Demographic parameters and soil turnover rates of a population of Atta cephalotes

Abstract. The distribution and abundance of a population of Atta cephalotes was surveyed along trails at La Selva Biological Field Station in Costa Rica during December 2000. Nests were mapped along trails passing through primary forest. This data was compared to an identical survey conducted between 1989 and 1991 (Perfecto and Vandermeer 1993). The two data sets were used to calculate percent natality and percent mortality of Atta nests, which then allowed a calculation of the overall population growth rate. A population growth rate of -.05 percent per annum was calculated, significantly different from the 22 percent per annum estimation of the previous survey. Based on the calculated demographic parameters and estimated average nest size, I estimate a complete forest soil turnover rate of 311 years. This is in concurrence with the Perfecto and Vandermeer (1993) estimation of 200-300 years. The apparent fluctuating population growth rate and rapid soil turnover rate is significant when considering the effects Atta nests have on the plant community and ecosystem of the rainforest. Studies have shown that Atta nests result in tremendous soil turnover; that the fungal gardens maintained within Atta nests create deep, nutrient rich chambers in the soil; and that the ground surface above the nests is kept relatively free of vegetation. Such clearing of space and change in resource availability creates "gaps" in the forest which effect understory plant composition during the life of a nest and after a colony dies or abandons a nest. The demographic parameters and soil turnover rates determined by this study indicate the magnitude at which these influences are taking place.