Influence of nests of leaf-cutting ants on plant species diversity in road verges of northern Patagonia

It has been suggested that ant nests are the most frequent small-scale disturbance that affect vegetation patterns. However, their effects on plant diversity are little studied. We document effects of nests of the leaf-cutting ant Acromyrmex lobicornis on physical-chemical soil properties and their influence on plant diversity near road verges in a desert steppe in NW Patagonia, Argentina. We analysed nest soils and controls for nitrogen, phosphorus, organic matter, moisture retention capacity and texture. We also analysed the vegetation on 42 nests (30 active and 12 abandoned or without life) and 42 areas without nests. Soil around nests had a greater nutrient content and capacity to retain moisture than control soils, which is mainly due to the presence of organic waste that the ants deposit on the soil surface. We found no association between the occurrence of nests and specific groups of plants, but plant diversity was higher at nest-sites than at nearby non-nest sites. This increased diversity - which is also found on abandoned nests - is mainly due to the occurrence of a larger number of native and exotic plant species on nest-sites that are uncommon elsewhere in the study area. The most abundant plant species showed similar cover values at nest and non-nest sites. This suggests that changes in diversity are associated to edaphic changes caused by nests rather than by changes in competitive balance caused by dominant species exclusion. We propose that the nests of Acromyrmex lobicornis, through increasing the availability of resources, generate favourable microsites that can function both as 'refuges' for less frequent native species, and as 'stepping stones' for less frequent exotic plant species.