Biologia, 3 Depto. Last instar male larvae were found on August 5, in a six-year-old colony with approximately L of fungus garden distributed in 21 pots. Thirty to forty days later, queen larvae started to be seen. The adult sexual forms were comparable in size with those found in the field. Two apparent failed attempts of a nuptial flight were observed during the last week of October, followed by the appearance of dead males and females in the garbage piles of the colony.
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Atta sexdens Hymenoptera: Formicidae nests are located under higher canopy cover in colombian amazon rainforests.
Abstract: Leafcutter ants, Atta and Acromyrmex spp. Some rainforest leafcutter ant species seem to be found in higher densities at forest edges or gaps, which in some cases are actively created by themselves. This implies the presence of a lower than average canopy cover which, in addition to several ant activities, may lead to vegetation diversification during and after colony life.
This study compared canopy cover at active Atta sexdens nests and at random sites in three habitat types in the Colombian Amazon for the years , and Canopy cover at nests and random sites was measured with a densiometer in and estimated by means of satellite images in and Contrary to expectation, canopy cover was found consistently higher at A.
This and other nest aspects differentiate A. Therefore, it is argued how A. Resumen: Se consideran las hormigas arrieras Atta y Acromyrmex spp. Algunas especies de hormiga arriera de la selva lluviosa parecen encontrarse en densidades mayores en los bordes de la selva o en aperturas, las cuales en algunos casos son creadas activamente por ellas mismas.
Palabras clave: Attini. Hormiga arriera. Imagen satelital. Leafcutter ants, genera Atta and Acromyrmex, are considered important ecosystem engineers in the Neotropics Fowler et al ; Holldobler and Wilson They are also of great relevance as key herbivores during successional processes Vasconcelos and Cherrett ; Vasconcelos and as pest species in planted forest such as agroforestry and restoration programs, and agricultural land Della-Lucia Their nests are heterogeneously distributed Jaffe and Vilela ; Lapointe et al.
For example, Atta cephalotes L. The palatable forage hypothesis argues that leafcutter ants are favored by these pioneer plant species due to their lower chemical and higher nutritional content Farji-Brener Many rainforest Atta species also actively create gaps by removing all vegetation on and around the nest Goncalves Gap creation, soil perturbation Alvarado et al.
Contrary to site selection of most Atta species, Atta sexdens Linneaus, nests in the Colombian Amazon were observed only in habitat types with high canopy cover van Gils et al. This study aimed to evaluate whether nests within these habitat types are located under relatively higher as seems to be suggested by van Gils et al. This study contributes to our knowledge of the impact of each leafcutter ant species on different, and especially human-disturbed, ecosystems Wirth et al.
Study area. Mean annual rainfall in the study area is mm. There is a distinct dry season from June to September and a wet season from October to May. The study area is part of the hilly sedimentary Amazon plain, which dates from the Tertiary Riano The soil is of marine and continental origin and mostly classified as Typic Hapludult Ultisol Brady and Weil Since the beginning of last century, human activity has also created patches of secondary forest, agricultural plots and housing sites in the area Riano ; Pinilla Three habitat types were included in this study: OF old rainforest , AF secondary rainforest - abandoned agricultural plots and GF secondary rainforest - abandoned grassland.
In each habitat type, the location of active A. The locations were determined indirectly for , and and they were mapped on satellite images as described in van Gils et al. Only the and satellite images were considered sufficiently free of cloud formation to be used in the current study. Canopy cover determination. In , canopy cover was determined with a Robert E. Measurements were carried out at each active A. Four measurements were taken towards the north, east, south and west on the most central nest mound or at each random site.
The same procedure was repeated 15 m to the north, east, south and west. This way, a canopy area of approximately 45 x 45 m was covered. Twenty measurements were taken at each nest or random site. The average was multiplied by 1. To get a relative indication of vegetation cover on active A. On these two images, all active nests of the respective years were projected.
Pixel values, therefore, do not represent actual canopy cover percentages, but give a relative indication: Low canopy cover reflects more light and it causes brighter grey pixels with higher values. In contrast, high canopy cover reflects less light and causes darker grey pixels with lower values. On the satellite images, it was aimed to determine pixel values of a canopy area similar to that covered by the densiometer method i.
