Ecosystem engineering by leaf‐cutting ants: nests of Atta cephalotes drastically alter forest structure and microclimate

1. The role played by Atta species as ecosystem engineers remains poorly investigated despite previous evidence that their nests can impact plant assemblages. 2. In a large remnant of Atlantic forest, we compared forest structure at 36 Atta cephalotes nests to control sites and assessed shifts in microclimate along transects from nests up to 24 m into the forest (11 representative colonies). 3. Nests (average size: 55 m²) were virtually free of understorey vegetation with a high proportion of dead stems (up to 70%). 4. Canopy openness above colonies increased by roughly 40% compared with controls (5.3% at colony vs. 3.7% at control sites). 5. At nest centres, about 6% of the total radiation penetrated through the sparse canopy. Light levels declined exponentially, reaching a third (2%) in the unaffected forest understorey. 6. Likewise, maximum soil temperatures and daily amplitudes declined exponentially from 25 to 23 °C and 1.6 to 0.8 °C, respectively. Soil moisture increased significantly along transects, yet the effect was small and no differences were detected for air temperature and humidity. 7. We extrapolated that individual A. cephalotes nests modify the microclimate in an area of almost 200 m² on average. For the population, this amounts to 6% of the area along forest edges, where colonies are strongly aggregated, compared with only 0.6% in the forest interior. 8. Nests changed microclimate to an extent that has been reported to impact seed germination, plant growth, and survival of seedlings, conclusively demonstrating that leaf‐cutting ants act as ecosystem engineers.     

A Density Labelling Method for the Quantitation of Radioactive Label Recycling in Studies on Individual Protein Turnover

Recycling of radioactive label from degraded protein is a majordrawback in pulse-chase experiments for the measurement of proteinturnover, because it leads to underestimations of degradationconstants. This paper describes a density labelling method forthe quantitation of label recycling into individual proteins,whereby underestimated degradation constants can be corrected.The method requires a density difference between native (light)and density-labelled (heavy) protein of less than 10 g dm^–3and, therefore, allows using ²H₂O concentrations of only 50%(v/v) or even considerably lower. Plants are pulse-labelledwith ^l4C and grown on ²H₂O either during the pulse or followingthe chase. Distribution of ¹⁴C-label between light and heavyprotein is determined from a ¹⁴C-superimposition curve obtainedby isopycnic centrifugation of the purified protein in a CsClgradient. The superimposition curve mathematically is describedby the sum of two single Gaussian curves. It proved impossibleto determine all six unknown parameters of this function byan algorithm. Therefore, mean values (i.e. the peak positionsin the gradient) of light and heavy protein are determined frominternal markers (³H-labelled light or heavy protein). Evaluationof data is performed by a computerized curve fitting programwhich provides a numerical and a graphical result of label recycling.The detection limit of the method is 10% of the minor componentin a heterogenous population of light and heavy protein differingin density by only 8.0 g dm^–3. Key words: Density labelling, deuterium oxide, label recycling, protein degradation, protein turnover