Spatial patterns of the ant Crematogaster scutellaris (Hymenoptera, Formicidae) in a model ecosystem

1. The spatial arrangement of individuals and populations may have deep influences on all the biotic interactions within a community. 2. The spatial distribution of nests of the ant Crematogaster scutellaris Olivier was analysed in an olive orchard in central Italy. As this species nests inside tree trunks, the regular structure of this simplified ecosystem may help to reduce the confounding effect of habitat heterogeneity on the spatial distribution of nests. In total, 531 trees were mapped and their shape (size and structure of the trunk) recorded. The presence of C. scutellaris nests in each tree was assessed in spring-summer and autumn 2006 and 2007. 3. The number of occupied trees changed in time, from a maximum of 129 (summer 2006) to a minimum of 60 (autumn 2007). Occupancy of tree was related to its shape, with larger trees being more frequently and more steadily occupied than smaller ones. 4. Nests were spatially aggregated, forming well-defined clusters, but aggregation was not explained by a corresponding clumping of larger trees. Nests belonging to the same cluster were usually not aggressive to each other, whereas aggression was more common between nests belonging to different clusters. The dynamic nature of the system coupled with the clustered distribution of nests, is consistent with a hypothesis of seasonal polydomy, and suggest that whereas some trees are steadily occupied (core) others are opportunistically colonised when new resources are discovered (satellites). 5. Clusters size distribution was shown to follow a truncated power-law, a finding consistent with the idea that clusters are self-organised structures dependent on local interactions. These results suggest that spatial self-organisation in ant colonies may in principle be more common than previously thought, although the mechanisms generating these patterns still need to be clarified.