In the same image, the values of an equal number of randomly located pixels in each habitat type were also determined. This way, a canopy area of 45 x 45 m was covered. Then, the average of these nine pixels was calculated to obtain one value per nest. In the same image, the average values of an equal number of randomly located groups of such nine pixels in each habitat type were also determined. Statistical analysis. Canopy cover percentages of and average pixel values of and were compared per habitat type and for all habitat types together with Kolmogorov-Smirnov tests carried out in statistics software program SPSS In and , nest pixel values were significantly lower in OF, GF and all habitat types together Table 1.
In AF, nest pixel values showed the same trend, but differences were not significant. Contrary to the nests of other common rainforest Atta species, this study showed that A. The lack of a clear pattern and significant differences in abandoned agricultural plot secondary forest AF may be due to a relatively large variability in canopy cover in this habitat or to the low sample number. It seems unlikely that the higher canopy cover at nests was caused only by non-removal of vegetation from A.
Although A. Atta worker ants frequently travel as far as m to reach suitable forage Eidmann ; Cherrett Therefore, high canopy cover micro-sites should not limit the access of ants to palatable pioneer plants.
Nests of other common rainforest leafcutter ant species, such as A. Partly due to this activity, both species strongly affect the surrounding vegetation structure and composition Garrettson et al. Farji-Brener ; Meyer ; Correa et al. However, colonization patterns and fine plant root growth of plants growing near nest mounds differ between nests of these two leafcutter ant species due to the above vs.
Another study on A. These studies clearly illustrate how the overall impact of each Atta species on the surrounding rainforest ecosystem is defined differently according to their particular behavior and nest-building characteristics Fowler et al. Contrary to A. This implies that less light reaches the forest floor at nest locations, even after colony death.
Furthermore, A. Increased light and nutrient availability and a bare forest floor are the principal ingredients for vegetation diversification and growth, especially for shade-intolerant plant species Correa et al. However, none of these are attributes of A. It seems that A. Therefore, general seedling germination and plant growth may even be reduced on and around these nests. The influence of A.
Further studies should elucidate the extent of A. I thank the community of Palmeras and the Barbosa family for allowing access to their lands. I am greatly indebted with. Andreas Gaigl and Cas Vanderwoude for comments made on an earlier version of the manuscript. Leaf-cutter ant Atta cephalotes influence on the morphology of Andepts in Costa Rica. Soil Science Society of America Journal The nature and properties of soils. Pearson Education, U. Some aspects of the distribution of pest species of the leaf-cutting ants in the Caribbean.
How leaf-cutting ants impact forests: drastic nest effects on light environment and plant assemblages. Oecologia Evaluation of digital and film hemispherical photography and spherical densiometry for measuring forest light environments. Canadian Journal of Forest Research Why are leaf-cutting ants more common in early secondary forests than in old-growth tropical forests? An evaluation of the palatable forage hypothesis.
Oikos The effect of abandoned leaf-cutting ant nests on plant assemblage composition in a tropical rainforest of Costa Rica. Do leaf-cutting ants make "bottom-up" gaps in neotropical forests?
Ecology Letters 3: The importance of where to dump the refuse: Seed banks and fine roots in nests of the leaf-cutting ants Atta cephalotes and Atta colombica. Biotropica Leaf-cutter ant assemblies: effects of latitude, vegetation and behaviors. En: Huxley C. A pest is a pest is a pest?
Colonies of Atta sexdens are found scattered throughout forest floors and fields - anywhere with an appropriate climate and abundant foliage to harvest. Rather than building upward, A. Atta sexdens divides its colony members into four major castes, each differentiated by a range of head widths. Gardener-Nurses: The smallest of the four castes, gardener-nurses have an average head width of 1.
Atta sexdens Hymenoptera: Formicidae nests are located under higher canopy cover in colombian amazon rainforests. Abstract: Leafcutter ants, Atta and Acromyrmex spp. Some rainforest leafcutter ant species seem to be found in higher densities at forest edges or gaps, which in some cases are actively created by themselves. This implies the presence of a lower than average canopy cover which, in addition to several ant activities, may lead to vegetation diversification during and after colony life. This study compared canopy cover at active Atta sexdens nests and at random sites in three habitat types in the Colombian Amazon for the years , and Canopy cover at nests and random sites was measured with a densiometer in and estimated by means of satellite images in and