Positive ecosystem engineering effects of the ant Atta vollenweideri on the shrub Grabowskia duplicata

Question How does the ant Atta vollenweideri indirectly affect the shrub Grabowskia duplicata? Does environmental modification induced by nest building affect the life history and population structure of this dominant shrub? Location Halophytic savanna of western Uruguay, South America. Methods We compared the density of three shrub size classes, seedlings, saplings and adults, among patches created by ant nest building and in non-modified areas. We studied key soil properties for plant growth in both live nests and non-modified areas. We performed a greenhouse experiment to study the effects of soils (live nest versus non-modified area) on shrub germination probability. We also assessed vegetative propagation by measuring the incidence of root suckers and clonal offshoots in live nests and non-modified areas through a field survey. Results Seedling-size class was more abundant on live nests than in non-modified areas, suggesting that environmental conditions of this type of habitat (i.e. increased sodium and moisture content of soil, and decreased soil compaction) could facilitate shrub recruitment. We did not find any effects of soil type (live nests versus non-modified areas) on shrub germination probability. However, the vegetative propagation incidence was higher in the environment with live nests. Conclusions We found a strong indirect relationship between the ant A. vollenweideri and the shrub G. duplicata. This relationship seems to be mediated by an allogenic engineering process, i.e. soil bioturbation caused by nest building. The environmental conditions of live nests seem to facilitate shrub recruitment, mainly by increasing vegetative propagation.