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.