaf-cutting ants. Inclusively, for the species Pouteria sp., the mortality by leaf-cutting ants tends to decrease with the distance from the colony border. Our results suggest that in forest patches, mainly those with regeneration age less than 30 years, a higher herbivory rate by the leaf-cutting ants is found because of two additive effects: a higher leaf-cutting ant density together with the reduction of foraging areas, what implies a concentration of herbivory in a smaller area. The leaf-cutting ants can also affect seed germination and limit the recruitment success of new seedlings; effects that could inhibit the regeneration of forests and, also, reduce plant species diversity. In conclusion, at secondary forest patches, leaf-cutting ants are amplifying the already negative effects of fragmentation on the plant community

ASSUNTO(S)

botanica floresta atlÃntica nordestina formigas cortadeiras herbivory regeneration establishment of seedlings forest succession secondary forests fragmentaÃÃo florestal atta spp.

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